Dynamic Symmetric Key Provisioning Protocol (DSKPP)
draft-ietf-keyprov-dskpp-01.txt

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Abstract

DSKPP is a client-server protocol for initialization (and configuration) of symmetric keys to locally and remotely accessible cryptographic modules. The protocol can be run with or without private-key capabilities in the cryptographic modules, and with or without an established public-key infrastructure.

Three variations of the protocol support multiple usage scenarios. The four-pass (i.e., two round-trip) variant enables key generation in near real-time. With the four-pass variant, keys are mutually generated by the provisioning server and cryptographic module; provisioned keys are not transferred over-the-wire or over-the-air. Two- and one-pass variants enable secure and efficient download and installation of symmetric keys to a cryptographic module in environments where near real-time communication may not be possible.

This document builds on information contained in [RFC4758] (RSA, The Security Division of EMC, “Cryptographic Token Key Initialization Protocol (CT-KIP),” November 2006.), adding specific enhancements in response to implementation experience and liaison requests. It is intended, therefore, that this document or a successor version thereto will become the basis for subsequent progression of a symmetric key provisioning protocol specification on the standards track.

Table of Contents

1.  Introduction
    1.1.  Scope
    1.2.  Background
2.  Requirements Notation and Terminology
3.  Use Cases
    3.1.  Single Key Request
    3.2.  Multiple Key Requests
    3.3.  Session Time-Out Policy
    3.4.  Outsourced Provisioning
    3.5.  Key Renewal
    3.6.  Pre-Loaded Key Replacement
    3.7.  Pre-Shared Transport Key
    3.8.  SMS-Based Key Transport
    3.9.  Non-Protected Transport Layer
    3.10.  Non-Authenticated Transport Layer
4.  DSKPP Overview
    4.1.  Entities
    4.2.  Overview of Protocol Usage
    4.3.  Four-Pass Protocol Usage
        4.3.1.  Message Flow
        4.3.2.  Generation of Symmetric Keys for Cryptographic Modules
        4.3.3.  Client Authentication
        4.3.4.  Key Confirmation
        4.3.5.  Server Authentication
    4.4.  Two-Pass Protocol Usage
        4.4.1.  Message Flow
        4.4.2.  Key Confirmation
        4.4.3.  Server Authentication
    4.5.  One-Pass Protocol Usage
        4.5.1.  Message Flow
        4.5.2.  Key Confirmation
        4.5.3.  Server Authentication
5.  Methods Common to More Than One Protocol Variant
    5.1.  The DSKPP One-Way Pseudorandom Function, DSKPP-PRF
        5.1.1.  Introduction
        5.1.2.  Declaration
    5.2.  Encryption of Pseudorandom Nonces Sent from the DSKPP Client (Applicable to Four-Pass and Two-Pass DSKPP)
    5.3.  Client Authentication Mechanisms (Applicable to Four- and Two-Pass DSKPP)
        5.3.1.  Device Certificate
        5.3.2.  Device Identifier
        5.3.3.  Authentication Code
    5.4.  Client Authentication Examples
        5.4.1.  Example Using a MAC from an Authentication Code
        5.4.2.  Example Using a Device Certificate
6.  Four-Pass Protocol
    6.1.  XML Basics
    6.2.  Round-Trip #1: <KeyProvClientHello> and <KeyProvServerHello>
        6.2.1.  Examples
        6.2.2.  Components of the <KeyProvClientHello> Request
        6.2.3.  Components of the <KeyProvServerHello> Response
    6.3.  Round-Trip #2: <KeyProvClientNonce> and <KeyProvServerFinished>
        6.3.1.  Examples
        6.3.2.  Components of a <KeyProvClientNonce> Request
        6.3.3.  Components of a <KeyProvServerFinished> Response
    6.4.  DSKPP Server Results: The StatusCode Type
7.  Two-Pass Protocol
    7.1.  XML Basics
    7.2.  Round-Trip #1: <KeyProvClientHello> and <KeyProvServerFinished>
        7.2.1.  Examples
        7.2.2.  Components of the <KeyProvClientHello> Request
        7.2.3.  Components of a <KeyProvServerFinished> Response
    7.3.  DSKPP Server Results: The StatusCode Type
8.  One-Pass Protocol
    8.1.  XML Basics
    8.2.  Server to Client Only: <KeyProvServerFinished>
        8.2.1.  Example
        8.2.2.  Components of a <KeyProvServerFinished> Response
9.  Trigger
    9.1.  XML Basics
    9.2.  Example
    9.3.  Components of the <KeyProvTrigger> Message
10.  Extensibility
    10.1.  The ClientInfoType Type
    10.2.  The ServerInfoType Type
    10.3.  The KeyInitializationDataType Type
11.  Key Initialization Profiles of Two- and One-Pass DSKPP
    11.1.  Introduction
    11.2.  Key Transport Profile
        11.2.1.  Introduction
        11.2.2.  Identification
        11.2.3.  Payloads
    11.3.  Key Wrap Profile
        11.3.1.  Introduction
        11.3.2.  Identification
        11.3.3.  Payloads
    11.4.  Passphrase-Based Key Wrap Profile
        11.4.1.  Introduction
        11.4.2.  Identification
        11.4.3.  Payloads
12.  Protocol Bindings
    12.1.  General Requirements
    12.2.  HTTP/1.1 Binding for DSKPP
        12.2.1.  Introduction
        12.2.2.  Identification of DSKPP Messages
        12.2.3.  HTTP Headers
        12.2.4.  HTTP Operations
        12.2.5.  HTTP Status Codes
        12.2.6.  HTTP Authentication
        12.2.7.  Initialization of DSKPP
        12.2.8.  Example Messages
13.  DSKPP Schema
14.  Security Considerations
    14.1.  General
    14.2.  Active Attacks
        14.2.1.  Introduction
        14.2.2.  Message Modifications
        14.2.3.  Message Deletion
        14.2.4.  Message Insertion
        14.2.5.  Message Replay
        14.2.6.  Message Reordering
        14.2.7.  Man-in-the-Middle
    14.3.  Passive Attacks
    14.4.  Cryptographic Attacks
    14.5.  Attacks on the Interaction between DSKPP and User Authentication
    14.6.  Additional Considerations Specific to 2- and 1-pass DSKPP
        14.6.1.  Client Contributions to K_TOKEN Entropy
        14.6.2.  Key Confirmation
        14.6.3.  Server Authentication
        14.6.4.  Client Authentication
        14.6.5.  Key Protection in the Passphrase Profile
15.  Internationalization Considerations
16.  IANA Considerations
17.  Intellectual Property Considerations
18.  Contributors
19.  Acknowledgements
20.  References
    20.1.  Normative references
    20.2.  Informative references
Appendix A.  Integration with PKCS #11
    A.1.  The 4-pass Variant
    A.2.  The 2-pass Variant
    A.3.  The 1-pass Variant
Appendix B.  Example of DSKPP-PRF Realizations
    B.1.  Introduction
    B.2.  DSKPP-PRF-AES
        B.2.1.  Identification
        B.2.2.  Definition
        B.2.3.  Example
    B.3.  DSKPP-PRF-SHA256
        B.3.1.  Identification
        B.3.2.  Definition
        B.3.3.  Example
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

1.1.  Scope

This document describes a client-server protocol for initialization (and configuration) of symmetric keys to locally and remotely accessible cryptographic modules. The protocol can be run with or without private-key capabilities in the cryptographic modules, and with or without an established public-key infrastructure. The objectives of this protocol are to:

• Provide a secure method of initializing cryptographic modules with symmetric keys without exposing generated, secret material to any other entities than the server and the cryptographic module itself.
• Provide a secure method of generating and transporting symmetric keys to a cryptographic module in environments where near real-time communication is not possible.
• Provide a secure method of transporting pre-generated (i.e., legacy) keys to a cryptographic module.
• Provide a solution that is easy to administer and scales well.

The mechanism is intended for general use within computer and communications systems employing symmetric key cryptographic modules that are locally (i.e., over-the-wire) or remotely (i.e., over-the-air) accessible.

1.2.  Background

A locally accessible symmetric key cryptographic module may be hosted by, for example, a hardware device connected to a personal computer through an electronic interface, such as USB, or a software application resident on a personal computer. A remotely accessible symmetric key cryptographic module may be hosted by, for example, any device that can support over-the-air communication, such as a hand-held hardware device (e.g., a mobile phone). The cryptographic module itself offers symmetric key cryptographic functionality that may be used to authenticate a user towards some service, perform data encryption, etc. Increasingly, these modules enable their programmatic initialization as well as programmatic retrieval of their output values. This document intends to meet the need for an open and inter-operable mechanism to programmatically initialize and configure symmetric keys to locally and remotely accessible cryptographic modules.

The target mechanism makes use of a symmetric key provisioning server. In an ideal deployment scenario, near real-time communication is possible between the provisioning server and the cryptographic module. In such an environment, it is possible for the cryptographic module and provisioning server to mutually generate a symmetric key, and to ensure that keys are not transported between them.

There are, however, several deployment scenarios that make mutual key generation less suitable. Specifically, scenarios where near real-time communication between the symmetric key provisioning server and the cryptographic module is not possible, and scenarios with significant design constraints. Examples include work-flow constraints (e.g., policies that require incremental administrative approval), network design constraints that create network latency, and budget constraints that sustain reliance upon legacy systems that already have supplies of pre-generated keys. In these situations, the cryptographic module is required to download and install a symmetric key from the provisioning server in a secure and efficient manner.

This document tries to meet the needs of these scenarios by describing three variations to DSKPP for the provisioning of symmetric keys in two round trips or less. The four-pass (i.e., two round-trip) variant enables key generation in near real-time. With this variant, keys are mutually generated by the provisioning server and cryptographic module; provisioned keys are not transferred over-the-wire or over-the-air. In contrast, two- and one-pass variants enable secure and efficient download and installation of symmetric keys to a cryptographic module in environments where near real-time communication is not possible.

2.  Requirements Notation and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (, “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

The following notations are used in this document:

||

String concatenation

[x]

Optional element x

A ^ B

Exclusive-OR operation on strings A and B (where A and B are of equal length)

ENC_X(Y)

Encryption of message Y with symmetric key X, using a defined block cipher

ENC_PX(Y)

Encryption using message Y with a public key X

KDF_X(Y)

Key derivation function that generates an arbitrary number of octets of output using secret X and seed Y

DSKPP-PRF_X(Y,Z)

Pseudo random function that generates a fixed number Z of octets using secret X and seed Y (used in DSKPP methods for MAC computations and key derivation)

MAC_X(Y)

Keyed message authentication code computed over Y with symmetric key X

SIGN_x(Y)

Function that provides authentication and integrity protection of message content Y using private key x

B64(X)

Base 64 encoding of string X

H(X)

Hash function applied to X

Alg_List

List of encryption and MAC algorithms supported by the client

Alg_Sel

Algorithms list selected by the server for the DSKPP protocol run

DSKPP client

Manages communication between the symmetric key cryptographic module and the DSKPP server

DSKPP server

The symmetric key provisioning server that participates in the DSKPP protocol run

Issuer

The organization that issues or authorizes issuance of the symmetric key to the end user of the symmetric key cryptographic module (e.g., a bank who issues one-time password authentication tokens to their retail banking users)

ID_C

Identifier for DSKPP client

ID_S

Identifier for DSKPP server

AUTHCODE

Client Authentication Code comprised of a string of numeric characters known to the device and the server and containing an identifier and a password (the AUTHCODE may be used to derive the AUTHDATA during the DSKPP protocol exchange)

AUTHDATA

Client Authentication Data that may be derived from the AUTHCODE or using the client private key, k_CLIENT

K

Key used to encrypt R_C (either K_SERVER or K_SHARED)

K_AUTHCODE

Secret key that is derived from AUTHCODE and used for client authentication purposes

k_CLIENT

Private key of the DSKPP client

K_CLIENT

Public key of the DSKPP client

K_DERIVED

Secret key derived from a passphrase that is known to both the DSKPP client or user and the DSKPP server

K_MAC

Secret key used for key confirmation and server authentication purposes, and generated in DSKPP

K_MAC'

A second secret key used for server authentication purposes in 2- and 1-pass DSKPP

K_SERVER

Public key of the DSKPP server

K_SHARED

Secret key shared between the DSKPP client and the DSKPP server

K_TOKEN

Secret key used for cryptographic module computations, and generated in DSKPP

K_CONFDATA

Key configuration data carried within the key container

R

Pseudorandom value chosen by the DSKPP client and used for MAC computations, which is mandatory for 2-pass DSKPP and optional for 4-pass

R_C

Pseudorandom value chosen by the DSKPP client and used as input to the generation of K_TOKEN

R_S

Pseudorandom value chosen by the DSKPP server and used as input to the generation of K_TOKEN

URL_S

Server address as a URL

I

Unsigned integer representing a counter value that is monotonically increasing and guaranteed not to be used again by the server towards the cryptographic module

I'

Similar to I except I' is always higher than I

The following typographical convention is used in the body of the text: <XMLElement>.

3.  Use Cases

This section describes typical use cases.

3.1.  Single Key Request

A cryptographic module hosted by a device, such as a mobile phone, makes a request for a symmetric key from a provisioning server. Depending upon how the system is deployed, the provisioning server may generate a new key on-the-fly or use a pre-generated key, e.g., one provided by a legacy back-end issuance server. The provisioning server assigns a unique key ID to the symmetric key and provisions it to the cryptographic module.

3.2.  Multiple Key Requests

A cryptographic module makes multiple requests for symmetric keys from the same provisioning server. The symmetric keys may or may not be of the same type, i.e., the keys may be used with different symmetric key cryptographic algorithms, including one-time password authentication algorithms, and AES encryption algorithm.

3.3.  Session Time-Out Policy

Once a cryptographic module initiates a symmetric key request, the provisioning server may require that any subsequent actions to complete the provisioning cycle occur within a certain time window. For example, an issuer may provide a time-limited authentication code to a user during registration, which the user will input into the cryptographic module to authenticate themselves with the provisioning server. If the user inputs a valid authentication code within the fixed time period established by the issuer, the server will allow a key to be provisioned to the cryptographic module hosted by the user's device.

3.4.  Outsourced Provisioning

A symmetric key issuer outsources its key provisioning to a third party key provisioning server provider. The issuer is responsible for authenticating and granting rights to users to acquire keys while acting as a proxy to the cryptographic module to acquire symmetric keys from the provisioning server; the cryptographic module communicates with the issuer proxy server, which forwards provisioning requests to the provisioning server.

3.5.  Key Renewal

A cryptographic module requests renewal of a symmetric key using the same key ID already associated with the key. Such a need may occur in the case when a user wants to upgrade her device that houses the cryptographic module or when a key has expired. When a user uses the same cryptographic module to, for example, perform strong authentication at multiple Web login sites, keeping the same key ID removes the need for the user to register a new key ID at each site.

3.6.  Pre-Loaded Key Replacement

This use case represents a special case of symmetric key renewal in which a local administrator can authenticate the user procedurally before initiating the provisioning process. It also allows for an issuer to pre-load a key onto a cryptographic module with a restriction that the key is replaced with a new key prior to use of the cryptographic module. Another variation of this use case is the issuer who recycles devices. In this case, an issuer would provision a new symmetric key to a cryptographic module hosted on a device that was previously owned by another user.

Note that this use case is essentially the same as the last use case wherein the same key ID is used for renewal.

3.7.  Pre-Shared Transport Key

A cryptographic module is loaded onto a smart card after the card is issued to a user. The symmetric key for the cryptographic module will then be provisioned using a secure channel mechanism present in many smart card platforms. This allows a direct secure channel to be established between the smart card chip and the provisioning server. For example, the card commands (i.e., Application Protocol Data Units, or APDUs) are encrypted with a pre-shared transport key and sent directly to the smart card chip, allowing secure post-issuance in-the-field provisioning. This secure flow can pass Transport Layer Security (TLS) and other transport security boundaries.

Note that two pre-conditions for this use case are for the protocol to be tunneled and the provisioning server to know the correct pre-established transport key.

3.8.  SMS-Based Key Transport

A mobile device supports Short Message Service (SMS) but is not able to support a data service allowing for HTTP or HTTPS transports. In addition, an application may use a cryptographic module to enforce an acceptable level of protection for download of the symmetric key via SMS. In such a case, the cryptographic module hosted by the mobile device may initiate a symmetric key request from a desktop computer and ask the server to send the key to the mobile device through SMS. User authentication is carried out via the online communication established between the desktop computer and the provisioning server.

3.9.  Non-Protected Transport Layer

Some devices are not able to support a secure transport channel such as SSL or TLS to provide data confidentiality. A cryptographic module hosted by such a device requests a symmetric key from the provisioning server. It is up to DSKPP to ensure data confidentiality over non-secure networks.

3.10.  Non-Authenticated Transport Layer

Some devices are not able to use a transport protocol that provides server authentication such as SSL or TLS. A cryptographic module hosted by such a device wants to be sure that it sends a request for a symmetric key to a legitimate provisioning server. It is up to DSKPP to provide proper client and server authentication.

4.  DSKPP Overview

4.1.  Entities

In principle, the protocol involves a DSKPP client and a DSKPP server. The DSKPP client manages communication between the cryptographic module and the provisioning server. The DSKPP server herein represents the provisioning server.

A high-level object model that describes the client-side entities and how they relate to each other is shown in Figure 1 (Object Model).

-----------          -------------
| User    |          | Device    |
|---------|*  owns  *|-----------|
| UserID  |--------->| DeviceID  |
| ...     |          | ...       |
-----------          -------------
                         | 1
                         |
                         | contains
                         |
                         | *
                         V
                   -----------------------
                   |Cryptographic Module |
                   |---------------------|
                   |CryptoModuleID
                   |Encryption Algorithms|
                   |MAC Algorithms       |
                   |...                  |
                   -----------------------
                         | 1
                         |
                         | contains
                         |
                         | *
                         V
                   -----------------------
                   |Key Container        |
                   |---------------------|
                   |KeyID                |
                   |Key Type             |
                   |...                  |
                   -----------------------

Conceptually, each entity represents the following:

User:

The person or client to whom devices are issued

UserID:

A unique identifier for the user or client

Device:

A physical piece of hardware or software framework that hosts symmetric key cryptographic modules

DeviceID:

A unique identifier for the device

Cryptographic Module:

A component of an application, which enables symmetric key cryptographic functionality

CryptoModuleID:

A unique identifier for an instance of the cryptographic module

Encryption Algorithms:

Encryption algorithms supported by the cryptographic module

MAC Algorithms:

MAC algorithms supported by the cryptographic module

Key Container:

An object that encapsulates a symmetric key and its configuration data

KeyID:

A unique identifier for the symmetric key

Key Type:

The type of symmetric key cryptographic methods for which the key will be used (e.g., OATH HOTP or RSA SecurID authentication, AES encryption, etc.)

It is assumed that a device will host an application layered above the cryptographic module, and this application will manage communication between the DSKPP client and cryptographic module. The manner in which the communicating application will transfer DSKPP protocol elements to and from the cryptographic module is transparent to the DSKPP server. One method for this transfer is described in [CT‑KIP‑P11] (RSA Laboratories, “PKCS #11 Mechanisms for the Cryptographic Token Key Initialization Protocol,” December 2005.).

4.2.  Overview of Protocol Usage

DSKPP enables symmetric key provisioning between a DSKPP server and DSKPP client. The DSKPP protocol supports the following types of requests and responses:

<KeyProvClientHello>

With this request, a DSKPP client initiates contact with the DSKPP server, indicating what protocol versions and variants, key types, encryption and MAC algorithms that it supports. In addition, the request may include client authentication data that the DSKPP server uses to verify proof-of-possession of the device.

<KeyProvServerHello>

Upon reception of a <KeyProvClientHello> request, the DSKPP server uses the <KeyProvServerHello> response to specify which protocol version and variant, key type, encryption algorithm, and MAC algorithm that will be used by the DSKPP server and DSKPP client during the protocol run. The decision of which variant, key type, and cryptographic algorithms to pick is policy- and implementation-dependent and therefore outside the scope of this document.

The <KeyProvServerHello> response includes the DSKPP server's random nonce, R_S. The response also consists of information about either a shared secret key, or its own public key, that the DSKPP client uses when sending its protected random nonce, R_C, in the <KeyProvClientNonce> request (see below).

Optionally, the DSKPP server may provide a MAC that the DSKPP client may use for server authentication.

<KeyProvClientNonce>

With this request, a DSKPP client and DSKPP server securely exchange protected data, e.g., the protected random nonce R_C. In addition, the request may include client authentication data that the DSKPP server uses to verify proof-of-possession of the device.

<KeyProvServerFinished>

The <KeyProvServerFinished> response is a confirmation message that includes a key container that holds configuration data, and may also contain protected key material (this depends on the protocol variant, as discussed below).

Optionally, the DSKPP server may provide a MAC that the DSKPP client may use for server authentication.

To initiate a DSKPP session:

1.

A user may use a browser to connect to a web server that is running on some host. The user may then identify (and optionally authenticate) herself (through some means that essentially are out of scope for this document) and request a symmetric key.

2.

A client application may request a symmetric key by invoking the DSKPP client.

3.

A DSKPP server may send a trigger message to a client application, which would then invoke the DSKPP client.

To contact the DSKPP server:

1.

A user may indicate how the DSKPP client is to contact a certain DSKPP server during a browsing session.

2.

A DSKPP client may be pre-configured to contact a certain DSKPP server.

3.

A user may be informed out-of-band about the location of the DSKPP server.

Once the location of the DSKPP server is known, the DSKPP client and the DSKPP server engage in a 4-pass, 2-pass, or 1-pass protocol. Depending upon the policy and implementation, a DSKPP server selects which variant of the protocol to use: 4-pass, 2-pass, or 1-pass. With the four-pass variant, keys are mutually generated by the DSKPP server and DSKPP client; provisioned keys are not transferred over-the-wire or over-the-air. Two- and one-pass variants enable secure and efficient download and installation of symmetric keys to a DSKPP client in environments where near real-time communication may not be possible.Figure 2 (The DSKPP protocol (with OPTIONAL preceding trigger)) shows which messages get exchanged during each type of protocol run (4-pass, 2-pass, or 1-pass).

+---------------+                            +---------------+
|               |                            |               |
|  DSKPP client |                            |  DSKPP server |
|               |                            |               |
+---------------+                            +---------------+
        |                                            |
        |        [ <---- DSKPP trigger ----- ]       |
        |                                            |
        |        ------- Client Hello ------->       |
        |        (Applicable to 4- and 2-pass)       |
        |                                            |
        |        <------ Server Hello --------       |
        |         (Applicable to 4-pass only)        |
        |                                            |
        |        ------- Client Nonce ------->       |
        |         (Applicable to 4-pass only)        |
        |                                            |
        |        <----- Server Finished ------       |
        |      (Applicable to 4-, 2-, and 1-pass)    |
        |                                            |

The table below identifies which protocol variants may be applied to the use cases from Section 3 (Use Cases):

----------------------------------------------------------
Protocol   Applicable                Applicable
Variant    Use Cases                 Deployment Scenarios
----------------------------------------------------------
4-pass     All but 3.6 and           Near real-time
           3.8 if mutual key         communication is
           generation is desired;    possible
           none if transport of
           a pre-generated key

2-pass     All                       Either near real-time
                                     or non real-time
                                     communication may be
                                     possible

1-pass     All but 3.8               Either near real-time
                                     or non real-time
                                     communication may be
                                     possible

4.3.  Four-Pass Protocol Usage

The 4-pass protocol flow is suitable for environments wherein there is near real-time communication possible between the DSKPP client and DSKPP server. It is not suitable for environments wherein administrative approval is a required step in the flow, nor for provisioning of pre-generated keys.

The full four-pass protocol exchange is as follows:

[<Trigger>]:

[ID_Device], [ID_K], [URL_S], [R_S]

<KeyProvClientHello>:

[ID_Device], [ID_K], [R_S], Alg_List

<KeyProvServerHello>:

R_S, Alg_Sel, [K_SERVER], [DSKPP-PRF_K_MAC'("MAC 1 Computation" || [R] || R_S, len(R_S))

<KeyProvClientNonce>:

AUTHDATA, ENC_PK_SERVER(R_C) OR AUTHDATA, ENC_K_SHARED(R_C)

<KeyProvServerFinished>:

K_CONFDATA, DSKPP-PRF_K_MAC("MAC 2 Computation"||R_C, len(R_C))

The following subsections describe the exchange in more detail.

4.3.1.  Message Flow

The 4-pass protocol flow consists of two round trips between the DSKPP client and DSKPP server (see Figure 2 (The DSKPP protocol (with OPTIONAL preceding trigger))), where each round-trip involves two "passes", i.e., one request message and one response message:

Round-trip #1:

Pass 1 = <KeyProvClientHello>, Pass 2 = <KeyProvServerHello>

Round-trip #2:

Pass 3 = <KeyProvClientNonce>, Pass 4 = <KeyProvServerFinished>

4.3.1.1.  Round-trip #1: <KeyProvClientHello> and <KeyProvServerHello>

The DSKPP client sends a <KeyProvClientHello> message to the DSKPP server. The message provides information to the DSKPP server about the DSKPP versions, protocol variants, key types, encryption and MAC algorithms supported by the cryptographic module for the purposes of this protocol.

The DSKPP server responds to the DSKPP client with a <KeyProvServerHello> message, whose content includes a random nonce, R_S, along with information about the type of key to generate, and the encryption algorithm chosen to protect sensitive data sent in the protocol. The length of the nonce R_S may depend on the selected key type. The <KeyProvServerHello> message also provides information about either a shared secret key to use for encrypting the cryptographic module's random nonce (see description of <KeyProvClientNonce> below), or its own public key. Optionally, <KeyProvServerHello> may include a MAC that the DSKPP client may use for server authentication during key replacement.

4.3.1.2.  Round-trip #2: <KeyProvClientNonce> and <KeyProvServerFinished>

Based on information contained in the <KeyProvServerHello> message, the cryptographic module generates a random nonce, R_C. The length of the nonce R_C may depend on the selected key type. The cryptographic module encrypts R_C using the selected encryption algorithm and with a key, K, that is either the DSKPP server's public key, K_SERVER, or a shared secret key, K_SHARED, as indicated by the DSKPP server. If K is equivalent to K_SERVER, then the cryptographic module SHOULD verify the server's certificate before using it to encrypt R_C in accordance with [RFC3280] (Housley, R., Polk, W., Ford, W., and D. Solo, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” April 2002.). The DSKPP client then sends the encrypted random nonce to the DSKPP server in a <KeyProvClientNonce> message, and may include client authentication data, such as a certificate or MAC derived from an authentication code and R_C. Finally, the cryptographic module calculates a symmetric key, K_TOKEN, of the selected type from the combination of the two random nonces R_S and R_C, the encryption key K, and possibly some other data, using the DSKPP-PRF function defined in Section 5.1 (The DSKPP One-Way Pseudorandom Function, DSKPP-PRF).

The DSKPP server decrypts R_C, calculates K_TOKEN from the combination of the two random nonces R_S and R_C, the encryption key K, and possibly some other data, using the DSKPP-PRF function defined in Section 5.1 (The DSKPP One-Way Pseudorandom Function, DSKPP-PRF). The server then associates K_TOKEN with the cryptographic module in a server-side data store. The intent is that the data store later on will be used by some service that needs to verify or decrypt data produced by the cryptographic module and the key.

Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called <KeyProvServerFinished>. Optionally, <KeyProvServerFinished> may include a MAC that the DSKPP client may use for server authentication. The confirmation message includes a key container that holds an identifier for the generated key (but not the key itself) and additional configuration information, e.g., the identity of the DSKPP server. The default symmetric key container format that is used in the <KeyProvServerFinished> message is based on the Portable Symmetric Key Container (PSKC) defined in [PSKC] (, “Portable Symmetric Key Container,” 2005.). Alternative formats MAY include PKCS#12 [PKCS‑12] (, “Personal Information Exchange Syntax Standard,” 2005.) or PKCS#5 XML [PKCS‑5‑XML] (RSA Laboratories, “XML Schema for PKCS #5 Version 2.0,” October 2006.) format.

Upon receipt of the DSKPP server's confirmation message, the cryptographic module associates the provided key container with the generated key K_TOKEN, and stores any provided configuration data.

4.3.2.  Generation of Symmetric Keys for Cryptographic Modules

With 4-pass DSKPP, the symmetric key that is the target of provisioning, is generated on-the-fly without being transferred between the DSKPP client and DSKPP server. A sample data flow depicting how this works followed by computational information are provided in the subsections below.

4.3.2.1.  Data Flow

A sample data flow showing key generation during the 4-pass protocol is shown in Figure 4 (Principal data flow for DSKPP key generation - using public server key).

+----------------------+    +-------+     +----------------------+
|    +------------+    |    |       |     |                      |
|    | Server key |    |    |       |     |                      |
| +<-|  Public    |------>------------->-------------+---------+ |
| |  |  Private   |    |    |       |     |          |         | |
| |  +------------+    |    |       |     |          |         | |
| |        |           |    |       |     |          |         | |
| V        V           |    |       |     |          V         V |
| |   +---------+      |    |       |     |        +---------+ | |
| |   | Decrypt |<-------<-------------<-----------| Encrypt | | |
| |   +---------+      |    |       |     |        +---------+ | |
| |      |  +--------+ |    |       |     |            ^       | |
| |      |  | Server | |    |       |     |            |       | |
| |      |  | Random |--->------------->------+  +----------+  | |
| |      |  +--------+ |    |       |     |   |  | Client   |  | |
| |      |      |      |    |       |     |   |  | Random   |  | |
| |      |      |      |    |       |     |   |  +----------+  | |
| |      |      |      |    |       |     |   |        |       | |
| |      V      V      |    |       |     |   V        V       | |
| |   +------------+   |    |       |     | +------------+     | |
| +-->|  DSKPP PRF |   |    |       |     | |  DSKPP PRF |<----+ |
|     +------------+   |    |       |     | +------------+       |
|           |          |    |       |     |       |              |
|           V          |    |       |     |       V              |
|       +-------+      |    |       |     |   +-------+          |
|       |  Key  |      |    |       |     |   |  Key  |          |
|       +-------+      |    |       |     |   +-------+          |
|       +-------+      |    |       |     |   +-------+          |
|       |Key Id |-------->------------->------|Key Id |          |
|       +-------+      |    |       |     |   +-------+          |
+----------------------+    +-------+     +----------------------+
      DSKPP Server         DSKPP Client         DSKPP Client
                            (PC Host)      (cryptographic module)

Note: Conceptually, although R_C is one pseudorandom string, it may be viewed as consisting of two components, R_C1 and R_C2, where R_C1 is generated during the protocol run, and R_C2 can be pre-generated and loaded on the cryptographic module before the device is issued to the user. In that case, the latter string, R_C2, SHOULD be unique for each cryptographic module.

The inclusion of the two random nonces R_S and R_C in the key generation provides assurance to both sides (the cryptographic module and the DSKPP server) that they have contributed to the key's randomness and that the key is unique. The inclusion of the encryption key K ensures that no man-in-the-middle may be present, or else the cryptographic module will end up with a key different from the one stored by the legitimate DSKPP server.

Note: A man-in-the-middle (in the form of corrupt client software or a mistakenly contacted server) may present his own public key to the cryptographic module. This will enable the attacker to learn the client's version of K_TOKEN. However, the attacker is not able to persuade the legitimate server to derive the same value for K_TOKEN, since K_TOKEN is a function of the public key involved, and the attacker's public key must be different than the correct server's (or else the attacker would not be able to decrypt the information received from the client). Therefore, once the attacker is no longer "in the middle," the client and server will detect that they are "out of sync" when they try to use their keys. In the case of encrypting R_C with K_SERVER, it is therefore important to verify that K_SERVER really is the legitimate server's key. One way to do this is to independently validate a newly generated K_TOKEN against some validation service at the server (e.g. by using a connection independent from the one used for the key generation).

4.3.2.2.  Computing the Symmetric Key

In DSKPP, keys are generated using the DSKPP-PRF function defined in Section 5.1 (The DSKPP One-Way Pseudorandom Function, DSKPP-PRF), a secret random value R_C chosen by the DSKPP client, a random value R_S chosen by the DSKPP server, and the key k used to encrypt R_C. The input parameter s of DSKPP-PRF is set to the concatenation of the (ASCII) string "Key generation", k, and R_S, and the input parameter dsLen is set to the desired length of the key, K_TOKEN (the length of K_TOKEN is given by the key's type):

dsLen = (desired length of K_TOKEN)

K_TOKEN = DSKPP-PRF (R_C, "Key generation" || k || R_S, dsLen)

When computing K_TOKEN above, the output of DSKPP-PRF MAY be subject to an algorithm-dependent transform before being adopted as a key of the selected type. One example of this is the need for parity in DES keys.

4.3.3.  Client Authentication

To ensure that a generated key K_TOKEN ends up associated with the correct cryptographic module and user, the DSKPP client using any of the methods described in Section 5.3 (Client Authentication Mechanisms (Applicable to Four- and Two-Pass DSKPP) ). Whatever the method, the DSKPP server MUST ensure that a generated key is associated with the correct cryptographic module, and if applicable, the correct user.

4.3.4.  Key Confirmation

In four-pass DSKPP, the client includes a nonce R_C in the <KeyProvClientHello> message. The MAC value in the <KeyProvServerFinished> message MUST be computed on the (ASCII) string "MAC 2 computation", the client nonce R_C using a MAC key K_MAC. This key MUST be generated together with K_TOKEN using R_C and R_S.

The MAC value in <KeyProvServerFinished> MAY be computed by using the DSKPP-PRF function of Section 5.1 (The DSKPP One-Way Pseudorandom Function, DSKPP-PRF), in which case the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 2 computation", R_C, the parameter dsLen MUST be set to the length of R_C:

dsLen = len(R_C)

MAC = DSKPP-PRF (K_MAC, "MAC 2 computation" || R_C, dsLen)

4.3.5.  Server Authentication

A DSKPP server MUST authenticate itself to avoid a false "Commit" of a symmetric key that which could cause the cryptographic module to end up in an initialized state for which the server does not know the stored key. To do this, the DSKPP server authenticates itself by including a MAC value in the <KeyProvServerHello> message when replacing a existing key. The MAC value is generated using the existing the MAC key K_MAC' (the MAC key that existed before this protocol run). The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes. In addition, a DSKPP server can leverage transport layer authentication if it is available.

When the MAC value is used for server authentication, the value MAY be computed by using the DSKPP-PRF function of Section 5.1 (The DSKPP One-Way Pseudorandom Function, DSKPP-PRF), in which case the input parameter s MUST be set to the concatenation of the (ASCII) string "MAC 1 computation", R (if sent by the client), and R_S, and k MUST be set to the existing MAC key K_MAC' . The input parameter dsLen MUST be set to the length of R_S:

dsLen = len(R_S)

MAC = DSKPP-PRF (K_MAC', "MAC 1 computation" || [R ||] R_S, dsLen)

4.4.  Two-Pass Protocol Usage

The 2-pass protocol flow is suitable for environments wherein near real-time communication between the DSKPP client and server may not be possible. It is also suitable for environments wherein administrative approval is a required step in the flow, and for provisioning of pre-generated keys. In the 2-pass protocol flow, the client's initial <KeyProvClientHello> message is directly followed by a <KeyProvServerFinished> message. There is no exchange of the <KeyProvServerHello> message or the <KeyProvClientNonce> message. However, as the two-pass variant of DSKPP consists of one round trip to the server, the client is still able to include its random nonce, R_C, algorithm preferences and supported key types in the <KeyProvClientHello> message. Note that by including R_C in <KeyProvClientHello>, the DSKPP client is able to ensure the server is alive before "committing" the key. Also note that the DSKPP "trigger" message MAY be used to trigger the client's sending of the <KeyProvClientHello> message.

Essentially, two-pass DSKPP is a transport of key material from the DSKPP server to the DSKPP client. Two-pass DSKPP supports multiple key initialization methods that ensure K_TOKEN is not exposed to any other entity than the DSKPP server and the cryptographic module itself. Currently, three such key initialization methods are defined (refer to Section 11 (Key Initialization Profiles of Two- and One-Pass DSKPP)), each supporting a different usage of 2-pass DSKPP:

Key Transport

This profile is intended for PKI-capable devices. Key transport is carried out using a public key, K_CLIENT, whose private key part resides in the cryptographic module as the transport key.

Key Wrap

This profile is ideal for pre-keyed devices, e.g., SIM cards. Key wrap is carried out using a symmetric key-wrapping key, K_SHARED, which is known in advance by both the cryptographic module and the DSKPP server.

Passphrase-Based Key Wrap

This profile is a variation of the Key Wrap Profile. It is applicable to constrained devices with keypads, e.g., mobile phones. Key wrap is carried out using a passphrase-derived key-wrapping key, K_DERIVED, which is known in advance by both the cryptographic module and DSKPP server.

The full 2-pass protocol exchange when the key is transported using the client public key is as follows:

[<Trigger>]:

[ID_Device], [ID_K], [URL_S],[R_S]

<KeyProvClientHello>:

[ID_Device], ID_K, R_S, R_C, AUTHDATA, Alg_List

<KeyProvServerFinished>:

ENC_K_CLIENT ( K_TOKEN || K_MAC)), K_CONFDATA, ID_S, DSKPP-PRF_K_MAC("MAC 1 Computation" || ID_S || R_C, len(R_C) ), [ DSKPP-PRF_K_MAC'("MAC 1 Computation" || ID_S || R_C), 16]

The full 2-pass protocol exchange when the key is wrapped using a shared key is as follows:

[<Trigger>]:

[ID_Device], [ID_K], [URL_S],[R_S]

<KeyProvClientHello>:

[ID_Device], ID_K, R_S, R_C, AUTHDATA, Alg_List

<KeyProvServerFinished>:

ENC_K_SHARED(K_TOKEN || K_MAC), K_CONFDATA, ID_S, DSKPP-PRF_K_MAC("MAC 1 Computation" || ID_S || R_C), [ DSKPP-PRF_K_MAC'("MAC 1 Computation "|| ID_S||R_C)]

The full 2-pass protocol when the key is wrapped using a passphrase based derived key is as follows:

[<Trigger>]:

[ID_Device], [ID_K], [URL_S],[R_S]

<KeyProvClientHello>:

[ID_Device], ID_K, R_S, R_C, AUTHDATA, Alg_List

<KeyProvServerFinished>:

ENC_K_DERIVED(K_TOKEN || K_MAC), K_CONFDATA, ID_S, DSKPP-PRF_K_MAC("MAC 1 Computation" || ID_S || R_C), [ DSKPP-PRF_K_MAC'("MAC 1 Computation" || ID_S || R_C)]

The following subsections describe these exchanges in more detail.

4.4.1.  Message Flow

The 2-pass protocol flow consists of one round trip between the DSKPP client and DSKPP server, which consists of two "passes", i.e., one request message and one response message:

Round-trip #1: Pass 1=<KeyProvClientHello>, Pass 2=<KeyProvServerFinished>

a.

The DSKPP client sends a <KeyProvClientHello> message to the DSKPP server. The message provides the client nonce, R_C, and information about the DSKPP versions, protocol variants, key types, encryption and MAC algorithms supported by the cryptographic module for the purposes of this protocol. The message may also include client authentication data, such as device certificate or MAC derived from authentication code and R_C. Authentication code is sent in clear only when underlying transport layer can ensure data confidentiality. Unlike 4-pass DSKPP, 2-pass DSKPP client uses the <KeyProvClientHello> message to declare which key initialization method it supports, providing required payload information, e.g., K_CLIENT for the Key Transport Profile.

b.

The DSKPP server generates a key K from which two keys, K_TOKEN and K_MAC are derived. (Alternatively, the key K may have been pre-generated as described in Section 3.1 (Single Key Request). K is either transported or wrapped in accordance with the key initialization method specified by the DSKPP client in the <KeyProvClientHello> message. The server then associates K_TOKEN with the cryptographic module in a server-side data store. The intent is that the data store later on will be used by some service that needs to verify or decrypt data produced by the cryptographic module and the key.

c.

Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called <KeyProvServerFinished>. The confirmation message includes a key container that holds an identifier for the key, the key K from which K_TOKEN and K_MAC are derived, and additional configuration information (note that the latter MUST include the identity of the DSKPP server for authentication purposes). In addition, <KeyProvServerFinished> MUST include two MACs whose values are calculated with contribution from the client nonce, R_C, provided in the <KeyProvClientHello> message. The data will allow the cryptographic module to perform key confirmation and server authentication before "committing" the key. Note that the second MAC value that is intended for key confirmation MAY only be used for replacing and existing key.

d.

Upon receipt of the DSKPP server's confirmation message, the cryptographic module extracts the key data from the provided key container, uses the provided MAC values to perform key confirmation and server authentication, and stores the key material locally.

4.4.2.  Key Confirmation

In two-pass DSKPP, the client is REQUIRED to include a nonce R in the <KeyProvClientHello> message. Further, the server is REQUIRED to include an identifier, ID_S, for itself (via the key container) in the <KeyProvServerFinished> message. The MAC value in the <KeyProvServerFinished> message MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and R using a MAC key K_MAC. This key MUST be provided together with K_TOKEN to the cryptographic module.

If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation" and R, and the parameter dsLen MUST be set to the length of R:

dsLen = len(R)

MAC = DSKPP-PRF (K_MAC, "MAC 1 computation" || ID_S || R, dsLen)

4.4.3.  Server Authentication

A server MUST authenticate itself when attempting to replace an existing K_TOKEN. In 2-pass DSKPP, servers authenticate themselves by including a second MAC value in the AuthenticationDataType element of <KeyProvServerFinished>. The MAC value in the AuthenticationDataType element MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and R, using the existing MAC key K_MAC' (the MAC key that existed before this protocol run). The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes.

If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation" ID_S, and R. The parameter dsLen MUST be set to at least 16 (i.e. the length of the MAC MUST be at least 16 octets):

dsLen >= 16

MAC = DSKPP-PRF (K_MAC', "MAC 1 computation" || ID_S || R, dsLen)

4.5.  One-Pass Protocol Usage

The one-pass protocol flow is suitable for environments wherein near real-time communication between the DSKPP client and server may not be possible. It is also suitable for environments wherein administrative approval is a required step in the flow, and for provisioning of pre-generated keys. In one-pass DSKPP, the server simply sends a <KeyProvServerFinished> message to the DSKPP client. In this case, there is no exchange of the <KeyProvClientHello>, <KeyProvServerHello>, and <KeyProvClientNonce> DSKPP messages, and hence there is no way for the client to express supported algorithms or key types. Before attempting one-pass DSKPP, the server MUST therefore have prior knowledge not only that the client is able and willing to accept this variant of DSKPP, but also of algorithms and key types supported by the client.

Essentially, one-pass DSKPP is a transport of key material from the DSKPP server to the DSKPP client. As with two-pass DSKPP, the one-pass variant relies on key initialization methods that ensure K_TOKEN is not exposed to any other entity than the DSKPP server and the cryptographic module itself. The same key initialization profiles are defined as described in Section 4.4 (Two-Pass Protocol Usage) and Section 11 (Key Initialization Profiles of Two- and One-Pass DSKPP).

Outside the specific cases where one-pass DSKPP is desired, clients SHOULD be constructed and configured to only accept DSKPP server messages in response to client-initiated transactions.

The 1-pass protocol when the key is transported using the client public Key is as follows:

<KeyProvServerFinished>:

ENC_K_CLIENT ( K_TOKEN || K_MAC)), K_CONFDATA, DSKPP-PRF_K_MAC ("MAC 1 Computation" || ID_S || I), [ DSKPP-PRF_K_MAC'("MAC 2 Computation"||ID_S||I')]

The 1-pass protocol when the key is wrapped using a shared key is as follows:

<KeyProvServerFinished>:

ENC_K_SHARED (K_TOKEN || K_MAC), K_CONFDATA, DSKPP-PRF_K_MAC("MAC 1 Computation" || ID_S || I), [ PRF_K_MAC'("MAC 2 Computation" || ID_S || I')]

The 1-pass protocol when the key is wrapped using a passphrase derived key is as follows:

<KeyProvServerFinished>:

ENC_K_DERIVED(K_TOKEN || K_MAC), K_CONFDATA, DSKPP-PRF_K_MAC("MAC 1 Computation" || ID_S || I), [DSKPP-PRF_K_MAC'("MAC 2 Computation" || ID_S || I')]

The subsections below describe the 1-pass protocol in more detail.

4.5.1.  Message Flow

The 1-pass protocol flow consists of one "pass", i.e., a single message sent from the DSKPP server to the DSKPP client:

Pass 1: <KeyProvServerFinished>

a.

The DSKPP server generates a key K from which two keys, K_TOKEN and K_MAC are derived. K is either transported or wrapped in accordance with the key initialization method known in advance by the DSKPP server. The server then associates K_TOKEN with the cryptographic module in a server-side data store. The intent is that the data store later on will be used by some service that needs to verify or decrypt data produced by the cryptographic module and the key.

b.

Once the association has been made, the DSKPP server sends a confirmation message to the DSKPP client called <KeyProvServerFinished>. The confirmation message includes a key container that holds an identifier for the key, the key K from which K_TOKEN and K_MAC are derived, and additional configuration information (note that the latter MUST include the identity of the DSKPP server for authentication purposes). In addition, <KeyProvServerFinished> MUST include two MACs, which will allow the cryptographic module to perform key confirmation and server authentication before "commuting" the key. Note that unlike two-pass DSKPP, in the one-pass variant, the server does not have the client nonce, R_C, and therefore the MACs values are calculated with contribution from an unsigned integer, I, generated by the server during the protocol run.

c.

Upon receipt of the DSKPP server's confirmation message, the cryptographic module extracts the key data from the provided key container, uses the two MAC values to perform key confirmation and server authentication, and stores the key material locally.

4.5.2.  Key Confirmation

In one-pass DSKPP, the server MUST include an identifier, ID_S, for itself (via the key container) in the <KeyProvServerFinished> message. The MAC value in the <KeyProvServerFinished> message MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and an unsigned integer value I, using a MAC key K_MAC. The value I MUST be monotonically increasing and guaranteed not to be used again by this server towards this cryptographic module. It could for example be the number of seconds since some point in time with sufficient granularity, a counter value, or a combination of the two where the counter value is reset for each new time value. In contrast to the MAC calculation in four-pass DSKPP, the MAC key K_MAC MUST be provided together with K_TOKEN to the cryptographic module.

Note: The integer I does not necessarily need to be maintained by the DSKPP server on a per cryptographic module basis (it is enough if the server can guarantee that the same value is never being sent twice to the same cryptographic module).

If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation", ID_S, and I. The parameter dsLen MUST be set to at least 16 (i.e. the length of the MAC MUST be at least 16 octets):

dsLen >= 16

MAC = DSKPP-PRF (K_MAC, "MAC 1 computation" || ID_S || I, dsLen)

The server MUST provide I to the client in the Nonce attribute of the <Mac> element of the <KeyProvServerFinished> message using the AuthenticationCodeMacType defined in Section 6.2.2.4 (Selecting a Client and Server Authentication Mechanism: The AuthenticationDataType Type).

4.5.3.  Server Authentication

As discussed in , servers need to authenticate themselves when attempting to replace an existing K_TOKEN. In 1-pass DSKPP, servers authenticate themselves by including a second MAC value in the AuthenticationDataType element of <KeyProvServerFinished>. The MAC value in the AuthenticationDataType element MUST be computed on the (ASCII) string "MAC 1 computation", the server identifier ID_S, and a new value I', I' > I, using the existing MAC key K_MAC' (the MAC key that existed before this protocol run). The MAC algorithm MUST be the same as the algorithm used for key confirmation purposes.

If DSKPP-PRF is used as the MAC algorithm, then the input parameter s MUST consist of the concatenation of the (ASCII) string "MAC 1 computation" ID_S, and I'. The parameter dsLen MUST be set to at least 16 (i.e. the length of the MAC MUST be at least 16 octets):

dsLen >= 16

MAC = DSKPP-PRF (K_MAC', "MAC 1 computation" || ID_S || I', dsLen)

The server MUST provide I' to the client in the Nonce attribute of the <Mac> element of the AuthenticationDataType extension. If the protocol run is successful, the client stores I' as the new value of I for this server.

5.  Methods Common to More Than One Protocol Variant

The mechanisms contained in this section are used in more than one variant of DSKPP.

5.1.  The DSKPP One-Way Pseudorandom Function, DSKPP-PRF

5.1.1.  Introduction

All of the protocol variants depend on DSKPP-PRF. The general requirements on DSKPP-PRF are the same as on keyed hash functions: It MUST take an arbitrary length input, and be one-way and collision-free (for a definition of these terms, see, e.g., [FAQ] (RSA Laboratories, “Frequently Asked Questions About Today's Cryptography,” 2000.)). Further, the DSKPP-PRF function MUST be capable of generating a variable-length output, and its output MUST be unpredictable even if other outputs for the same key are known.

It is assumed that any realization of DSKPP-PRF takes three input parameters: A secret key k, some combination of variable data, and the desired length of the output. The combination of variable data can, without loss of generalization, be considered as a salt value (see PKCS#5 Version 2.0 [PKCS‑5] (RSA Laboratories, “Password-Based Cryptography Standard,” March 1999.), Section 4), and this characterization of DSKPP-PRF SHOULD fit all actual PRF algorithms implemented by cryptographic modules. From the point of view of this specification, DSKPP-PRF is a "black-box" function that, given the inputs, generates a pseudorandom value.

Separate specifications MAY define the implementation of DSKPP-PRF for various types of cryptographic modules. Appendix B (Example of DSKPP-PRF Realizations) contains two example realizations of DSKPP-PRF.

5.1.2.  Declaration

DSKPP-PRF (k, s, dsLen)

Input:

k

secret key in octet string format

s

octet string of varying length consisting of variable data distinguishing the particular string being derived

dsLen

desired length of the output

Output:

DS

pseudorandom string, dsLen-octets long

For the purposes of this document, the secret key k MUST be at least 16 octets long.

5.2.  Encryption of Pseudorandom Nonces Sent from the DSKPP Client (Applicable to Four-Pass and Two-Pass DSKPP)

During 4- and 2-pass message exchanges, DSKPP client random nonce(s) are either encrypted with the public key provided by the DSKPP server or by a shared secret key. For example, in the case of a public RSA key, an RSA encryption scheme from PKCS #1 [PKCS‑1] (RSA Laboratories, “RSA Cryptography Standard,” June 2002.) MAY be used.

In the case of a shared secret key, to avoid dependence on other algorithms, the DSKPP client MAY use the DSKPP-PRF function described herein with the shared secret key K_SHARED as input parameter k (in this case, K_SHARED SHOULD be used solely for this purpose), the concatenation of the (ASCII) string "Encryption" and the server's nonce R_S as input parameter s, and dsLen set to the length of R_C:

dsLen = len(R_C)

DS = DSKPP-PRF(K_SHARED, "Encryption" || R_S, dsLen)

This will produce a pseudorandom string DS of length equal to R_C. Encryption of R_C MAY then be achieved by XOR-ing DS with R_C:

Enc-R_C = DS ^ R_C

The DSKPP server will then perform the reverse operation to extract R_C from Enc-R_C.

5.3.  Client Authentication Mechanisms (Applicable to Four- and Two-Pass DSKPP)

To ensure that a generated K_TOKEN ends up associated with the correct cryptographic module and user, the DSKPP server MAY couple an initial user authentication to the DSKPP execution in several ways, as discussed in the following sub-sections. Whatever the method, the DSKPP server MUST ensure that a generated key is associated with the correct cryptographic module, and if applicable, the correct user. For a further discussion of this, and threats related to man-in-the-middle attacks in this context, see Section 14 (Security Considerations).

5.3.1.  Device Certificate

Instead of requiring an Authentication Code for in-band authentication, a device private key and certificate could be used, which was supplied with the cryptographic module by its issuer for client authentication at the transport layer e.g TLS/HTTPS. When the Device certificate is available and client authentication is not provided in the transport layer, the DSKPP client may include a device's certificate signed data for the authentication data.

5.3.2.  Device Identifier

The DSKPP server could be pre-configured with a unique device identifier corresponding to a particular cryptographic module. The DSKPP server MAY then include this identifier in the DSKPP initialization trigger, and the DSKPP client would include it in its message(s) to the DSKPP server for authentication. Note that it is also legitimate for a DSKPP client to initiate the DSKPP protocol run without having received an initialization message from a server, but in this case any provided device identifier MUST NOT be accepted by the DSKPP server unless the server has access to a unique key for the identified device and that key will be used in the protocol.

5.3.3.  Authentication Code

As shown in Figure 5 (User Authentication with One-Time Code), a key issuer may provide a one-time value, called an Authentication Code, to the user or device out-of-band and require this value to be used by the DSKPP client when contacting the DSKPP server. The DSKPP client MAY include the authentication data in its <KeyProvClientHello> (and <KeyProvClientNonce> for four-pass) message, and the DSKPP server MUST verify the data before continuing with the protocol run.

Note: An alternate method for getting the Authentication Code to the client, is for the DSKPP server to place the value in the <TriggerNonce> element of the DSKPP initialization trigger (if triggers are used; see Section 12.2.7 (Initialization of DSKPP)) . When this method is used, a transport providing privacy and integrity MUST be used to deliver the DSKPP initialization trigger from the DSKPP server to the DSKPP client, e.g. HTTPS.

+------------+  Get Authentication Code  +------------+
|    User    |<------------------------->|   Issuer   |
+------------+                           +------------+
       |                                        |
       |                                        |
       |                                        |
       V                                        V
+--------------+                        +--------------+
|    DSKPP     |   Authentication Data  |    DSKPP     |
|    Client    |----------------------->|    Server    |
+--------------+                        +--------------+

The Authentication Code, AUTHCODE, may be considered as a special form of a shared secret between a User and a DSKPP server. The Issuer may generate the Authentication Code as follows:

AUTHCODE = passwordLen || identifier || password || checksum

where

passwordLen

: 1 digit indicating the 'password' length. The maximum length of the password is 10. A passwordLen value '0' indicates a password of 10 digits.

identifier

: A globally unique identifier of the user’s order for token provisioning. The length of the identifier may be fixed e.g. 10 digits or variable e.g. 1 to 20 digits. The identifier may be generated as a sequence number.

password

: 6 to 10 digits. The password should be generated by the system as a random number to make the AUTHCODE more difficult to guess.

checksum

: 1 digit calculated from the remaining digits in the code.

The Authentication Data, AUTHDATA, may be derived from the AUTHCODE and other information as follows:

MAC = DSKPP-PRF-AES(K_AUTHCODE, AUTHCODE->Identifier || URL_S || [R_S], 16)

where

Refer to Section 5.1 (The DSKPP One-Way Pseudorandom Function, DSKPP-PRF) for a description of DSKPP-PRF in general and Appendix B (Example of DSKPP-PRF Realizations) for a description of DSKPP-PRF-AES.

In four-pass DSKPP, the cryptographic module uses the client nonce R_C, the server nonce R_S, and the server URL URL_S to calculate the MAC. In two-pass DSKPP, the cryptographic module does not have access to the server nonce R_S therefore only the client nonce R_C is used in combination with the server URL URL_S to produce the MAC.

The K_AUTHCODE MAY be derived from AUTHCODE>password as follows:

K_AUTHCODE = truncate( Hash( Hash(...n times...( AUTHCODE->password ||R_C||[K]) ) ) )

where

K is optional and MAY be one of the following:

K_CLIENT: The device public key when a device certificate is available and used for key transport in 2-pass

K_SHARED: The shared key between the Client and the Server when it is used for key wrap in two-pass or for R_C protection in four-pass

K_DERIVED: when a passphrase derived key is used for key wrap in two-pass.

'truncate()' returns the first 16 bytes from the result of the last hash iteration, and n is the number of hash iterations. n may be any number between 10 and 1000.

Notes:

1

Authentication data MAY be omitted if client certificate authentication has been provided by the transport channel such as TLS.

2

When an issuer delegates symmetric key provisioning to a third party provisioning service provider, both client authentication and issuer authentication are required by the provisioning server. Client authentication to the issuer MAY be in-band or out-of-band as described above. The issuer acts as a proxy for the provisioning server. The issuer authenticates to the provisioning service provider either using a certificate or a pre-established secret key.

5.4.  Client Authentication Examples

5.4.1.  Example Using a MAC from an Authentication Code

  <AuthenticationData>
    <ClientID>31300257</ClientID>
    <AuthenticationCodeMac>
      <IterationCount>512</IterationCount>
      <Mac>4bRJf9xXd3KchKoTenHJiw==</Mac>
    </AuthenticationCodeMac>
              </AuthenticationData>

5.4.2.  Example Using a Device Certificate

  <AuthenticationData>
    <DigitalSignature>
      <ds:SignedInfo>
        <ds:CanonicalizationMethod
          Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
        <ds:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
        <ds:Reference URI="#Nonce">
          <ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
          <ds:DigestValue></ds:DigestValue>
        </ds:Reference>
      </ds:SignedInfo>
      <ds:SignatureValue></ds:SignatureValue>
      <ds:KeyInfo>
        <ds:X509Data>
          <ds:X509Certificate>miib</ds:X509Certificate>
        </ds:X509Data>
      </ds:KeyInfo>
      <ds:Object Id="Nonce">xwQzwEl0CjPAiQeDxwRJdQ==</ds:Object>
              </DigitalSignature>

6.  Four-Pass Protocol

In this section, example messages are used to describe parameters, encoding and semantics in a 4-pass DSKPP exchanges. The examples are written using XML. While they are syntactically correct, MAC and cipher values are fictitious.

6.1.  XML Basics

The DSKPP XML schema can be found in Section 13 (DSKPP Schema). Some DSKPP elements rely on the parties being able to compare received values with stored values. Unless otherwise noted, all elements in this document that have the XML Schema "xs:string" type, or a type derived from it, MUST be compared using an exact binary comparison. In particular, DSKPP implementations MUST NOT depend on case-insensitive string comparisons, normalization or trimming of white space, or conversion of locale-specific formats such as numbers.

Implementations that compare values that are represented using different character encodings MUST use a comparison method that returns the same result as converting both values to the Unicode character encoding, Normalization Form C [UNICODE] (Davis, M. and M. Duerst, “Unicode Normalization Forms,” March 2001.), and then performing an exact binary comparison.

No collation or sorting order for attributes or element values is defined. Therefore, DSKPP implementations MUST NOT depend on specific sorting orders for values.

6.2.  Round-Trip #1: <KeyProvClientHello> and <KeyProvServerHello>

6.2.1.  Examples

6.2.1.1.  Example Without a Preceding Trigger

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvClientHello Version="1.0"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
    keyprov-dskpp-1.0.xsd">
  <DeviceIdentifierData>
    <DeviceId>
      <pskc:Manufacturer>ManufacturerABC</pskc:Manufacturer>
      <pskc:SerialNo>XL0000000001234</pskc:SerialNo>
      <pskc:Model>U2</pskc:Model>
    </DeviceId>
  </DeviceIdentifierData>
  <SupportedKeyTypes>
    <Algorithm>urn:ietf:params:xml:schema:keyprov:otpalg#HOTP</Algorithm>
    <Algorithm>
      http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst#SecurID-AES
    </Algorithm>
  </SupportedKeyTypes>
  <SupportedEncryptionAlgorithms>
    <Algorithm>http://www.w3.org/2001/05/xmlenc#rsa_1_5</Algorithm>
    <Algorithm>
      urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
    </Algorithm>
  </SupportedEncryptionAlgorithms>
  <SupportedMacAlgorithms>
    <Algorithm>
      urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
    </Algorithm>
  </SupportedMacAlgorithms>
  <SupportedProtocolVariants><FourPass/></SupportedProtocolVariants>
  <SupportedKeyContainers>
    <KeyContainerFormat>
      urn:ietf:params:xml:schema:keyprov:container#KeyContainer
    </KeyContainerFormat>
  </SupportedKeyContainers>
</dskpp:KeyProvClientHello>

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvServerHello Version="1.0" SessionID="4114" Status="Success"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
  keyprov-dskpp-1.0.xsd">
  <KeyType>
    urn:ietf:params:xml:schema:keyprov:otpalg#SecurID-AES
  </KeyType>
  <EncryptionAlgorithm>
    urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
  </EncryptionAlgorithm>
  <MacAlgorithm>
    urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
  </MacAlgorithm>
  <EncryptionKey>
    <ds:KeyName>KEY-1</ds:KeyName>
  </EncryptionKey>
  <KeyContainerFormat>
    urn:ietf:params:xml:schema:keyprov:container#KeyContainer
  </KeyContainerFormat>
  <Payload>
    <Nonce>qw2ewasde312asder394jw==</Nonce>
  </Payload>
</dskpp:KeyProvServerHello>

6.2.1.2.  Example Assuming a Preceding Trigger

<?xml version="1.0" encoding="UTF-8"?>

<dskpp:KeyProvClientHello Version="1.0"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
    keyprov-dskpp-1.0.xsd">
  <DeviceIdentifierData>
    <DeviceId>
      <pskc:Manufacturer>ManufacturerABC</pskc:Manufacturer>
      <pskc:SerialNo>XL0000000001234</pskc:SerialNo>
      <pskc:Model>U2</pskc:Model>
    </DeviceId>
  </DeviceIdentifierData>
  <KeyID>SE9UUDAwMDAwMDAx</KeyID>
  <TriggerNonce>112dsdfwf312asder394jw==</TriggerNonce>
  <SupportedKeyTypes>
    <Algorithm>urn:ietf:params:xml:schema:keyprov:otpalg#HOTP</Algorithm>
    <Algorithm>
      http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst#SecurID-AES
    </Algorithm>
  </SupportedKeyTypes>
  <SupportedEncryptionAlgorithms>
    <Algorithm>http://www.w3.org/2001/05/xmlenc#rsa_1_5</Algorithm>
    <Algorithm>
      urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
    </Algorithm>
  </SupportedEncryptionAlgorithms>
  <SupportedMacAlgorithms>
    <Algorithm>
      urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
    </Algorithm>
  </SupportedMacAlgorithms>
  <SupportedProtocolVariants><FourPass/></SupportedProtocolVariants>
  <SupportedKeyContainers>
    <KeyContainerFormat>
      urn:ietf:params:xml:schema:keyprov:container#KeyContainer
    </KeyContainerFormat>
  </SupportedKeyContainers>
</dskpp:KeyProvClientHello>

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvServerHello Version="1.0" SessionID="4114" Status="Success"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
    keyprov-dskpp-1.0.xsd">
  <KeyType>
    http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst#SecurID-AES
  </KeyType>
  <EncryptionAlgorithm>
    urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
  </EncryptionAlgorithm>
  <MacAlgorithm>
    urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
  </MacAlgorithm>
  <EncryptionKey>
    <ds:KeyName>KEY-1</ds:KeyName>
  </EncryptionKey>
  <KeyContainerFormat>
    urn:ietf:params:xml:schema:keyprov:container#KeyContainer
  </KeyContainerFormat>
  <Payload>
    <Nonce>qw2ewasde312asder394jw==</Nonce>
  </Payload>
  <Mac MacAlgorithm=
    "urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes">
    cXcycmFuZG9tMzEyYXNkZXIzOTRqdw==
  </Mac>
</dskpp:KeyProvServerHello>

6.2.2.  Components of the <KeyProvClientHello> Request

The components of this message have the following meaning:

• Version: (attribute inherited from the AbstractRequestType type) The highest version of this protocol the client supports. Only version one ("1.0") is currently specified.
• <DeviceIdentifierData>: An identifier for the cryptographic module as defined in Section 5.3 (Client Authentication Mechanisms (Applicable to Four- and Two-Pass DSKPP) ) above. The identifier MUST only be present if such shared secrets exist or if the identifier was provided by the server in a <KeyProvTrigger> element (see Section 12.2.7 (Initialization of DSKPP) below). In the latter case, it MUST have the same value as the identifier provided in that element.
• <KeyID>: An identifier for the key that will be overwritten if the protocol run is successful. The identifier MUST only be present if the key exists or if the identifier was provided by the server in a <KeyProvTrigger> element, in which case, it MUST have the same value as the identifier provided in that element (see a (Trigger) and Section 12.2.7 (Initialization of DSKPP) below).
• <KeyProvClientNonce>: This is the nonce R, which, when present, MUST be used by the server when calculating MAC values (see below). It is RECOMMENDED that clients include this element whenever the <KeyID> element is present.
• <TriggerNonce>: This OPTIONAL element MUST be present if and only if the DSKPP run was initialized with a <KeyProvTrigger> message (see Section 12.2.7 (Initialization of DSKPP) below), and MUST, in that case, have the same value as the <TriggerNonce> child of that message. A server using nonces in this way MUST verify that the nonce is valid and that any device or key identifier values provided in the <KeyProvTrigger> message match the corresponding identifier values in the <KeyProvClientHello> message.
• <SupportedKeyTypes>: A sequence of URIs indicating the key types for which the cryptographic module is willing to generate keys through DSKPP.
• <SupportedEncryptionAlgorithms>: A sequence of URIs indicating the encryption algorithms supported by the cryptographic module for the purposes of DSKPP. The DSKPP client MAY indicate the same algorithm both as a supported key type and as an encryption algorithm.
• <SupportedMacAlgorithms>: A sequence of URIs indicating the MAC algorithms supported by the cryptographic module for the purposes of DSKPP. The DSKPP client MAY indicate the same algorithm both as an encryption algorithm and as a MAC algorithm (e.g., urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes defined in Appendix B (Example of DSKPP-PRF Realizations)).
• <SupportedProtocolVariants>: This OPTIONAL element is used by the DSKPP client to indicate support for four-pass or two-pass DSKPP. If two-pass support is specified, then <KeyProvClientNonce> MUST be set to nonce R in the <KeyProvClientHello> message unless <TriggerNonce> is already present.
• <SupportedKeyContainers>: This OPTIONAL element is a sequence of URIs indicating the key container formats supported by the DSKPP client. If this element is not provided, then the DSKPP server MUST proceed with "urn:ietf:params:xml:schema:keyprov:container#KeyContainer" (see [PSKC] (, “Portable Symmetric Key Container,” 2005.)).
• <AuthenticationData>: This OPTIONAL element contains data that the DSKPP client uses to authenticate the user or device to the DSKPP server. The element is set as specified in Section 5.3 (Client Authentication Mechanisms (Applicable to Four- and Two-Pass DSKPP) ).
• <Extensions>: A sequence of extensions. One extension is defined for this message in this version of DSKPP: the ClientInfoType (see Section 10 (Extensibility)).

6.2.2.1.  The DSKPP Client: The DeviceIdentifierDataType Type

The DeviceIdentifierDataType type is used to uniquely identify the device that houses the cryptographic module, e.g., a mobile phone. The device identifier allows the DSKPP server to find, e.g., a pre-shared transport key for 2-pass DSKPP and/or the correct shared secret for MAC'ing purposes. The default DeviceIdentifierDataType is defined in [PSKC] (, “Portable Symmetric Key Container,” 2005.).

6.2.2.2.  Selecting a Protocol Variant: The ProtocolVariantsType Type

The ProtocolVariantsType type is OPTIONAL for a DSKPP client, who MAY use it to indicate the number of passes of the DSKPP protocol that it supports. The ProtocolVariantsType MAY be used to indicate support for 4-pass or 2-pass DSKPP. Because 1-pass DSKPP does not include a client request to the server, the ProtocolVariantsType type MAY NOT be used to indicate support for 1-pass DSKPP. If the ProtocolVariantsType is not used, then the DSKPP server will proceed with ordinary 4-pass DSKPP. However, it does not support 4-pass DSKPP, then the server MUST find a suitable two-pass variant or else the protocol run will fail.

The TwoPassSupportType type signals client support for the 2-pass version of DSKPP, informs the server of supported two-pass variants, and provides OPTIONAL payload data to the DSKPP server. The payload is sent in an opportunistic fashion, and MAY be discarded by the DSKPP server if the server does not support the two-pass variant the payload is associated with. The elements of this type have the following meaning:

• <SupportedKeyInitializationMethod>: A two-pass key initialization method supported by the DSKPP client. Multiple supported methods MAY be present, in which case they MUST be listed in order of precedence.
• <Payload>: An OPTIONAL payload associated with each supported key initialization method.

A DSKPP client that indicates support for two-pass DSKPP MUST also include the nonce R in its <KeyProvClientHello> message (this will enable the client to verify that the DSKPP server it is communicating with is alive).

6.2.2.3.  Selecting a Key Container Format: The KeyContainersFormatType Type

The OPTIONAL KeyContainersFormatType type is a list of type-value pairs that a DSKPP client or server MAY use to define key container formats it supports. Key container formats are identified through URIs, e.g., the PSKC KeyContainer URI "urn:ietf:params:xml:schema:keyprov:container#KeyContainer" (see [PSKC] (, “Portable Symmetric Key Container,” 2005.)).

6.2.2.4.  Selecting a Client and Server Authentication Mechanism: The AuthenticationDataType Type

The OPTIONAL AuthenticationDataType type is used by DSKPP clients and server to carry authentication values in DSKPP messages. The element MAY contain a device certificate or MAC derived from an authentication code as follows:

a.

A DSKPP client MAY include a one-time use AuthenticationCode that was given by the issuer to the user for acquiring a symmetric key. An AuthenticationCode MAY or MAY NOT contain alphanumeric characters in addition to numeric digits depending on the device type and policy of the issuer. For example, if the device is a mobile phone, a code that the user enters on the keypad would typically be restricted to numeric digits for ease of use. An authentication code MAY be sent to the DSKPP server as MAC data calculated according to section Section 5.3.3 (Authentication Code).

b.

A DSKPP client MAY contain Authentication Data consisting of signed data of client Nonce with a client certificate's private key. A service provider may have a policy to issue symmetric keys for a device only if it has a trusted device certificate. An authentication code isn't required in this case.

c.

A DSKPP server MAY use the AuthenticationDataType element AuthenticationCodeMac to carry a MAC for authenticating itself to the client. For example, when a successful 1- or 2-pass DSKPP protocol run will result in an existing key being replaced, then the DSKPP server MUST include a MAC proving to the DSKPP client that the server knows the value of the key it is about to replace.

The element of the AuthenticationDataType type have the following meaning:

• <ClientID>: A requester's identifier. The value MAY be a user ID, a device ID, or a keyID associated with the requester's authentication value. When the authentication data is based on a certificate, <ClientID> can be omitted, as the certificate itself is typically sufficient to identify the requester. Also, if a <KeyProvTrigger> message was provided by the server to initiate the DSKPP protocol run, <ClientID> can be omitted, as the DeviceID, KeyID, and/or nonce provided in the <InitializationTriggerType> element ought to be sufficient to identify the requester.
• <AuthenticationCodeMac>: An authentication MAC and OPTIONAL additional information (e.g., MAC algorithm). The value could be a one-time use value sent as a MAC value to the DSKPP server; or, it could be a MAC value sent to the DSKPP client. Refer to section Section 5.3.3 (Authentication Code) for calculation of MAC with an authentication code.
• <DigitalSignature>: Client nonce R_C signed using the device certificate and sent in KeyProvClientHello for two-pass protocol or in KeyProvClientNonce for four-pass protocol.

6.2.3.  Components of the <KeyProvServerHello> Response

This message is the first message sent from the DSKPP server to the DSKPP client (assuming a trigger message has not been sent to initiate the protocol, in which case, this message is the second message sent from the DSKPP server to the DSKPP client). It is sent upon reception of a <KeyProvClientHello> message. The components of this message have the following meaning:

• Version: (attribute inherited from the AbstractResponseType type) The version selected by the DSKPP server. MAY be lower than the version indicated by the DSKPP client, in which case, local policy at the client MUST determine whether or not to continue the session.
• SessionID: (attribute inherited from the AbstractResponseType type) An identifier for this session.
• Status: (attribute inherited from the AbstractResponseType type) Return code for the <KeyProvClientHello>. If Status is not "Continue", only the Status and Version attributes will be present; otherwise, all the other element MUST be present as well.
• <KeyType>: The type of the key to be generated.
• <EncryptionAlgorithm>: The encryption algorithm to use when protecting R_C.
• <MacAlgorithm>: The MAC algorithm to be used by the DSKPP server.
• <EncryptionKey>: Information about the key to use when encrypting R_C. It will either be the server's public key (the <ds:KeyValue> alternative of ds:KeyInfoType) or an identifier for a shared secret key (the <ds:KeyName> alternative of ds:KeyInfoType).
• <KeyContainerFormat>: The key container format type to be used by the DSKPP server. The default setting relies on the KeyContainerType element defined in "urn:ietf:params:xml:schema:keyprov:container" [PSKC] (, “Portable Symmetric Key Container,” 2005.).
• <Payload>: The actual payload. For this version of the protocol, only one payload is defined: the pseudorandom string R_S.
• <Extensions>: A list of server extensions. Two extensions are defined for this message in this version of DSKPP: the ClientInfoType and the ServerInfoType (see Section 10 (Extensibility)).
• <Mac>: The MAC MUST be present if the DSKPP run will result in the replacement of an existing symmetric key with a new one (i.e., if the <KeyID> element was present in the <ClientHello message). In this case, the DSKPP server MUST prove to the cryptographic module that it is authorized to replace it.

The DSKPP client MUST verify the MAC if the successful execution of the protocol will result in the replacement of an existing symmetric key with a newly generated one. The DSKPP client MUST terminate the DSKPP session if the MAC does not verify, and MUST delete any nonces, keys, and/or secrets associated with the failed run of the DSKPP protocol.

The MacType's MacAlgorithm attribute MUST, when present, identify the negotiated MAC algorithm.

6.3.  Round-Trip #2: <KeyProvClientNonce> and <KeyProvServerFinished>

6.3.1.  Examples

6.3.1.1.  Example Using Default Encryption

This message contains the nonce chosen by the cryptographic module, R_C, encrypted by the specified encryption key and encryption algorithm.

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvClientNonce Version="1.0" SessionID="4114"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
    keyprov-dskpp-1.0.xsd">
  <EncryptedNonce>VXENc+Um/9/NvmYKiHDLaErK0gk=</EncryptedNonce>
  <AuthenticationData>
    <ClientID>31300257</ClientID>
    <AuthenticationCodeMac>
      <IterationCount>512</IterationCount>
      <Mac>4bRJf9xXd3KchKoTenHJiw==</Mac>
    </AuthenticationCodeMac>
  </AuthenticationData>
</dskpp:KeyProvClientNonce>

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvServerFinished Version="1.0" SessionID="4114" Status="Success"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
    keyprov-dskpp-1.0.xsd">
  <KeyContainer>
    <KeyContainer Version="1.0">
      <pskc:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1"/>
      <pskc:Device>
        <pskc:Key
          KeyAlgorithm=
          "http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst#SecurID-AES"
          KeyId="XL0000000001234">
          <pskc:Issuer>CredentialIssuer</pskc:Issuer>
          <pskc:Usage otp="true">
            <pskc:ResponseFormat format="DECIMAL" length="6"/>
          </pskc:Usage>
          <pskc:FriendlyName>MyFirstToken</pskc:FriendlyName>
          <pskc:Data Name="TIME">
            <pskc:Value>AAAAADuaygA=</pskc:Value>
          </pskc:Data>
          <pskc:Expiry>10/30/2012</pskc:Expiry>
        </pskc:Key>
      </pskc:Device>
    </KeyContainer>
  </KeyContainer>
  <Mac
    MacAlgorithm="urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes">
    miidfasde312asder394jw==
  </Mac>
</dskpp:KeyProvServerFinished>

6.3.2.  Components of a <KeyProvClientNonce> Request

The components of this message have the following meaning:

• Version: (inherited from the AbstractRequestType type) MUST be the same version as in the <KeyProvServerHello> message.
• <SessionID>: MUST have the same value as the SessionID attribute in the received <KeyProvServerHello> message.
• <EncryptedNonce>: The nonce generated and encrypted by the cryptographic module. The encryption MUST be made using the selected encryption algorithm and identified key, and as specified in Section 5.1 (The DSKPP One-Way Pseudorandom Function, DSKPP-PRF).
• <AuthenticationData>: The authentication data value MUST be set as specified in Section 5.3 (Client Authentication Mechanisms (Applicable to Four- and Two-Pass DSKPP) ) and Section 6.2.2.4 (Selecting a Client and Server Authentication Mechanism: The AuthenticationDataType Type).
• <Extensions>: A list of extensions. Two extensions are defined for this message in this version of DSKPP: the ClientInfoType and the ServerInfoType (see Section 10 (Extensibility))

6.3.3.  Components of a <KeyProvServerFinished> Response

This message is the last message of the DSKPP protocol run. In a 4-pass exchange, the DSKPP server sends this message in response to a <KeyProvClientNonce> message, whereas in a 2-pass exchange, the DSKPP server sends this message in response to a <KeyProvClientHello> message. In a 1-pass exchange, the DSKPP server sends only this message to the client. The components of this message have the following meaning:

• Version: (inherited from the AbstractResponseType type) The DSKPP version used in this session.
• SessionID: (inherited from the AbstractResponseType type) The previously established identifier for this session.
• Status: (inherited from the AbstractResponseType type) Return code for the <KeyProvServerFinished> message. If Status is not "Success", only the Status, SessionID, and Version attributes will be present (the presence of the SessionID attribute is dependent on the type of reported error); otherwise, all the other elements MUST be present as well. In this latter case, the <KeyProvServerFinished> message can be seen as a "Commit" message, instructing the cryptographic module to store the generated key and associate the given key identifier with this key.
• <KeyContainer>: The key container containing symmetric key values (in the case of a 2- or 1-pass exchange) and configuration data. The default container format is based on the KeyContainerType type from PSKC, as defined in [PSKC] (, “Portable Symmetric Key Container,” 2005.).
• <Extensions>: A list of extensions chosen by the DSKPP server. For this message, this version of DSKPP defines one extension, the ClientInfoType (see Section 10 (Extensibility)).
• <Mac>: To avoid a false "Commit" message causing the cryptographic module to end up in an initialized state for which the server does not know the stored key, <KeyProvServerFinished> messages MUST always be authenticated with a MAC. The MAC MUST be made using the already established MAC algorithm.

When receiving a <KeyProvServerFinished> message with Status="Success" for which the MAC verifies, the DSKPP client MUST associate the generated key K_TOKEN with the provided key identifier and store this data permanently. After this operation, it MUST NOT be possible to overwrite the key unless knowledge of an authorizing key is proven through a MAC on a later <KeyProvServerHello> (and <KeyProvServerFinished>) message.

The DSKPP client MUST verify the MAC. The DSKPP client MUST terminate the DSKPP session if the MAC does not verify, and MUST, in this case, also delete any nonces, keys, and/or secrets associated with the failed run of the DSKPP protocol.

The MacType's MacAlgorithm attribute MUST, when present, identify the negotiated MAC algorithm.

6.4.  DSKPP Server Results: The StatusCode Type

The StatusCode type enumerates all possible return codes. Upon transmission or receipt of a message for which the Status attribute's value is not "Success" or "Continue", the default behavior, unless explicitly stated otherwise below, is that both the DSKPP server and the DSKPP client MUST immediately terminate the DSKPP session. DSKPP servers and DSKPP clients MUST delete any secret values generated as a result of failed runs of the DSKPP protocol. Session identifiers MAY be retained from successful or failed protocol runs for replay detection purposes, but such retained identifiers MUST NOT be reused for subsequent runs of the protocol.

When possible, the DSKPP client SHOULD present an appropriate error message to the user.

These status codes are valid in all 4-Pass DSKPP Response messages unless explicitly stated otherwise:

• "Continue" indicates that the DSKPP server is ready for a subsequent request from the DSKPP client. It cannot be sent in the server's final message.
• "Success" indicates successful completion of the DSKPP session. It can only be sent in the server's final message.
• "Abort" indicates that the DSKPP server rejected the DSKPP client's request for unspecified reasons.
• "AccessDenied" indicates that the DSKPP client is not authorized to contact this DSKPP server.
• "MalformedRequest" indicates that the DSKPP server failed to parse the DSKPP client's request.
• "UnknownRequest" indicates that the DSKPP client made a request that is unknown to the DSKPP server.
• "UnknownCriticalExtension" indicates that a critical DSKPP extension (see below) used by the DSKPP client was not supported or recognized by the DSKPP server.
• "UnsupportedVersion" indicates that the DSKPP client used a DSKPP protocol version not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message.
• "NoSupportedKeyTypes" indicates that the DSKPP client only suggested key types that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message.
• "NoSupportedEncryptionAlgorithms" indicates that the DSKPP client only suggested encryption algorithms that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message.
• "NoSupportedMacAlgorithms" indicates that the DSKPP client only suggested MAC algorithms that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message.
• "NoProtocolVariants" indicates that the DSKPP client only suggested a protocol variant (either 2-pass or 4-pass) that is not supported by the DSKPP server. This error is only valid in the DSKPP server's first response messagei
• "NoSupportedKeyContainers" indicates that the DSKPP client only suggested key container formats that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message.
• "AuthenticationDataMissing" indicates that the DSKPP client didn't provide authentication data that the DSKPP server required.
• "AuthenticationDataInvalid" indicates that the DSKPP client supplied user or device authentication data that the DSKPP server failed to validate.
• "InitializationFailed" indicates that the DSKPP server could not generate a valid key given the provided data. When this status code is received, the DSKPP client SHOULD try to restart DSKPP, as it is possible that a new run will succeed.
• "ProvisioningPeriodExpired" indicates that the provisioning period set by the DSKPP server has expired. When the status code is received, the DSKPP client SHOULD report the key initialization failure reason to the user and the user MUST register with the DSKPP server to initialize a new key.

7.  Two-Pass Protocol

In this section, example messages are used to describe parameters, encoding and semantics in a 2-pass DSKPP exchanges. The examples are written using XML. While they are syntactically correct, MAC and cipher values are fictitious.

7.1.  XML Basics

The DSKPP XML schema can be found in Section 13 (DSKPP Schema). Some DSKPP elements rely on the parties being able to compare received values with stored values. Unless otherwise noted, all elements in this document that have the XML Schema "xs:string" type, or a type derived from it, MUST be compared using an exact binary comparison. In particular, DSKPP implementations MUST NOT depend on case-insensitive string comparisons, normalization or trimming of white space, or conversion of locale-specific formats such as numbers.

Implementations that compare values that are represented using different character encodings MUST use a comparison method that returns the same result as converting both values to the Unicode character encoding, Normalization Form C [UNICODE] (Davis, M. and M. Duerst, “Unicode Normalization Forms,” March 2001.), and then performing an exact binary comparison.

No collation or sorting order for attributes or element values is defined. Therefore, DSKPP implementations MUST NOT depend on specific sorting orders for values.

7.2.  Round-Trip #1: <KeyProvClientHello> and <KeyProvServerFinished>

7.2.1.  Examples

7.2.1.1.  Example Using the Key Transport Profile

The client indicates support all the Key Transport, Key Wrap, and Passphrase-Based Key Wrap profiles (see Section 11 (Key Initialization Profiles of Two- and One-Pass DSKPP)):

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvClientHello Version="1.0"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
  keyprov-dskpp-1.0.xsd">
  <DeviceIdentifierData>
    <DeviceId>
      <pskc:Manufacturer>ManufacturerABC</pskc:Manufacturer>
      <pskc:SerialNo>XL0000000001234</pskc:SerialNo>
      <pskc:Model>U2</pskc:Model>
    </DeviceId>
  </DeviceIdentifierData>
  <ClientNonce>xwQzwEl0CjPAiQeDxwRJdQ==</ClientNonce>
  <SupportedKeyTypes>
    <Algorithm>urn:ietf:params:xml:schema:keyprov:otpalg#HOTP</Algorithm>
    <Algorithm>
      http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst#SecurID-AES
    </Algorithm>
  </SupportedKeyTypes>
  <SupportedEncryptionAlgorithms>
    <Algorithm>http://www.w3.org/2001/05/xmlenc#rsa_1_5</Algorithm>
    <Algorithm>http://www.w3.org/2001/04/xmlenc#kw-aes128</Algorithm>
    <Algorithm>urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes</Algorithm>
  </SupportedEncryptionAlgorithms>
  <SupportedMacAlgorithms>
    <Algorithm>urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes</Algorithm>
  </SupportedMacAlgorithms>
  <SupportedProtocolVariants>
    <TwoPass>
      <SupportedKeyInitializationMethod>
        urn:ietf:params:xml:schema:keyprov:protocol#wrap
      </SupportedKeyInitializationMethod>
      <Payload xsi:type="ds:KeyInfoType">
        <ds:KeyName>Key_001</ds:KeyName>
      </Payload>
      <SupportedKeyInitializationMethod>
        urn:ietf:params:xml:schema:keyprov:protocol#transport
      </SupportedKeyInitializationMethod>
      <SupportedKeyInitializationMethod>
        urn:ietf:params:xml:schema:keyprov:protocol#passphrase-wrap
      </SupportedKeyInitializationMethod>
      <Payload xsi:type="ds:KeyInfoType">
        <ds:X509Data>
          <ds:X509Certificate>miib</ds:X509Certificate>
        </ds:X509Data>
      </Payload>
    </TwoPass>
  </SupportedProtocolVariants>
  <SupportedKeyContainers>
    <KeyContainerFormat>
      urn:ietf:params:xml:schema:keyprov:container#KeyContainer
    </KeyContainerFormat>
  </SupportedKeyContainers>
  <AuthenticationData>
    <DigitalSignature>
      <ds:SignedInfo>
        <ds:CanonicalizationMethod
          Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
        <ds:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
        <ds:Reference URI="#Nonce">
          <ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
          <ds:DigestValue></ds:DigestValue>
        </ds:Reference>
      </ds:SignedInfo>
      <ds:SignatureValue></ds:SignatureValue>
      <ds:KeyInfo>
        <ds:X509Data>
          <ds:X509Certificate>miib</ds:X509Certificate>
        </ds:X509Data>
      </ds:KeyInfo>
      <ds:Object Id="Nonce">xwQzwEl0CjPAiQeDxwRJdQ==</ds:Object>
    </DigitalSignature>
  </AuthenticationData>
</dskpp:KeyProvClientHello>

In this example, the server responds to the previous request using the key transport profile.

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvServerFinished Version="1.0" SessionID="4114" Status="Success"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xmlns:xenc="http://www.w3.org/2001/04/xmlenc#"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
    keyprov-dskpp-1.0.xsd">
  <KeyContainer>
    <KeyContainer Version="1.0">
      <pskc:EncryptionMethod
        Algorithm="http://www.w3.org/2001/05/xmlenc#rsa_1_5">
        <pskc:KeyInfo>
          <ds:X509Data>
            <ds:X509Certificate>miib</ds:X509Certificate>
          </ds:X509Data>
        </pskc:KeyInfo>
      </pskc:EncryptionMethod>
      <pskc:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1"/>
      <Device xmlns="urn:ietf:params:xml:ns:keyprov:1.0:container">
        <Key KeyAlgorithm="urn:ietf:params:xml:schema:keyprov:otpalg#HOTP"
          KeyId="SDU312345678">
          <Issuer>CredentialIssuer</Issuer>
          <Usage otp="true">
            <ResponseFormat format="DECIMAL" length="6"/>
          </Usage>
          <FriendlyName>MyFirstToken</FriendlyName>
          <Data Name="SECRET">
            <Value>
              7JHUyp3azOkqJENSsh6b2vxXzwGBYypzJxEr+ikQAa229KV/BgZhGA==
            </Value>
            <ValueDigest>
              i8j+kpbfKQsSlwmJYS99lQ==
            </ValueDigest>
          </Data>
          <Data Name="COUNTER">
            <Value>AAAAAAAAAAA=</Value>
          </Data>
          <Expiry>10/30/2012</Expiry>
        </Key>
      </Device>
    </KeyContainer>
  </KeyContainer>
  <Mac
    MacAlgorithm="urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes">
    miidfasde312asder394jw==
  </Mac>
  <AuthenticationData>
    <AuthenticationCodeMac>
      <Mac>4bRJf9xXd3KchKoTenHJiw==</Mac>
    </AuthenticationCodeMac>
  </AuthenticationData>
</dskpp:KeyProvServerFinished>

7.2.1.2.  Example Using the Key Wrap Profile

The client sends a request that specifies a shared key to protect the K_TOKEN, and the server responds using the Key Wrap Profile. Authentication data in this example is basing on an authentication code rather than a device certificate.

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvClientHello Version="1.0"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xmlns:pkcs-5="http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5v2-0#"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
  keyprov-dskpp-1.0.xsd">
  <DeviceIdentifierData>
    <DeviceId>
      <pskc:Manufacturer>ManufacturerABC</pskc:Manufacturer>
      <pskc:SerialNo>XL0000000001234</pskc:SerialNo>
      <pskc:Model>U2</pskc:Model>
    </DeviceId>
  </DeviceIdentifierData>
  <ClientNonce>xwQzwEl0CjPAiQeDxwRJdQ==</ClientNonce>
  <SupportedKeyTypes>
    <Algorithm>urn:ietf:params:xml:schema:keyprov:otpalg#HOTP</Algorithm>
    <Algorithm>
      http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst#SecurID-AES
    </Algorithm>
  </SupportedKeyTypes>
  <SupportedEncryptionAlgorithms>
    <Algorithm>http://www.w3.org/2001/05/xmlenc#rsa_1_5</Algorithm>
    <Algorithm>http://www.w3.org/2001/04/xmlenc#kw-aes128</Algorithm>
    <Algorithm>
      http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbes2
    </Algorithm>
    <Algorithm>
      urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
    </Algorithm>
  </SupportedEncryptionAlgorithms>
  <SupportedMacAlgorithms>
    <Algorithm>
      urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
    </Algorithm>
  </SupportedMacAlgorithms>
  <SupportedProtocolVariants>
    <TwoPass>
      <SupportedKeyInitializationMethod>
        urn:ietf:params:xml:schema:keyprov:protocol#wrap
      </SupportedKeyInitializationMethod>
      <Payload xsi:type="ds:KeyInfoType">
        <ds:KeyName>Key_001</ds:KeyName>
      </Payload>
    </TwoPass>
  </SupportedProtocolVariants>
  <SupportedKeyContainers>
    <KeyContainerFormat>
      urn:ietf:params:xml:schema:keyprov:container#KeyContainer
    </KeyContainerFormat>
  </SupportedKeyContainers>
  <AuthenticationData>
    <ClientID>31300257</ClientID>
    <AuthenticationCodeMac>
      <IterationCount>512</IterationCount>
      <Mac>4bRJf9xXd3KchKoTenHJiw==</Mac>
    </AuthenticationCodeMac>
  </AuthenticationData>
</dskpp:KeyProvClientHello>

In this example, the server responds to the previous request using the key wrap profile.

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvServerFinished Version="1.0" Status="Success"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xmlns:xenc="http://www.w3.org/2001/04/xmlenc#"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
  keyprov-dskpp-1.0.xsd">
  <KeyContainer>
    <ServerID>https://www.somedskppservice.com/</ServerID>
    <KeyContainer Version="1.0">
      <EncryptionMethod Algorithm="http://www.w3.org/2001/04/xmlenc#kw-aes128"
        xmlns="urn:ietf:params:xml:ns:keyprov:1.0:container">
        <KeyInfo>
           <ds:KeyName>Key-001</ds:KeyName>
         </KeyInfo>
      </EncryptionMethod>
      <pskc:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1"/>
      <Device xmlns="urn:ietf:params:xml:ns:keyprov:1.0:container">
        <Key KeyAlgorithm="urn:ietf:params:xml:schema:keyprov:otpalg#HOTP"
          KeyId="SDU312345678">
          <Issuer>CredentialIssuer</Issuer>
          <Usage otp="true">
            <ResponseFormat format="DECIMAL" length="6"/>
          </Usage>
          <FriendlyName>MyFirstToken</FriendlyName>
          <Data Name="SECRET">
            <Value>
              JSPUyp3azOkqJENSsh6b2hdXz1WBYypzJxEr+ikQAa22M6V/BgZhRg==
            </Value>
            <ValueDigest>
              i8j+kpbfKQsSlwmJYS99lQ==
            </ValueDigest>
          </Data>
          <Data Name="COUNTER">
            <Value>AAAAAAAAAAA=</Value>
          </Data>
          <Expiry>10/30/2012</Expiry>
        </Key>
      </Device>
    </KeyContainer>
  </KeyContainer>
  <Mac MacAlgorithm="urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes">
    miidfasde312asder394jw==
  </Mac>
  <AuthenticationData>
    <AuthenticationCodeMac>
      <Mac>4bRJf9xXd3KchKoTenHJiw==</Mac>
    </AuthenticationCodeMac>
  </AuthenticationData>
</dskpp:KeyProvServerFinished>

7.2.1.3.  Example Using the Passphrase-Based Key Wrap Profile

The client sends a request similar to that in Section 7.2.1.1 (Example Using the Key Transport Profile) with authentication data basing on an authentication code, and the server responds using the Passphrase-Based Key Wrap Profile. The authentication data is set in clear text when it is sent over a secure transport channel such as TLS.

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvClientHello Version="1.0"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xmlns:pkcs-5="http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5v2-0#"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
  keyprov-dskpp-1.0.xsd">
  <DeviceIdentifierData>
    <DeviceId>
      <pskc:Manufacturer>ManufacturerABC</pskc:Manufacturer>
      <pskc:SerialNo>XL0000000001234</pskc:SerialNo>
      <pskc:Model>U2</pskc:Model>
    </DeviceId>
  </DeviceIdentifierData>
  <ClientNonce>xwQzwEl0CjPAiQeDxwRJdQ==</ClientNonce>
  <SupportedKeyTypes>
    <Algorithm>urn:ietf:params:xml:schema:keyprov:otpalg#HOTP</Algorithm>
    <Algorithm>
      http://www.rsa.com/rsalabs/otps/schemas/2005/09/otps-wst#SecurID-AES
    </Algorithm>
  </SupportedKeyTypes>
  <SupportedEncryptionAlgorithms>
    <Algorithm>http://www.w3.org/2001/05/xmlenc#rsa_1_5</Algorithm>
    <Algorithm>http://www.w3.org/2001/04/xmlenc#kw-aes128</Algorithm>
    <Algorithm>
      http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbes2
    </Algorithm>
    <Algorithm>
      urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
    </Algorithm>
  </SupportedEncryptionAlgorithms>
  <SupportedMacAlgorithms>
    <Algorithm>
      urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes
    </Algorithm>
  </SupportedMacAlgorithms>
  <SupportedProtocolVariants>
    <TwoPass>
      <SupportedKeyInitializationMethod>
        urn:ietf:params:xml:schema:keyprov:protocol#wrap
      </SupportedKeyInitializationMethod>
      <Payload xsi:type="ds:KeyInfoType">
        <ds:KeyName>Key_001</ds:KeyName>
      </Payload>
      <SupportedKeyInitializationMethod>
        urn:ietf:params:xml:schema:keyprov:protocol#passphrase-wrap
      </SupportedKeyInitializationMethod>
    </TwoPass>
  </SupportedProtocolVariants>
  <SupportedKeyContainers>
    <KeyContainerFormat>
      urn:ietf:params:xml:schema:keyprov:container#KeyContainer
    </KeyContainerFormat>
  </SupportedKeyContainers>
  <AuthenticationData>
    <ClientID>31300257</ClientID>
    <AuthenticationCodeMac>
      <IterationCount>512</IterationCount>
      <Mac>4bRJf9xXd3KchKoTenHJiw==</Mac>
    </AuthenticationCodeMac>
  </AuthenticationData>
</dskpp:KeyProvClientHello>

In this example, the server responds to the previous request using the Passphrase-Based Key Wrap Profile.

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvServerFinished Version="1.0" SessionID="4114" Status="Success"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xmlns:xenc="http://www.w3.org/2001/04/xmlenc#"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
    keyprov-dskpp-1.0.xsd">
  <KeyContainer>
    <KeyContainer Version="1.0">
      <EncryptionMethod
        Algorithm="http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbes2"
        xmlns="urn:ietf:params:xml:ns:keyprov:1.0:container">
          <PBEEncryptionParam
             EncryptionAlgorithm="http://www.w3.org/2001/04/xmlenc#kw-aes128-cbc">
            <PBESalt>y6TzckeLRQw=</PBESalt>
            <PBEIterationCount>1024</PBEIterationCount>
          </PBEEncryptionParam>
          <IV>c2FtcGxlaXY=</IV>
        </EncryptionMethod>
      <pskc:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1"/>
      <Device xmlns="urn:ietf:params:xml:ns:keyprov:1.0:container">
        <Key KeyAlgorithm="urn:ietf:params:xml:schema:keyprov:otpalg#HOTP"
          KeyId="SDU312345678">
          <Issuer>CredentialIssuer</Issuer>
          <Usage otp="true">
            <ResponseFormat format="DECIMAL" length="6"/>
          </Usage>
          <FriendlyName>MyFirstToken</FriendlyName>
          <Data Name="SECRET">
            <Value>
              JSPUyp3azOkqJENSsh6b2hdXz1WBYypzJxEr+ikQAa22M6V/BgZhRg==
            </Value>
            <ValueDigest>
              i8j+kpbfKQsSlwmJYS99lQ==
            </ValueDigest>
          </Data>
          <Data Name="COUNTER">
            <Value>AAAAAAAAAAA=</Value>
          </Data>
          <Expiry>10/30/2012</Expiry>
        </Key>
      </Device>
    </KeyContainer>
  </KeyContainer>
  <Mac MacAlgorithm="urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes">
    miidfasde312asder394jw==
  </Mac>
  <AuthenticationData>
    <AuthenticationCodeMac>
      <Mac>4bRJf9xXd3KchKoTenHJiw==</Mac>
    </AuthenticationCodeMac>
  </AuthenticationData>
</dskpp:KeyProvServerFinished>

7.2.2.  Components of the <KeyProvClientHello> Request

The components of this message have the following meaning:

• Version: (attribute inherited from the AbstractRequestType type) The highest version of this protocol the client supports. Only version one ("1.0") is currently specified.
• <DeviceIdentifierData>: An identifier for the cryptographic module as defined in Section 5.3 (Client Authentication Mechanisms (Applicable to Four- and Two-Pass DSKPP) ) above. The identifier MUST only be present if such shared secrets exist or if the identifier was provided by the server in a <KeyProvTrigger> element (see Section 12.2.7 (Initialization of DSKPP) below). In the latter case, it MUST have the same value as the identifier provided in that element.
• <KeyID>: An identifier for the key that will be overwritten if the protocol run is successful. The identifier MUST only be present if the key exists or the identifier was provided by the server in a <KeyProvTrigger> element (see Section 12.2.7 (Initialization of DSKPP) below). In the latter case, it MUST have the same value as the identifier provided in that element.
• <KeyProvClientNonce>: This is the nonce R, which, when present, MUST be used by the server when calculating MAC values (see below). It is RECOMMENDED that clients include this element whenever the <KeyID> element is present.
• <TriggerNonce>: This OPTIONAL element MUST be present if and only if the DSKPP run was initialized with a <KeyProvTrigger> message (see Section 12.2.7 (Initialization of DSKPP) below), and MUST, in that case, have the same value as the <TriggerNonce> child of that message. A server using nonces in this way MUST verify that the nonce is valid and that any device or key identifier values provided in the <KeyProvTrigger> message match the corresponding identifier values in the <KeyProvClientHello> message.
• <SupportedKeyTypes>: A sequence of URIs indicating the key types for which the cryptographic module is willing to generate keys through DSKPP.
• <SupportedEncryptionAlgorithms>: A sequence of URIs indicating the encryption algorithms supported by the cryptographic module for the purposes of DSKPP. The DSKPP client MAY indicate the same algorithm both as a supported key type and as an encryption algorithm.
• <SupportedMacAlgorithms>: A sequence of URIs indicating the MAC algorithms supported by the cryptographic module for the purposes of DSKPP. The DSKPP client MAY indicate the same algorithm both as an encryption algorithm and as a MAC algorithm (e.g., urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes defined in Appendix B (Example of DSKPP-PRF Realizations)).
• <SupportedProtocolVariants>: This OPTIONAL element is used by the DSKPP client to indicate support for four-pass or two-pass DSKPP. If two-pass support is specified, then <KeyProvClientNonce> MUST be set to nonce R in the <KeyProvClientHello> message unless <TriggerNonce> is already present.
• <SupportedKeyContainers>: This OPTIONAL element is a sequence of URIs indicating the key container formats supported by the DSKPP client. If this element is not provided, then the DSKPP server MUST proceed with "urn:ietf:params:xml:schema:keyprov:container#KeyContainer" (see [PSKC] (, “Portable Symmetric Key Container,” 2005.).
• <AuthenticationData>: This OPTIONAL element contains data that the DSKPP client uses to authenticate the user or device to the DSKPP server. The element is set as specified in Section 5.3 (Client Authentication Mechanisms (Applicable to Four- and Two-Pass DSKPP) ).
• <Extensions>: A sequence of extensions. One extension is defined for this message in this version of DSKPP: the ClientInfoType (see Section 10 (Extensibility)).

7.2.3.  Components of a <KeyProvServerFinished> Response

This message is the last message of the DSKPP protocol run. In a 4-pass exchange, the DSKPP server sends this message in response to a <KeyProvClientNonce> message, whereas in a 2-pass exchange, the DSKPP server sends this message in response to a <KeyProvClientHello> message. In a 1-pass exchange, the DSKPP server sends only this message to the client. The components of this message have the following meaning:

• Version: (inherited from the AbstractResponseType type) The DSKPP version used in this session.
• SessionID: (inherited from the AbstractResponseType type) The previously established identifier for this session.
• Status: (inherited from the AbstractResponseType type) Return code for the <KeyProvServerFinished> message. If Status is not "Success", only the Status, SessionID, and Version attributes will be present (the presence of the SessionID attribute is dependent on the type of reported error); otherwise, all the other elements MUST be present as well. In this latter case, the <KeyProvServerFinished> message can be seen as a "Commit" message, instructing the cryptographic module to store the generated key and associate the given key identifier with this key.
• <KeyContainer>: The key container containing symmetric key values (in the case of a 2- or 1-pass exchange) and configuration data. The default container format is based on the KeyContainerType type from PSKC, as defined in [PSKC] (, “Portable Symmetric Key Container,” 2005.).
• <Extensions>: A list of extensions chosen by the DSKPP server. For this message, this version of DSKPP defines one extension, the ClientInfoType (see Section 10 (Extensibility)).
• <Mac>: To avoid a false "Commit" message causing the cryptographic module to end up in an initialized state for which the server does not know the stored key, <KeyProvServerFinished> messages MUST always be authenticated with a MAC. The MAC MUST be made using the already established MAC algorithm.
• <AuthenticationData>: This OPTIONAL element contains data that allows the DSKPP client to authenticate the DSKPP server. The MAC value is calculated with K_MAC' as specified in Section 4.4.3 (Server Authentication).

When receiving a <KeyProvServerFinished> message with Status="Success" for which the MAC verifies, the DSKPP client MUST associate the generated key K_TOKEN with the provided key identifier and store this data permanently. After this operation, it MUST not be possible to overwrite the key unless knowledge of an authorizing key is proven through a MAC on a later <KeyProvServerHello> (and <KeyProvServerFinished>) message.

The DSKPP client MUST verify the MAC. The DSKPP client MUST terminate the DSKPP session if the MAC does not verify, and MUST, in this case, also delete any nonces, keys, and/or secrets associated with the failed run of the DSKPP protocol.

The MacType's MacAlgorithm attribute MUST, when present, identify the negotiated MAC algorithm.

7.3.  DSKPP Server Results: The StatusCode Type

The StatusCode type enumerates all possible return codes. Upon transmission or receipt of a message for which the Status attribute's value is not "Success" or "Continue", the default behavior, unless explicitly stated otherwise below, is that both the DSKPP server and the DSKPP client MUST immediately terminate the DSKPP session. DSKPP servers and DSKPP clients MUST delete any secret values generated as a result of failed runs of the DSKPP protocol. Session identifiers MAY be retained from successful or failed protocol runs for replay detection purposes, but such retained identifiers MUST not be reused for subsequent runs of the protocol.

When possible, the DSKPP client SHOULD present an appropriate error message to the user.

These status codes are valid in all DSKPP Response messages unless explicitly stated otherwise:

• "Continue" indicates that the DSKPP server is ready for a subsequent request from the DSKPP client. It cannot be sent in the server's final message.
• "Success" indicates successful completion of the DSKPP session. It can only be sent in the server's final message.
• "Abort" indicates that the DSKPP server rejected the DSKPP client's request for unspecified reasons.
• "AccessDenied" indicates that the DSKPP client is not authorized to contact this DSKPP server.
• "MalformedRequest" indicates that the DSKPP server failed to parse the DSKPP client's request.
• "UnknownRequest" indicates that the DSKPP client made a request that is unknown to the DSKPP server.
• "UnknownCriticalExtension" indicates that a critical DSKPP extension (see below) used by the DSKPP client was not supported or recognized by the DSKPP server.
• "UnsupportedVersion" indicates that the DSKPP client used a DSKPP protocol version not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message.
• "NoSupportedKeyTypes" indicates that the DSKPP client only suggested key types that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message.
• "NoSupportedEncryptionAlgorithms" indicates that the DSKPP client only suggested encryption algorithms that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested encryption algorithms.
• "NoSupportedMacAlgorithms" indicates that the DSKPP client only suggested MAC algorithms that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested MAC algorithms.
• "NoProtocolVariants" indicates that the DSKPP client only suggested a protocol variant (either 2-pass or 4-pass) that is not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested protocol variants.
• "NoSupportedKeyContainers" indicates that the DSKPP client only suggested key container formats that are not supported by the DSKPP server. This error is only valid in the DSKPP server's first response message. Note that the error will only occur if the DSKPP server does not support any of the DSKPP client's suggested key container formats.
• "AuthenticationDataMissing" indicates that the DSKPP client didn't provide authentication data that the DSKPP server required.
• "AuthenticationDataInvalid" indicates that the DSKPP client supplied user or device authentication data that the DSKPP server failed to validate.
• "InitializationFailed" indicates that the DSKPP server could not generate a valid key given the provided data. When this status code is received, the DSKPP client SHOULD try to restart DSKPP, as it is possible that a new run will succeed.
• "ProvisioningPeriodExpired" indicates that the provisioning period set by the DSKPP server has expired. When the status code is received, the DSKPP client SHOULD report the key initialization failure reason to the user and the user MUST register with the DSKPP server to initialize a new key.

8.  One-Pass Protocol

In this section, example messages are used to describe parameters, encoding and semantics in a 1-pass DSKPP protocol. The examples are written using XML. While they are syntactically correct, MAC and cipher values are fictitious.

8.1.  XML Basics

The DSKPP XML schema can be found in Section 13 (DSKPP Schema). Some DSKPP elements rely on the parties being able to compare received values with stored values. Unless otherwise noted, all elements in this document that have the XML Schema "xs:string" type, or a type derived from it, MUST be compared using an exact binary comparison. In particular, DSKPP implementations MUST NOT depend on case-insensitive string comparisons, normalization or trimming of white space, or conversion of locale-specific formats such as numbers.

Implementations that compare values that are represented using different character encodings MUST use a comparison method that returns the same result as converting both values to the Unicode character encoding, Normalization Form C [UNICODE] (Davis, M. and M. Duerst, “Unicode Normalization Forms,” March 2001.), and then performing an exact binary comparison.

No collation or sorting order for attributes or element values is defined. Therefore, DSKPP implementations MUST NOT depend on specific sorting orders for values.

8.2.  Server to Client Only: <KeyProvServerFinished>

8.2.1.  Example

The Server sends a provisioned key to a client with prior knowledge about the client's capabilities:

<?xml version="1.0" encoding="UTF-8"?>
<dskpp:KeyProvServerFinished Version="1.0" SessionID="4114" Status="Success"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
  xmlns:xenc="http://www.w3.org/2001/04/xmlenc#"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
    keyprov-dskpp-1.0.xsd">
  <KeyContainer>
    <KeyContainer Version="1.0">
      <pskc:EncryptionMethod
        Algorithm="http://www.w3.org/2001/05/xmlenc#rsa_1_5">
        <pskc:KeyInfo>
          <ds:X509Data>
            <ds:X509Certificate>miib</ds:X509Certificate>
          </ds:X509Data>
        </pskc:KeyInfo>
      </pskc:EncryptionMethod>
      <pskc:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1"/>
      <Device xmlns="urn:ietf:params:xml:ns:keyprov:1.0:container">
        <Key KeyAlgorithm="urn:ietf:params:xml:schema:keyprov:otpalg#HOTP"
          KeyId="SDU312345678">
          <Issuer>CredentialIssuer</Issuer>
          <Usage otp="true">
            <ResponseFormat format="DECIMAL" length="6"/>
          </Usage>
          <FriendlyName>MyFirstToken</FriendlyName>
          <Data Name="SECRET">
            <Value>
              7JHUyp3azOkqJENSsh6b2vxXzwGBYypzJxEr+ikQAa229KV/BgZhGA==
            </Value>
            <ValueDigest>
              i8j+kpbfKQsSlwmJYS99lQ==
            </ValueDigest>
          </Data>
          <Data Name="COUNTER">
            <Value>AAAAAAAAAAA=</Value>
          </Data>
          <Expiry>10/30/2009</Expiry>
        </Key>
      </Device>
    </KeyContainer>
  </KeyContainer>
  <Mac MacAlgorithm="urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes">
    miidfasde312asder394jw==
  </Mac>
  <AuthenticationData>
    <AuthenticationCodeMac>
      <Mac>4bRJf9xXd3KchKoTenHJiw==</Mac>
    </AuthenticationCodeMac>
  </AuthenticationData>
</dskpp:KeyProvServerFinished>

8.2.2.  Components of a <KeyProvServerFinished> Response

This message is the last message of the DSKPP protocol run. In a 4-pass exchange, the DSKPP server sends this message in response to a <KeyProvClientNonce> message, whereas in a 2-pass exchange, the DSKPP server sends this message in response to a <KeyProvClientHello> message. In a 1-pass exchange, the DSKPP server sends only this message to the client. The components of this message have the following meaning:

• Version: (inherited from the AbstractResponseType type) The DSKPP version used in this session.
• SessionID: (inherited from the AbstractResponseType type) The previously established identifier for this session.
• Status: (inherited from the AbstractResponseType type) Return code for the <KeyProvServerFinished> message. If Status is not "Success", only the Status, SessionID, and Version attributes will be present (the presence of the SessionID attribute is dependent on the type of reported error); otherwise, all the other elements MUST be present as well. In this latter case, the <KeyProvServerFinished> message can be seen as a "Commit" message, instructing the cryptographic module to store the generated key and associate the given key identifier with this key.
• <KeyContainer>: The key container containing symmetric key values (in the case of a 2- or 1-pass exchange) and configuration data. The default container format is based on the KeyContainerType type from PSKC, as defined in [PSKC] (, “Portable Symmetric Key Container,” 2005.).
• <Extensions>: A list of extensions chosen by the DSKPP server. For this message, this version of DSKPP defines one extension, the ClientInfoType (see Section 10 (Extensibility)).
• <Mac>: To avoid a false "Commit" message causing the cryptographic module to end up in an initialized state for which the server does not know the stored key, <KeyProvServerFinished> messages MUST always be authenticated with a MAC. The MAC MUST be made using the already established MAC algorithm.
• <AuthenticationData>: This OPTIONAL element contains data that allows the DSKPP client to authenticate the DSKPP server. The MAC value is calculated with K_MAC' as specified in Section 4.5.3 (Server Authentication).

When receiving a <KeyProvServerFinished> message with Status="Success" for which the MAC verifies, the DSKPP client MUST associate the generated key K_TOKEN with the provided key identifier and store this data permanently. After this operation, it MUST not be possible to overwrite the key unless knowledge of an authorizing key is proven through a MAC on a later <KeyProvServerHello> (and <KeyProvServerFinished>) message.

The DSKPP client MUST verify the MAC. The DSKPP client MUST terminate the DSKPP session if the MAC does not verify, and MUST, in this case, also delete any nonces, keys, and/or secrets associated with the failed run of the DSKPP protocol.

The MacType's MacAlgorithm attribute MUST, when present, identify the negotiated MAC algorithm.

9.  Trigger

In this section, an example is used to describe parameters, encoding and semantics in a DSKPP Trigger message. The example is written using XML.

9.1.  XML Basics

The DSKPP XML schema can be found in Section 13 (DSKPP Schema). Some DSKPP elements rely on the parties being able to compare received values with stored values. Unless otherwise noted, all elements in this document that have the XML Schema "xs:string" type, or a type derived from it, MUST be compared using an exact binary comparison. In particular, DSKPP implementations MUST NOT depend on case-insensitive string comparisons, normalization or trimming of white space, or conversion of locale-specific formats such as numbers.

Implementations that compare values that are represented using different character encodings MUST use a comparison method that returns the same result as converting both values to the Unicode character encoding, Normalization Form C [UNICODE] (Davis, M. and M. Duerst, “Unicode Normalization Forms,” March 2001.), and then performing an exact binary comparison.

No collation or sorting order for attributes or element values is defined. Therefore, DSKPP implementations MUST NOT depend on specific sorting orders for values.

9.2.  Example

<dskpp:KeyProvTrigger Version="1.0"
  xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
  xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="urn:ietf:params:xml:ns:keyprov:1.0:protocol
    keyprov-dskpp-1.0.xsd">
  <InitializationTrigger>
    <DeviceIdentifierData>
      <DeviceId>
        <pskc:Manufacturer>ManufacturerABC</pskc:Manufacturer>
        <pskc:SerialNo>XL0000000001234</pskc:SerialNo>
        <pskc:Model>U2</pskc:Model>
      </DeviceId>
    </DeviceIdentifierData>
    <KeyID>SE9UUDAwMDAwMDAx</KeyID>
    <TokenPlatformInfo KeyLocation="Hardware" AlgorithmLocation="Software"/>
    <TriggerNonce>112dsdfwf312asder394jw==</TriggerNonce>
    <DSKPPServerUrl>https://www.somekeyprovservice.com/</DSKPPServerUrl>
  </InitializationTrigger>
</dskpp:KeyProvTrigger>

9.3.  Components of the <KeyProvTrigger> Message

The DSKPP server MAY initialize the DSKPP protocol by sending a <KeyProvTrigger> message. This message MAY, e.g., be sent in response to a user requesting key initialization in a browsing session.

The <KeyProvTrigger> element is intended for the DSKPP client and MAY inform the DSKPP client about the identifier for the device that houses the cryptographic module to be initialized, and optionally of the identifier for the key on that module. The latter would apply to key renewal. The trigger always contains a nonce to allow the DSKPP server to couple the trigger with a later DSKPP <KeyProvClientHello> request. Finally, the trigger MAY contain a URL to use when contacting the DSKPP server. The <xs:any> elements are for future extensibility. Any provided <DeviceIdentifierData> or <KeyID> values MUST be used by the DSKPP client in the subsequent <KeyProvClientHello> request. The OPTIONAL <TokenPlatformInfo> element informs the DSKPP client about the characteristics of the intended cryptographic module platform, and applies in the public-key variant of DSKPP in situations when the client potentially needs to decide which one of several modules to initialize.

10.  Extensibility

10.1.  The ClientInfoType Type

present in a <KeyProvClientHello> or a <KeyProvClientNonce> message, the OPTIONAL ClientInfoType extension contains DSKPP client-specific information. DSKPP servers MUST support this extension. DSKPP servers MUST NOT attempt to interpret the data it carries and, if received, MUST include it unmodified in the current protocol run's next server response. Servers need not retain the ClientInfoType's data after that response has been generated.

10.2.  The ServerInfoType Type

When present, the OPTIONAL ServerInfoType extension contains DSKPP server-specific information. This extension is only valid in <KeyProvServerHello> messages for which Status = "Continue". DSKPP clients MUST support this extension. DSKPP clients MUST NOT attempt to interpret the data it carries and, if received, MUST include it unmodified in the current protocol run's next client request (i.e., the <KeyProvClientNonce> message). DSKPP clients need not retain the ServerInfoType's data after that request has been generated. This extension MAY be used, e.g., for state management in the DSKPP server.

10.3.  The KeyInitializationDataType Type

This extension is used for 2- and 1-pass DSKPP exchange; it carries an identifier for the selected key initialization method as well as key initialization method-dependent payload data.

Servers MAY include this extension in a <KeyProvServerFinished> message that is being sent in response to a received <KeyProvClientHello> message if and only if that <KeyProvClientHello> message selected TwoPassSupport as the ProtocolVariantType and the client indicated support for the selected key initialization method. Servers MUST include this extension in a <KeyProvServerFinished> message that is sent as part of a 1-pass DSKPP.

The elements of this type have the following meaning:

• <KeyInitializationMethod>: A two-pass key initialization method supported by the DSKPP client.
• <Payload>: A payload associated with the key initialization method. Since the syntax is a shorthand for <xs:element name="Payload" type="xs:anyType"/>, any well-formed payloads can be carried in this element.

11.  Key Initialization Profiles of Two- and One-Pass DSKPP

11.1.  Introduction

This appendix introduces three profiles of DSKPP for key initialization. They MAY all be used for two- as well as one-pass initialization of cryptographic modules. Further profiles MAY be defined by external entities or through the IETF process.

11.2.  Key Transport Profile

11.2.1.  Introduction

This profile initializes the cryptographic module with a symmetric key, K_TOKEN, through key transport and key derivation. The key transport is carried out using a public key, K_CLIENT, whose private key part resides in the cryptographic module as the transport key. A key K from which two keys, K_TOKEN and K_MAC are derived MUST be transported.

11.2.2.  Identification

This profile MUST be identified with the following URN:

urn:ietf:params:xml:schema:keyprov:protocol#transport

11.2.3.  Payloads

In the two-pass version of DSKPP, the client MUST send a payload associated with this key initialization method. The payload MUST be of type ds:KeyInfoType ([XMLDSIG] (W3C, “XML Signature Syntax and Processing,” February 2002.)), and only those choices of the ds:KeyInfoType that identify a public key are allowed. The ds:X509Certificate option of the ds:X509Data alternative is RECOMMENDED when the public key corresponding to the private key on the cryptographic module has been certified.

The server payload associated with this key initialization method MUST be of type xenc:EncryptedKeyType ([XMLENC] (W3C, “XML Encryption Syntax and Processing,” December 2002.)), and only those encryption methods utilizing a public key that are supported by the DSKPP client (as indicated in the <SupportedEncryptionAlgorithms> element of the <KeyProvClientHello> message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP) are allowed as values for the <xenc:EncryptionMethod> element. Further, in the case of 2-pass DSKPP, the <ds:KeyInfo> element MUST contain the same value (i.e. identify the same public key) as the <Payload> of the corresponding supported key initialization method in the <KeyProvClientHello> message that triggered the response. The <CarriedKeyName> element MAY be present, but MUST, when present, contain the same value as the <KeyID> element of the <KeyProvServerFinished> message. The Type attribute of the xenc:EncryptedKeyType MUST be present and MUST identify the type of the wrapped key. The type MUST be one of the types supported by the DSKPP client (as reported in the <SupportedKeyTypes> of the preceding <KeyProvClientHello> message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The transported key MUST consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN.

DSKPP servers and cryptographic modules supporting this profile MUST support the http://www.w3.org/2001/04/xmlenc#rsa-1_5 key-wrapping mechanism defined in [XMLENC] (W3C, “XML Encryption Syntax and Processing,” December 2002.).

When this profile is used, the MacAlgorithm attribute of the <Mac> element of the <KeyProvServerFinished> message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the <SupportedMacAlgorithms> element of the <KeyProvClientHello> message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The MAC MUST be calculated as described in Section 4.4 (Two-Pass Protocol Usage) for Two-Pass DSKPP and Section 4.5 (One-Pass Protocol Usage) for One-Pass DSKPP.

In addition, DSKPP servers MUST include the AuthenticationDataType element in their <KeyProvServerFinished> messages whenever a successful protocol run will result in an existing K_TOKEN being replaced.

11.3.  Key Wrap Profile

11.3.1.  Introduction

This profile initializes the cryptographic module with a symmetric key, K_TOKEN, through key wrap and key derivation. The key wrap MUST be carried out using a (symmetric) key-wrapping key, K_SHARED, known in advance by both the cryptographic module and the DSKPP server. A key K from which two keys, K_TOKEN and K_MAC are derived MUST be wrapped.

11.3.2.  Identification

This profile MUST be identified with the following URI:

urn:ietf:params:xml:schema:keyprov:protocol#wrap

11.3.3.  Payloads

In the 2-pass version of DSKPP, the client MUST send a payload associated with this key initialization method. The payload MUST be of type ds:KeyInfoType ([XMLDSIG] (W3C, “XML Signature Syntax and Processing,” February 2002.)), and only those choices of the ds:KeyInfoType that identify a symmetric key are allowed. The ds:KeyName alternative is RECOMMENDED.

The server payload associated with this key initialization method MUST be of type xenc:EncryptedKeyType ([XMLENC] (W3C, “XML Encryption Syntax and Processing,” December 2002.)), and only those encryption methods utilizing a symmetric key that are supported by the DSKPP client (as indicated in the <SupportedEncryptionAlgorithms> element of the <KeyProvClientHello> message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP) are allowed as values for the <xenc:EncryptionMethod> element. Further, in the case of 2-pass DSKPP, the <ds:KeyInfo> element MUST contain the same value (i.e. identify the same symmetric key) as the <Payload> of the corresponding supported key initialization method in the <KeyProvClientHello> message that triggered the response. The <CarriedKeyName> element MAY be present, and MUST, when present, contain the same value as the <KeyID> element of the <KeyProvServerFinished> message. The Type attribute of the xenc:EncryptedKeyType MUST be present and MUST identify the type of the wrapped key. The type MUST be one of the types supported by the DSKPP client (as reported in the <SupportedKeyTypes> of the preceding <KeyProvClientHello> message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The wrapped key MUST consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN.

DSKPP servers and cryptographic modules supporting this profile MUST support the http://www.w3.org/2001/04/xmlenc#kw-aes128 key-wrapping mechanism defined in [XMLENC] (W3C, “XML Encryption Syntax and Processing,” December 2002.).

When this profile is used, the MacAlgorithm attribute of the <Mac> element of the <KeyProvServerFinished> message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the <SupportedMacAlgorithms> element of the <KeyProvClientHello> message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The MAC MUST be calculated as described in Section 4.4 (Two-Pass Protocol Usage).

In addition, DSKPP servers MUST include the AuthenticationDataType element in their <KeyProvServerFinished> messages whenever a successful protocol run will result in an existing K_TOKEN being replaced.

11.4.  Passphrase-Based Key Wrap Profile

11.4.1.  Introduction

This profile is a variation of the key wrap profile. It initializes the cryptographic module with a symmetric key, K_TOKEN, through key wrap and key derivation, using a passphrase-derived key-wrapping key, K_DERIVED. The passphrase is known in advance by both the device user and the DSKPP server. To preserve the property of not exposing K_TOKEN to any other entity than the DSKPP server and the cryptographic module itself, the method SHOULD be employed only when the device contains facilities (e.g. a keypad) for direct entry of the passphrase. A key K from which two keys, K_TOKEN and K_MAC are derived MUST be wrapped.

11.4.2.  Identification

This profile MUST be identified with the following URI:

urn:ietf:params:xml:schema:keyprov:protocol#passphrase-wrap

11.4.3.  Payloads

In the 2-pass version of DSKPP, the client MUST send a payload associated with this key initialization method. The payload MUST be of type ds:KeyInfoType ([XMLDSIG] (W3C, “XML Signature Syntax and Processing,” February 2002.)). The ds:KeyName option MUST be used and the key name MUST identify the passphrase that will be used by the server to generate the key-wrapping key. As an example, the identifier could be a user identifier or a registration identifier issued by the server to the user during a session preceding the DSKPP protocol run.

The server payload associated with this key initialization method MUST be of type xenc:EncryptedKeyType ([XMLENC] (W3C, “XML Encryption Syntax and Processing,” December 2002.)), and only those encryption methods utilizing a passphrase to derive the key-wrapping key that are supported by the DSKPP client (as indicated in the <SupportedEncryptionAlgorithms> element of the <KeyProvClientHello> message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP) are allowed as values for the <xenc:EncryptionMethod> element. Further, in the case of 2-pass DSKPP, the <ds:KeyInfo> element MUST contain the same value (i.e. identify the same passphrase) as the <Payload> of the corresponding supported key initialization method in the <KeyProvClientHello> message that triggered the response. The <CarriedKeyName> element MAY be present, and MUST, when present, contain the same value as the <KeyID> element of the <KeyProvServerFinished> message. The Type attribute of the xenc:EncryptedKeyType MUST be present and MUST identify the type of the wrapped key. The type MUST be one of the types supported by the DSKPP client (as reported in the <SupportedKeyTypes> of the preceding <KeyProvClientHello> message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The wrapped key MUST consist of two parts of equal length. The first half constitutes K_MAC and the second half constitutes K_TOKEN. The length of K_TOKEN (and hence also the length of K_MAC) is determined by the type of K_TOKEN.

DSKPP servers and cryptographic modules supporting this profile MUST support the PBES2 password based encryption scheme defined in [PKCS‑5] (RSA Laboratories, “Password-Based Cryptography Standard,” March 1999.) (and identified as http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbes2 in [PKCS‑5‑XML] (RSA Laboratories, “XML Schema for PKCS #5 Version 2.0,” October 2006.)), the PBKDF2 passphrase-based key derivation function also defined in [PKCS‑5] (RSA Laboratories, “Password-Based Cryptography Standard,” March 1999.) (and identified as http://www.rsasecurity.com/rsalabs/pkcs/schemas/pkcs-5#pbkdf2 in [PKCS‑5‑XML] (RSA Laboratories, “XML Schema for PKCS #5 Version 2.0,” October 2006.)), and the http://www.w3.org/2001/04/xmlenc#kw-aes128 key-wrapping mechanism defined in [XMLENC] (W3C, “XML Encryption Syntax and Processing,” December 2002.).

When this profile is used, the MacAlgorithm attribute of the <Mac> element of the <KeyProvServerFinished> message MUST be present and MUST identify the selected MAC algorithm. The selected MAC algorithm MUST be one of the MAC algorithms supported by the DSKPP client (as indicated in the <SupportedMacAlgorithms> element of the <KeyProvClientHello> message in the case of 2-pass DSKPP, or as otherwise known in the case of 1-pass DSKPP). The MAC MUST be calculated as described in Section 4.4 (Two-Pass Protocol Usage).

In addition, DSKPP servers MUST include the AuthenticationDataType element in their <KeyProvServerFinished> messages whenever a successful protocol run will result in an existing K_TOKEN being replaced.

12.  Protocol Bindings

12.1.  General Requirements

DSKPP assumes a reliable transport.

12.2.  HTTP/1.1 Binding for DSKPP

12.2.1.  Introduction

This section presents a binding of the previous messages to HTTP/1.1 [RFC2616] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.). Note that the HTTP client normally will be different from the DSKPP client, i.e., the HTTP client will only exist to "proxy" DSKPP messages from the DSKPP client to the DSKPP server. Likewise, on the HTTP server side, the DSKPP server MAY receive DSKPP PDUs from a "front-end" HTTP server.

12.2.2.  Identification of DSKPP Messages

The MIME-type for all DSKPP messages MUST be

application/vnd.ietf.keyprov.dskpp+xml

12.2.3.  HTTP Headers

HTTP proxies MUST NOT cache responses carrying DSKPP messages. For this reason, the following holds:

• When using HTTP/1.1, requesters SHOULD:
  • Include a Cache-Control header field set to "no-cache, no-store".
  • Include a Pragma header field set to "no-cache".
• When using HTTP/1.1, responders SHOULD:
  • Include a Cache-Control header field set to "no-cache, no-must-revalidate, private".
  • Include a Pragma header field set to "no-cache".
  • NOT include a Validator, such as a Last-Modified or ETag header.

There are no other restrictions on HTTP headers, besides the requirement to set the Content-Type header value according to Section 12.2.2 (Identification of DSKPP Messages).

12.2.4.  HTTP Operations

Persistent connections as defined in HTTP/1.1 are assumed but not required. DSKPP requests are mapped to HTTP POST operations. DSKPP responses are mapped to HTTP responses.

12.2.5.  HTTP Status Codes

A DSKPP HTTP responder that refuses to perform a message exchange with a DSKPP HTTP requester SHOULD return a 403 (Forbidden) response. In this case, the content of the HTTP body is not significant. In the case of an HTTP error while processing a DSKPP request, the HTTP server MUST return a 500 (Internal Server Error) response. This type of error SHOULD be returned for HTTP-related errors detected before control is passed to the DSKPP processor, or when the DSKPP processor reports an internal error (for example, the DSKPP XML namespace is incorrect, or the DSKPP schema cannot be located). If the type of a DSKPP request cannot be determined, the DSKPP responder MUST return a 400 (Bad request) response.

In these cases (i.e., when the HTTP response code is 4xx or 5xx), the content of the HTTP body is not significant.

Redirection status codes (3xx) apply as usual.

Whenever the HTTP POST is successfully invoked, the DSKPP HTTP responder MUST use the 200 status code and provide a suitable DSKPP message (possibly with DSKPP error information included) in the HTTP body.

12.2.6.  HTTP Authentication

No support for HTTP/1.1 authentication is assumed.

12.2.7.  Initialization of DSKPP

The DSKPP server MAY initialize the DSKPP protocol by sending an HTTP response with Content-Type set according to Section 12.2.2 (Identification of DSKPP Messages) and response code set to 200 (OK). This message MAY, e.g., be sent in response to a user requesting key initialization in a browsing session. The initialization message MAY carry data in its body. If this is the case, the data MUST be a valid instance of a <KeyProvTrigger> element.

12.2.8.  Example Messages

a.

Initialization from DSKPP server:

HTTP/1.1 200 OK

Cache-Control: no-store
Content-Type: application/vnd.ietf.keyprov.dskpp+xml
Content-Length: <some value>

DSKPP initialization data in XML form...

b.

Initial request from DSKPP client:

POST http://example.com/cgi-bin/DSKPP-server HTTP/1.1
Cache-Control: no-store
Pragma: no-cache
Host: example.com
Content-Type: application/vnd.ietf.keyprov.dskpp+xml
Content-Length: <some value>

DSKPP data in XML form (supported version, supported algorithms...)

c.

Initial response from DSKPP server:

HTTP/1.1 200 OK

Cache-Control: no-store
Content-Type: application/vnd.ietf.keyprov.dskpp+xml
Content-Length: <some value>

DSKPP data in XML form (server random nonce, server public key, ...)

13.  DSKPP Schema

<?xml version="1.0" encoding="UTF-8"?>

 <xs:schema
   xmlns:dskpp="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
   xmlns:pskc="urn:ietf:params:xml:ns:keyprov:1.0:container"
   xmlns:xs="http://www.w3.org/2001/XMLSchema"
   xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
   targetNamespace="urn:ietf:params:xml:ns:keyprov:1.0:protocol"
   elementFormDefault="unqualified" attributeFormDefault="unqualified"
   version="1.0">

   <xs:import namespace="http://www.w3.org/2000/09/xmldsig#"
     schemaLocation="http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/xmldsig-core-schema.xsd"/>

   <xs:import namespace="urn:ietf:params:xml:ns:keyprov:1.0:container"
     schemaLocation="keyprov-pskc-1.0.xsd"/>

   <!-- Basic types -->
   <xs:complexType name="AbstractRequestType" abstract="true">
     <xs:attribute name="Version" type="dskpp:VersionType" use="required"/>
   </xs:complexType>

   <xs:complexType name="AbstractResponseType" abstract="true">
     <xs:attribute name="Version" type="dskpp:VersionType" use="required"/>
     <xs:attribute name="SessionID" type="dskpp:IdentifierType"/>
     <xs:attribute name="Status" type="dskpp:StatusCode" use="required"/>
   </xs:complexType>

   <xs:simpleType name="VersionType">
     <xs:restriction base="xs:string">
       <xs:pattern value="\d{1,2}\.\d{1,3}"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:simpleType name="IdentifierType">
     <xs:restriction base="xs:string">
       <xs:maxLength value="128"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:simpleType name="StatusCode">
     <xs:restriction base="xs:string">
       <xs:enumeration value="Continue"/>
       <xs:enumeration value="Success"/>
       <xs:enumeration value="Abort"/>
       <xs:enumeration value="AccessDenied"/>
       <xs:enumeration value="MalformedRequest"/>
       <xs:enumeration value="UnknownRequest"/>
       <xs:enumeration value="UnknownCriticalExtension"/>
       <xs:enumeration value="UnsupportedVersion"/>
       <xs:enumeration value="NoSupportedKeyTypes"/>
       <xs:enumeration value="NoSupportedEncryptionAlgorithms"/>
       <xs:enumeration value="NoSupportedMacAlgorithms"/>
       <xs:enumeration value="NoProtocolVariants"/>
       <xs:enumeration value="NoSupportedKeyContainers"/>
       <xs:enumeration value="AuthenticationDataMissing"/>
       <xs:enumeration value="AuthenticationDataInvalid"/>
       <xs:enumeration value="InitializationFailed"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="DeviceIdentifierDataType">
     <xs:choice>
       <xs:element name="DeviceId" type="pskc:DeviceIdType"/>
       <xs:any namespace="##other" processContents="strict"/>
     </xs:choice>
   </xs:complexType>

   <xs:simpleType name="PlatformType">
     <xs:restriction base="xs:string">
       <xs:enumeration value="Hardware"/>
       <xs:enumeration value="Software"/>
       <xs:enumeration value="Unspecified"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="TokenPlatformInfoType">
     <xs:attribute name="KeyLocation" type="dskpp:PlatformType"/>
     <xs:attribute name="AlgorithmLocation" type="dskpp:PlatformType"/>
   </xs:complexType>

   <xs:simpleType name="NonceType">
     <xs:restriction base="xs:base64Binary">
       <xs:minLength value="16"/>
     </xs:restriction>
   </xs:simpleType>

   <xs:complexType name="AlgorithmsType">
     <xs:sequence maxOccurs="unbounded">
       <xs:element name="Algorithm" type="dskpp:AlgorithmType"/>
     </xs:sequence>
   </xs:complexType>

   <xs:simpleType name="AlgorithmType">
     <xs:restriction base="xs:anyURI"/>
   </xs:simpleType>

   <xs:complexType name="ProtocolVariantsType">
     <xs:sequence>
       <xs:element name="FourPass" minOccurs="0"/>
       <xs:element name="TwoPass" type="dskpp:TwoPassSupportType"
         minOccurs="0"/>
       <xs:element name="OnePass" minOccurs="0"/>
     </xs:sequence>
   </xs:complexType>

   <xs:complexType name="TwoPassSupportType">
     <xs:sequence maxOccurs="unbounded">
       <xs:element name="SupportedKeyInitializationMethod"
         type="xs:anyURI"/>
       <xs:element name="Payload" minOccurs="0"/>
     </xs:sequence>
   </xs:complexType>

   <xs:complexType name="KeyContainersFormatType">
     <xs:sequence maxOccurs="unbounded">
       <xs:element name="KeyContainerFormat"
         type="dskpp:KeyContainerFormatType"/>
     </xs:sequence>
   </xs:complexType>

   <xs:simpleType name="KeyContainerFormatType">
     <xs:restriction base="xs:anyURI"/>
   </xs:simpleType>

   <xs:complexType name="AuthenticationDataType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Authentication data can consist of either authentication code
         for authenticating a user of the protocol, or an X.509 Certificate for
         authenticating a device. When a device certificate is used over a
         transport layer that is not secure, the Signature is calculated over
         a nonce value specified in ds:Signature/Object. When used in
         conjunction with the KeyProvServerFinished PDU, it contains a MAC
         authenticating the DSKPP server to the client.
       </xs:documentation>
     </xs:annotation>
     <xs:sequence>
       <xs:element name="ClientID" type="dskpp:IdentifierType"
                   minOccurs="0"/>
       <xs:choice minOccurs="0">
           <xs:element name="AuthenticationCodeMac"
                       type="dskpp:AuthenticationCodeMacType"/>
           <xs:element name="DigitalSignature"
                       type="ds:SignatureType"/>
       </xs:choice>
     </xs:sequence>
   </xs:complexType>

   <xs:complexType name="AuthenticationCodeMacType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         An authentication MAC calculated from an authentication code and
         optionally server information as well as nonce value if they are
         available.
       </xs:documentation>
     </xs:annotation>
     <xs:sequence>
       <xs:element name="Nonce" type="dskpp:NonceType" minOccurs="0"/>
       <xs:element name="IterationCount" type="xs:int" minOccurs="0"/>
       <xs:element name="Mac" type="dskpp:MacType"/>
     </xs:sequence>
   </xs:complexType>

   <xs:complexType name="MacType">
     <xs:simpleContent>
       <xs:extension base="xs:base64Binary">
         <xs:attribute name="MacAlgorithm" type="xs:anyURI"/>
       </xs:extension>
     </xs:simpleContent>
   </xs:complexType>

   <xs:complexType name="KeyContainerType">
     <xs:sequence>
       <xs:element name="ServerID" type="xs:anyURI" minOccurs="0"/>
       <xs:choice>
         <xs:element name="KeyContainer" type="pskc:KeyContainerType"/>
         <xs:any namespace="##other" processContents="strict"/>
       </xs:choice>
     </xs:sequence>
   </xs:complexType>

   <xs:complexType name="InitializationTriggerType">
     <xs:sequence>
       <xs:element name="DeviceIdentifierData"
         type="dskpp:DeviceIdentifierDataType" minOccurs="0"/>
       <xs:element name="KeyID" type="xs:base64Binary" minOccurs="0"/>
       <xs:element name="TokenPlatformInfo"
         type="dskpp:TokenPlatformInfoType" minOccurs="0"/>
       <xs:element name="TriggerNonce" type="dskpp:NonceType"/>
       <xs:element name="DSKPPServerUrl" type="xs:anyURI" minOccurs="0"/>
       <xs:any namespace="##other" processContents="strict"
         minOccurs="0"/>
     </xs:sequence>
   </xs:complexType>

   <!-- Extension types -->
   <xs:complexType name="ExtensionsType">
     <xs:sequence maxOccurs="unbounded">
       <xs:element name="Extension" type="dskpp:AbstractExtensionType"/>
     </xs:sequence>
   </xs:complexType>

   <xs:complexType name="AbstractExtensionType" abstract="true">
     <xs:attribute name="Critical" type="xs:boolean"/>
   </xs:complexType>

   <xs:complexType name="ClientInfoType">
     <xs:complexContent>
       <xs:extension base="dskpp:AbstractExtensionType">
         <xs:sequence>
           <xs:element name="Data"
             type="xs:base64Binary"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <xs:complexType name="ServerInfoType">
     <xs:complexContent>
       <xs:extension base="dskpp:AbstractExtensionType">
         <xs:sequence>
           <xs:element name="Data"
             type="xs:base64Binary"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <xs:complexType name="PayloadType">
     <xs:choice>
       <xs:element name="Nonce" type="dskpp:NonceType"/>
       <xs:any namespace="##other" processContents="strict"/>
     </xs:choice>
   </xs:complexType>

   <xs:complexType name="KeyInitializationDataType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         This extension is only valid in KeyProvServerFinished PDUs. It
         contains key initialization data and its presence results in a
         two-pass (or one-pass, if no KeyProvClientHello was sent) DSKPP
         exchange.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="dskpp:AbstractExtensionType">
         <xs:sequence>
           <xs:element name="KeyInitializationMethod" type="xs:anyURI"/>
           <xs:element name="Payload" type="dskpp:PayloadType"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <!-- DSKPP PDUs -->

   <xs:element name="KeyProvTrigger" type="dskpp:KeyProvTriggerType"/>

   <xs:complexType name="KeyProvTriggerType">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Message used to trigger the device to initiate a
         DSKPP protocol run.
       </xs:documentation>
     </xs:annotation>
     <xs:sequence>
       <xs:choice>
         <xs:element name="InitializationTrigger"
           type="dskpp:InitializationTriggerType"/>
         <xs:any namespace="##other" processContents="strict"/>
       </xs:choice>
     </xs:sequence>
     <xs:attribute name="Version" type="dskpp:VersionType"/>
   </xs:complexType>

   <!-- KeyProvClientHello PDU -->
   <xs:element name="KeyProvClientHello" type="dskpp:KeyProvClientHelloPDU"/>

   <xs:complexType name="KeyProvClientHelloPDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Message sent from DSKPP client to DSKPP server to initiate a
         DSKPP session.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="dskpp:AbstractRequestType">
         <xs:sequence>
           <xs:element name="DeviceIdentifierData"
             type="dskpp:DeviceIdentifierDataType" minOccurs="0"/>
           <xs:element name="KeyID" type="xs:base64Binary"
             minOccurs="0"/>
           <xs:element name="ClientNonce" type="dskpp:NonceType"
             minOccurs="0"/>
           <xs:element name="TriggerNonce" type="dskpp:NonceType"
             minOccurs="0"/>
           <xs:element name="SupportedKeyTypes"
             type="dskpp:AlgorithmsType"/>
           <xs:element name="SupportedEncryptionAlgorithms"
             type="dskpp:AlgorithmsType"/>
           <xs:element name="SupportedMacAlgorithms"
             type="dskpp:AlgorithmsType"/>
           <xs:element name="SupportedProtocolVariants"
             type="dskpp:ProtocolVariantsType" minOccurs="0"/>
           <xs:element name="SupportedKeyContainers"
             type="dskpp:KeyContainersFormatType" minOccurs="0"/>
           <xs:element name="AuthenticationData"
             type="dskpp:AuthenticationDataType" minOccurs="0"/>
           <xs:element name="Extensions" type="dskpp:ExtensionsType"
             minOccurs="0"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <!-- KeyProvServerHello PDU -->
   <xs:element name="KeyProvServerHello" type="dskpp:KeyProvServerHelloPDU"/>

   <xs:complexType name="KeyProvServerHelloPDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Message sent from DSKPP server to DSKPP client
         in response to a received KeyProvClientHello PDU.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="dskpp:AbstractResponseType">
         <xs:sequence minOccurs="0">
           <xs:element name="KeyType"
             type="dskpp:AlgorithmType"/>
           <xs:element name="EncryptionAlgorithm"
             type="dskpp:AlgorithmType"/>
           <xs:element name="MacAlgorithm"
             type="dskpp:AlgorithmType"/>
           <xs:element name="EncryptionKey"
             type="ds:KeyInfoType"/>
           <xs:element name="KeyContainerFormat"
             type="dskpp:KeyContainerFormatType"/>
           <xs:element name="Payload"
             type="dskpp:PayloadType"/>
           <xs:element name="Extensions"
             type="dskpp:ExtensionsType" minOccurs="0"/>
           <xs:element name="Mac" type="dskpp:MacType"
             minOccurs="0"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <!-- KeyProvClientNonce PDU -->
   <xs:element name="KeyProvClientNonce" type="dskpp:KeyProvClientNoncePDU"/>

   <xs:complexType name="KeyProvClientNoncePDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Second message sent from DSKPP client to
         DSKPP server in a DSKPP session.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="dskpp:AbstractRequestType">
         <xs:sequence>
           <xs:element name="EncryptedNonce"
             type="xs:base64Binary"/>
           <xs:element name="AuthenticationData"
             type="dskpp:AuthenticationDataType" minOccurs="0"/>
           <xs:element name="Extensions"
             type="dskpp:ExtensionsType" minOccurs="0"/>
         </xs:sequence>
         <xs:attribute name="SessionID" type="dskpp:IdentifierType"
           use="required"/>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

   <!-- KeyProvServerFinished PDU -->
   <xs:element name="KeyProvServerFinished" type="dskpp:KeyProvServerFinishedPDU"/>

   <xs:complexType name="KeyProvServerFinishedPDU">
     <xs:annotation>
       <xs:documentation xml:lang="en">
         Final message sent from DSKPP server to DSKPP client in a DSKPP
         session. A MAC value serves for key confirmation, and optional
         AuthenticationData servers for server authentication.
       </xs:documentation>
     </xs:annotation>
     <xs:complexContent>
       <xs:extension base="dskpp:AbstractResponseType">
         <xs:sequence minOccurs="0">
           <xs:element name="KeyContainer"
             type="dskpp:KeyContainerType"/>
           <xs:element name="Extensions"
             type="dskpp:ExtensionsType" minOccurs="0"/>
           <xs:element name="Mac"
             type="dskpp:MacType"/>
           <xs:element name="AuthenticationData"
             type="dskpp:AuthenticationDataType" minOccurs="0"/>
         </xs:sequence>
       </xs:extension>
     </xs:complexContent>
   </xs:complexType>

 </xs:schema>

14.  Security Considerations

14.1.  General

DSKPP is designed to protect generated key material from exposure. No other entities than the DSKPP server and the cryptographic module will have access to a generated K_TOKEN if the cryptographic algorithms used are of sufficient strength and, on the DSKPP client side, generation and encryption of R_C and generation of K_TOKEN take place as specified in the cryptographic module. This applies even if malicious software is present in the DSKPP client. However, as discussed in the following, DSKPP does not protect against certain other threats resulting from man-in-the-middle attacks and other forms of attacks. DSKPP SHOULD, therefore, be run over a transport providing privacy and integrity, such as HTTP over Transport Layer Security (TLS) with a suitable ciphersuite, when such threats are a concern. Note that TLS ciphersuites with anonymous key exchanges are not suitable in those situations.

14.2.  Active Attacks

14.2.1.  Introduction

An active attacker MAY attempt to modify, delete, insert, replay, or reorder messages for a variety of purposes including service denial and compromise of generated key material. Section 14.2.2 (Message Modifications) through Section 14.2.7 (Man-in-the-Middle).

14.2.2.  Message Modifications

Modifications to a <DSKPPTrigger> message will either cause denial-of-service (modifications of any of the identifiers or the nonce) or will cause the DSKPP client to contact the wrong DSKPP server. The latter is in effect a man-in-the-middle attack and is discussed further in Section 14.2.7 (Man-in-the-Middle).

An attacker may modify a <KeyProvClientHello> message. This means that the attacker could indicate a different key or device than the one intended by the DSKPP client, and could also suggest other cryptographic algorithms than the ones preferred by the DSKPP client, e.g., cryptographically weaker ones. The attacker could also suggest earlier versions of the DSKPP protocol, in case these versions have been shown to have vulnerabilities. These modifications could lead to an attacker succeeding in initializing or modifying another cryptographic module than the one intended (i.e., the server assigning the generated key to the wrong module), or gaining access to a generated key through the use of weak cryptographic algorithms or protocol versions. DSKPP implementations MAY protect against the latter by having strict policies about what versions and algorithms they support and accept. The former threat (assignment of a generated key to the wrong module) is not possible when the shared-key variant of DSKPP is employed (assuming existing shared keys are unique per cryptographic module), but is possible in the public-key variant. Therefore, DSKPP servers MUST NOT accept unilaterally provided device identifiers in the public-key variant. This is also indicated in the protocol description. In the shared-key variant, however, an attacker may be able to provide the wrong identifier (possibly also leading to the incorrect user being associated with the generated key) if the attacker has real-time access to the cryptographic module with the identified key. In other words, the generated key is associated with the correct cryptographic module but the module is associated with the incorrect user. See further Section 14.5 (Attacks on the Interaction between DSKPP and User Authentication) for a discussion of this threat and possible countermeasures.

An attacker may also modify a <KeyProvServerHello> message. This means that the attacker could indicate different key types, algorithms, or protocol versions than the legitimate server would, e.g., cryptographically weaker ones. The attacker may also provide a different nonce than the one sent by the legitimate server. Clients MAY protect against the former through strict adherence to policies regarding permissible algorithms and protocol versions. The latter (wrong nonce) will not constitute a security problem, as a generated key will not match the key generated on the legitimate server. Also, whenever the DSKPP run would result in the replacement of an existing key, the <Mac> element protects against modifications of R_S.

Modifications of <KeyProvClientNonce> messages are also possible. If an attacker modifies the SessionID attribute, then, in effect, a switch to another session will occur at the server, assuming the new SessionID is valid at that time on the server. It still will not allow the attacker to learn a generated K_TOKEN since R_C has been wrapped for the legitimate server. Modifications of the <EncryptedNonce> element, e.g., replacing it with a value for which the attacker knows an underlying R'C, will not result in the client changing its pre-DSKPP state, since the server will be unable to provide a valid MAC in its final message to the client. The server MAY, however, end up storing K'TOKEN rather than K_TOKEN. If the cryptographic module has been associated with a particular user, then this could constitute a security problem. For a further discussion about this threat, and a possible countermeasure, see Section 14.5 (Attacks on the Interaction between DSKPP and User Authentication) below. Note that use of Secure Socket Layer (SSL) or TLS does not protect against this attack if the attacker has access to the DSKPP client (e.g., through malicious software, "trojans").

Finally, attackers may also modify the <KeyProvServerFinished> message. Replacing the <Mac> element will only result in denial-of-service. Replacement of any other element may cause the DSKPP client to associate, e.g., the wrong service with the generated key. DSKPP SHOULD be run over a transport providing privacy and integrity when this is a concern.

14.2.3.  Message Deletion

Message deletion will not cause any other harm than denial-of-service, since a cryptographic module MUST NOT change its state (i.e., "commit" to a generated key) until it receives the final message from the DSKPP server and successfully has processed that message, including validation of its MAC. A deleted <KeyProvServerFinished> message will not cause the server to end up in an inconsistent state vis-a-vis the cryptographic module if the server implements the suggestions in Section 14.5 (Attacks on the Interaction between DSKPP and User Authentication).

14.2.4.  Message Insertion

An active attacker may initiate a DSKPP run at any time, and suggest any device identifier. DSKPP server implementations MAY receive some protection against inadvertently initializing a key or inadvertently replacing an existing key or assigning a key to a cryptographic module by initializing the DSKPP run by use of the <KeyProvTrigger>. The <TriggerNonce> element allows the server to associate a DSKPP protocol run with, e.g., an earlier user-authenticated session. The security of this method, therefore, depends on the ability to protect the <TriggerNonce> element in the DSKPP initialization message. If an eavesdropper is able to capture this message, he may race the legitimate user for a key initialization. DSKPP over a transport providing privacy and integrity, coupled with the recommendations in Section 14.5 (Attacks on the Interaction between DSKPP and User Authentication), is RECOMMENDED when this is a concern.

Insertion of other messages into an existing protocol run is seen as equivalent to modification of legitimately sent messages.

14.2.5.  Message Replay

During 4-pass DSKPP, attempts to replay a previously recorded DSKPP message will be detected, as the use of nonces ensures that both parties are live. For example, a DSKPP client knows that a server it is communicating with is "live" since the server MUST create a MAC on information sent by the client.

The same is true for 2-pass DSKPP thanks to the requirement that the client sends R in the <KeyProvClientHello> message and that the server includes R in the MAC computation.

In 1-pass DSKPP clients that record the latest I used by a particular server (as identified by ID_S) will be able to detect replays.

14.2.6.  Message Reordering

An attacker may attempt to re-order 4-pass DSKPP messages but this will be detected, as each message is of a unique type. Note: Message re-ordering attacks cannot occur in 2- and 1-pass DSKPP since each party sends at most one message each.

14.2.7.  Man-in-the-Middle

In addition to other active attacks, an attacker posing as a man in the middle may be able to provide his own public key to the DSKPP client. This threat and countermeasures to it are discussed in Section 4.3 (Four-Pass Protocol Usage). An attacker posing as a man-in-the-middle may also be acting as a proxy and, hence, may not interfere with DSKPP runs but still learn valuable information; see Section 14.3 (Passive Attacks).

14.3.  Passive Attacks

Passive attackers may eavesdrop on DSKPP runs to learn information that later on may be used to impersonate users, mount active attacks, etc.

If DSKPP is not run over a transport providing privacy, a passive attacker may learn:

• What cryptographic modules a particular user is in possession of;
• The identifiers of keys on those cryptographic modules and other attributes pertaining to those keys, e.g., the lifetime of the keys; and
• DSKPP versions and cryptographic algorithms supported by a particular DSKPP client or server.

Whenever the above is a concern, DSKPP SHOULD be run over a transport providing privacy. If man-in-the-middle attacks for the purposes described above are a concern, the transport SHOULD also offer server-side authentication.

14.4.  Cryptographic Attacks

An attacker with unlimited access to an initialized cryptographic module may use the module as an "oracle" to pre-compute values that later on may be used to impersonate the DSKPP server. Section 5.2 (Encryption of Pseudorandom Nonces Sent from the DSKPP Client (Applicable to Four-Pass and Two-Pass DSKPP)) and Section 4 (DSKPP Overview) contain discussions of this threat and steps RECOMMENDED to protect against it.

14.5.  Attacks on the Interaction between DSKPP and User Authentication

If keys generated in DSKPP will be associated with a particular user at the DSKPP server (or a server trusted by, and communicating with the DSKPP server), then in order to protect against threats where an attacker replaces a client-provided encrypted R_C with his own R'C (regardless of whether the public-key variant or the shared-secret variant of DSKPP is employed to encrypt the client nonce), the server SHOULD not commit to associate a generated K_TOKEN with the given cryptographic module until the user simultaneously has proven both possession of the device that hosts the cryptographic module containing K_TOKEN and some out-of-band provided authenticating information (e.g., a temporary password). For example, if the cryptographic module is a one-time password token, the user could be required to authenticate with both a one-time password generated by the cryptographic module and an out-of-band provided temporary PIN in order to have the server "commit" to the generated OTP value for the given user. Preferably, the user SHOULD perform this operation from another host than the one used to initialize keys on the cryptographic module, in order to minimize the risk of malicious software on the client interfering with the process.

Note: This scenario, wherein the attacker replaces a client-provided R_C with his own R'C, does not apply to 2- and 1-pass DSKPP as the client does not provide any entropy to K_TOKEN. The attack as such (and its countermeasures) still applies to 2- and 1-pass DSKPP, however, as it essentially is a man-in-the-middle attack.

Another threat arises when an attacker is able to trick a user to authenticate to the attacker rather than to the legitimate service before the DSKPP protocol run. If successful, the attacker will then be able to impersonate the user towards the legitimate service, and subsequently receive a valid DSKPP trigger. If the public-key variant of DSKPP is used, this may result in the attacker being able to (after a successful DSKPP protocol run) impersonate the user. Ordinary precautions MUST, therefore, be in place to ensure that users authenticate only to legitimate services.

14.6.  Additional Considerations Specific to 2- and 1-pass DSKPP

14.6.1.  Client Contributions to K_TOKEN Entropy

In 4-pass DSKPP, both the client and the server provide randomizing material to K_TOKEN , in a manner that allows both parties to verify that they did contribute to the resulting key. In the 1- and 2-pass DSKPP versions defined herein, only the server contributes to the entropy of K_TOKEN. This means that a broken or compromised (pseudo-)random number generator in the server may cause more damage than it would in the 4-pass variant. Server implementations SHOULD therefore take extreme care to ensure that this situation does not occur.

14.6.2.  Key Confirmation

4-pass DSKPP servers provide key confirmation through the MAC on R_C in the <KeyProvServerFinished> message. In the 1- and 2-pass DSKPP variants described herein, key confirmation is provided by the MAC including I (in the 1-pass case) or R (2-pass case), using K_MAC.

14.6.3.  Server Authentication

DSKPP servers MUST authenticate themselves whenever a successful DSKPP 1- or 2-pass protocol run would result in an existing K_TOKEN being replaced by a K_TOKEN', or else a denial-of-service attack where an unauthorized DSKPP server replaces a K_TOKEN with another key would be possible. In 1- and 2-pass DSKPP, servers authenticate by including the AuthenticationDataType extension containing a MAC as described in Section 4.4 (Two-Pass Protocol Usage) for Two-Pass DSKPP and Section 4.5 (One-Pass Protocol Usage) for One-Pass DSKPP.

14.6.4.  Client Authentication

A DSKPP server MUST authenticate a client to ensure that K_TOKEN is delivered to the intended device. The following measures SHOULD be considered:

• When a device certificate is used for client authentication, the DSKPP server SHOULD follow standard certificate verification processes to ensure that it is a trusted device.
• When an Authentication Code is used for client authentication, a password dictionary attack on the authentication data is possible.
• The length of the Authentication Code when used over a non-secure channel SHOULD be longer than what is used over a secure channel. When a device, e.g., some mobile phones with small screens, cannot handle a long Authentication Code in a user-friendly manner, DSKPP SHOULD rely on a secure channel for communication.
• In the case that a non-secure channel has to be used, the Authentication Code SHOULD be sent to the server MAC's as specified in Section 5.3 (Client Authentication Mechanisms (Applicable to Four- and Two-Pass DSKPP) ). The Authentication Code and nonce value MUST be strong enough to prevent offline brute-force recovery of the Authentication Code from the HMAC data. Given that the nonce value is sent in plaintext format over a non-secure transport, the cryptographic strength of the AuthenticationData depends more on the quality of the AuthenticationCode.
• When the AuthenticationCode is sent from the DSKPP server to the device in a DSKPP initialization trigger message, an eavesdropper may be able to capture this message and race the legitimate user for a key initialization. To prevent this, the transport layer used to send the DSKPP trigger MUST provide privacy and integrity e.g. secure browser session.

14.6.5.  Key Protection in the Passphrase Profile

The passphrase-based key wrap profile uses the PBKDF2 function from [PKCS‑5] (RSA Laboratories, “Password-Based Cryptography Standard,” March 1999.) to generate an encryption key from a passphrase and salt string. The derived key, K_DERIVED is used by the server to encrypt K_TOKEN and by the cryptographic module to decrypt the newly delivered K_TOKEN. It is important to note that passphrase-based encryption is generally limited in the security that it provides despite the use of salt and iteration count in PBKDF2 to increase the complexity of attack. Implementations SHOULD therefore take additional measures to strengthen the security of the passphrase-based key wrap profile. The following measures SHOULD be considered where applicable:

• The passphrase SHOULD be selected well, and usage guidelines such as the ones in [NIST‑PWD] (National Institute of Standards and Technology, “Password Usage,” May 1985.) SHOULD be taken into account.
• A different passphrase SHOULD be used for every key initialization wherever possible (the use of a global passphrase for a batch of cryptographic modules SHOULD be avoided, for example). One way to achieve this is to use randomly-generated passphrases.
• The passphrase SHOULD be protected well if stored on the server and/or on the cryptographic module and SHOULD be delivered to the device's user using secure methods.
• User pre-authentication SHOULD be implemented to ensure that K_TOKEN is not delivered to a rogue recipient.
• The iteration count in PBKDF2 SHOULD be high to impose more work for an attacker using brute-force methods (see [PKCS‑5] (RSA Laboratories, “Password-Based Cryptography Standard,” March 1999.) for recommendations). However, it MUST be noted that the higher the count, the more work is required on the legitimate cryptographic module to decrypt the newly delivered K_TOKEN. Servers MAY use relatively low iteration counts to accommodate devices with limited processing power such as some PDA and cell phones when other security measures are implemented and the security of the passphrase-based key wrap method is not weakened.
• Transport level security (e.g. TLS) SHOULD be used where possible to protect a 2-pass or 1-pass protocol run. Transport level security provides a second layer of protection for the newly generated K_TOKEN.

15.  Internationalization Considerations

The DSKPP protocol is mostly meant for machine-to-machine communications; as such, most of its elements are tokens not meant for direct human consumption. If these tokens are presented to the end user, some localization may need to occur. DSKPP exchanges information using XML. All XML processors are required to understand UTF-8 and UTF-16 encoding, and therefore all DSKPP clients and servers MUST understand UTF-8 and UTF-16 encoded XML. Additionally, DSKPP servers and clients MUST NOT encode XML with encodings other than UTF-8 or UTF-16.

16.  IANA Considerations

This document calls for registration of new URNs within the IETF sub-namespace per RFC3553 [RFC3553] (Mealling, M., Masinter, L., Hardie, T., and G. Klyne, “An IETF URN Sub-namespace for Registered Protocol Parameters,” June 2003.). The following URNs are RECOMMENDED:

• DSKPP XML schema: "urn:ietf:params:xml:schema:keyprov:protocol"
• DSKPP XML namespace: "urn:ietf:params:xml:ns:keyprov:protocol"

17.  Intellectual Property Considerations

RSA and RSA Security are registered trademarks or trademarks of RSA Security Inc. in the United States and/or other countries. The names of other products and services mentioned may be the trademarks of their respective owners.

18.  Contributors

This work is based on information contained in [RFC4758] (RSA, The Security Division of EMC, “Cryptographic Token Key Initialization Protocol (CT-KIP),” November 2006.), authored by Magnus Nystrom, with enhancements (esp. Client Authentication, and support for multiple key container formats) from an individual Internet-Draft co-authored by Mingliang Pei and Salah Machani.

We would like to thank Shuh Chang for contributing the DSKPP object model, and Philip Hoyer for his work in aligning DSKPP and PSKC schemas.

We would also like to thank Hannes Tschofenig for his draft reviews, feedback, and text contributions.

19.  Acknowledgements

We would like to thank the following for detailed review of previous DSKPP document versions:

• Dr. Ulrike Meyer (Review June 2007)
  
  
• Niklas Neumann (Review June 2007)
  
  
• Shuh Chang (Review June 2007)
  
  
• Hannes Tschofenig (Review June 2007 and again in August 2007)
  
  
• Sean Turner (Review August 2007)
  
  
• John Linn (Review August 2007)
  
  
• Philip Hoyer (Review September 2007)

We would also like to thank the following for their input to selected design aspects of the DSKPP protocol:

• Anders Rundgren (Key Container Format and Client Authentication Data)
  
  
• Hannes Tschofenig (HTTP Binding)
  
  
• Phillip Hallam-Baker (Registry for Algorithms)

Finally, we would like to thank Robert Griffin for opening communication channels for us with the IEEE P1619.3 Key Management Group, and facilitating our groups in staying informed of potential areas (esp. key provisioning and global key identifiers of collaboration) of collaboration.

20.  References

20.1. Normative references

20.2. Informative references

Appendix A.  Integration with PKCS #11

A DSKPP client that needs to communicate with a connected cryptographic module to perform a DSKPP exchange MAY use PKCS #11 [PKCS‑11] (RSA Laboratories, “Cryptographic Token Interface Standard,” June 2004.)as a programming interface.

A.1.  The 4-pass Variant

When performing 4-pass DSKPP with a cryptographic module using the PKCS #11 programming interface, the procedure described in [CT‑KIP‑P11] (RSA Laboratories, “PKCS #11 Mechanisms for the Cryptographic Token Key Initialization Protocol,” December 2005.), Appendix B, is RECOMMENDED.

A.2.  The 2-pass Variant

A suggested procedure to perform 2-pass DSKPP with a cryptographic module through the PKCS #11 interface using the mechanisms defined in [CT‑KIP‑P11] (RSA Laboratories, “PKCS #11 Mechanisms for the Cryptographic Token Key Initialization Protocol,” December 2005.) is as follows:

a.

On the client side,

1.

The client selects a suitable slot and token (e.g. through use of the <DeviceIdentifier> or the <PlatformInfo> element of the DSKPP trigger message).

2.

A nonce R is generated, e.g. by calling C_SeedRandom and C_GenerateRandom.

3.

The client sends its first message to the server, including the nonce R.

b.

On the server side,

1.

A generic key K = K_TOKEN | K _MAC (where '|' denotes concatenation) is generated, e.g. by calling C_GenerateKey (using key type CKK_GENERIC_SECRET). The template for K MUST allow it to be exported (but only in wrapped form, i.e. CKA_SENSITIVE MUST be set to CK_TRUE and CKA_EXTRACTABLE MUST also be set to CK_TRUE), and also to be used for further key derivation. From K, a token key K_TOKEN of suitable type is derived by calling C_DeriveKey using the PKCS #11 mechanism CKM_EXTRACT_KEY_FROM_KEY and setting the CK_EXTRACT_PARAMS to the first bit of the generic secret key (i.e. set to 0). Likewise, a MAC key K_MAC is derived from K by calling C_DeriveKey using the CKM_EXTRACT_KEY_FROM_KEY mechanism, this time setting CK_EXTRACT_PARAMS to the length of K (in bits) divided by two.

2.

The server wraps K with either the token's public key K_CLIENT, the shared secret key K_SHARED, or the derived shared secret key K_DERIVED by using C_WrapKey. If use of the DSKPP key wrap algorithm has been negotiated then the CKM_KIP_WRAP mechanism MUST be used to wrap K. When calling C_WrapKey, the hKey handle in the CK_KIP_PARAMS structure MUST be set to NULL_PTR. The pSeed parameter in the CK_KIP_PARAMS structure MUST point to the nonce R provided by the DSKPP client, and the ulSeedLen parameter MUST indicate the length of R. The hWrappingKey parameter in the call to C_WrapKey MUST be set to refer to the wrapping key.

3.

Next, the server needs to calculate a MAC using K_MAC. If use of the DSKPP MAC algorithm has been negotiated, then the MAC is calculated by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In the call to C_SignInit, K_MAC MUST be the signature key, the hKey parameter in the CK_KIP_PARAMS structure MUST be set to NULL_PTR, the pSeed parameter of the CT_KIP_PARAMS structure MUST be set to NULL_PTR, and the ulSeedLen parameter MUST be set to zero. In the call to C_Sign, the pData parameter MUST be set to the concatenation of the string ID_S and the nonce R, and the ulDataLen parameter MUST be set to the length of the concatenated string. The desired length of the MAC MUST be specified through the pulSignatureLen parameter and MUST be set to the length of R.

4.

If the server also needs to authenticate its message (due to an existing K_TOKEN being replaced), the server MUST calculate a second MAC. Again, if use of the DSKPP MAC algorithm has been negotiated, then the MAC is calculated by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In this call to C_SignInit, the K_MAC existing before this DSKPP protocol run MUST be the signature key, the hKey parameter in the CK_KIP_PARAMS structure MUST be set to NULL, the pSeed parameter of the CT_KIP_PARAMS structure MUST be set to NULL_PTR, and the ulSeeidLen parameter MUST be set to zero. In the call to C_Sign, the pData parameter MUST be set to the concatenation of the string ID_S and the nonce R, and the ulDataLen parameter MUST be set to the length of concatenated string. The desired length of the MAC MUST be specified through the pulSignatureLen parameter and MUST be set to the length of R.

5.

The server sends its message to the client, including the wrapped key K, the MAC and possibly also the authenticating MAC.

c.

On the client side,

1.

The client calls C_UnwrapKey to receive a handle to K. After this, the client calls C_DeriveKey twice: Once to derive K_TOKEN and once to derive K_MAC. The client MUST use the same mechanism (CKM_EXTRACT_KEY_FROM_KEY) and the same mechanism parameters as used by the server above. When calling C_UnwrapKey and C_DeriveKey, the pTemplate parameter MUST be used to set additional key attributes in accordance with local policy and as negotiated and expressed in the protocol. In particular, the value of the <KeyID> element in the server's response message MAY be used as CKA_ID for K_TOKEN. The key K MUST be destroyed after deriving K_TOKEN and K_MAC.

2.

The MAC is verified in a reciprocal fashion as it was generated by the server. If use of the CKM_KIP_MAC mechanism has been negotiated, then in the call to C_VerifyInit, the hKey parameter in the CK_KIP_PARAMS structure MUST be set to NULL_PTR, the pSeed parameter MUST be set to NULL_PTR, and ulSeedLen MUST be set to 0. The hKey parameter of C_VerifyInit MUST refer to K_MAC. In the call to C_Verify, pData MUST be set to the concatenation of the string ID_S and the nonce R, and the ulDataLen parameter MUST be set to the length of the concatenated string, pSignature to the MAC value received from the server, and ulSignatureLen to the length of the MAC. If the MAC does not verify the protocol session ends with a failure. The token MUST be constructed to not "commit" to the new K_TOKEN or the new K_MAC unless the MAC verifies.

3.

If an authenticating MAC was received (REQUIRED if the new K_TOKEN will replace an existing key on the token), then it is verified in a similar vein but using the K_MAC associated with this server and existing before the protocol run. Again, if the MAC does not verify the protocol session ends with a failure, and the token MUST be constructed no to "commit" to the new K_TOKEN or the new K_MAC unless the MAC verifies.

A.3.  The 1-pass Variant

A suggested procedure to perform 1-pass DSKPP with a cryptographic module through the PKCS #11 interface using the mechanisms defined in [CT‑KIP‑P11] (RSA Laboratories, “PKCS #11 Mechanisms for the Cryptographic Token Key Initialization Protocol,” December 2005.) is as follows:

a.

On the server side,

1.

A generic key K = K_TOKEN | K _MAC (where '|' denotes concatenation) is generated, e.g. by calling C_GenerateKey (using key type CKK_GENERIC_SECRET). The template for K MUST allow it to be exported (but only in wrapped form, i.e. CKA_SENSITIVE MUST be set to CK_TRUE and CKA_EXTRACTABLE MUST also be set to CK_TRUE), and also to be used for further key derivation. From K, a token key K_TOKEN of suitable type is derived by calling C_DeriveKey using the PKCS #11 mechanism CKM_EXTRACT_KEY_FROM_KEY and setting the CK_EXTRACT_PARAMS to the first bit of the generic secret key (i.e. set to 0). Likewise, a MAC key K_MAC is derived from K by calling C_DeriveKey using the CKM_EXTRACT_KEY_FROM_KEY mechanism, this time setting CK_EXTRACT_PARAMS to the length of K (in bits) divided by two.

2.

The server wraps K with either the token's public key, K_CLIENT, the shared secret key, K_SHARED, or the derived shared secret key, K_DERIVED by using C_WrapKey. If use of the DSKPP key wrap algorithm has been negotiated, then the CKM_KIP_WRAP mechanism MUST be used to wrap K. When calling C_WrapKey, the hKey handle in the CK_KIP_PARAMS structure MUST be set to NULL_PTR. The pSeed parameter in the CK_KIP_PARAMS structure MUST point to the octet-string representation of an integer I whose value MUST be incremented before each protocol run, and the ulSeedLen parameter MUST indicate the length of the octet-string representation of I. The hWrappingKey parameter in the call to C_WrapKey MUST be set to refer to the wrapping key.
Note: The integer-to-octet string conversion MUST be made using the I2OSP primitive from [PKCS‑1] (RSA Laboratories, “RSA Cryptography Standard,” June 2002.). There MUST be no leading zeros.

3.

For the server's message to the client, if use of the DSKPP MAC algorithm has been negotiated, then the MAC is calculated by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In the call to C_SignInit, K_MAC MUST be the signature key, the hKey parameter in the CK_KIP_PARAMS structure MUST be set to NULL_PTR, the pSeed parameter of the CT_KIP_PARAMS structure MUST be set to NULL_PTR, and the ulSeedLen parameter MUST be set to zero. In the call to C_Sign, the pData parameter MUST be set to the concatenation of the string ID_S and the octet-string representation of the integer I, and the ulDataLen parameter MUST be set to the length of concatenated string. The desired length of the MAC MUST be specified through the pulSignatureLen parameter as usual, and MUST be equal to, or greater than, sixteen (16).

4.

If the server also needs to authenticate its message (due to an existing K_TOKEN being replaced), the server calculates a second MAC. If the DSKPP MAC mechanism is used, the server does this by calling C_SignInit with the CKM_KIP_MAC mechanism followed by a call to C_Sign. In the call to C_SignInit, the K_MAC existing on the token before this protocol run MUST be the signature key, the hKey parameter in the CK_KIP_PARAMS structure MUST be set to NULL_PTR, the pSeed parameter of the CT_KIP_PARAMS structure MUST be set to NULL_PTR, and the ulSeedLen parameter MUST be set to zero. In the call to C_Sign, the pData parameter MUST be set to the concatenation of the string ID_S and the octet-string representation of the integer I+1 (i.e. I MUST be incremented before each use), and the ulDataLen parameter MUST be set to the length of the concatenated string. The desired length of the MAC MUST be specified through the pulSignatureLen parameter as usual, and MUST be equal to, or greater than, sixteen (16).

5.

The server sends its message to the client, including the MAC and possibly also the authenticating MAC.

b.

On the client side,

1.

The client calls C_UnwrapKey to receive a handle to K. After this, the client calls C_DeriveKey twice: Once to derive K_TOKEN and once to derive K_MAC. The client MUST use the same mechanism (CKM_EXTRACT_KEY_FROM_KEY) and the same mechanism parameters as used by the server above. When calling C_UnwrapKey and C_DeriveKey, the pTemplate parameter MUST be used to set additional key attributes in accordance with local policy and as negotiated and expressed in the protocol. In particular, the value of the <KeyID> element in the server's response message MAY be used as CKA_ID for K_TOKEN. The key K MUST be destroyed after deriving K_TOKEN and K_MAC.

2.

The MAC is verified in a reciprocal fashion as it was generated by the server. If use of the CKM_KIP_MAC mechanism has been negotiated, then in the call to C_VerifyInit, the hKey parameter in the CK_KIP_PARAMS structure MUST be set to NULL_PTR, the pSeed parameter MUST be set to NULL_PTR, and ulSeedLen MUST be set to 0. The hKey parameter of C_VerifyInit MUST refer to K_MAC. In the call to C_Verify, pData MUST be set to the concatenation of the string ID_S and the octet-string representation of the provided value for I, and the ulDataLen parameter MUST be set to the length of the concatenated string, pSignature to the MAC value received from the server, and ulSignatureLen to the length of the MAC. If the MAC does not verify or if the provided value of I is not larger than any stored value I' for the identified server ID_S the protocol session ends with a failure. The token MUST be constructed to not "commit" to the new K_TOKEN or the new K_MAC unless the MAC verifies. If the verification succeeds, the token MUST store the provided value of I as a new I' for ID_S.

3.

If an authenticating MAC was received (REQUIRED if K_TOKEN will replace an existing key on the token), it is verified in a similar vein but using the K_MAC existing before the protocol run. Again, if the MAC does not verify the protocol session ends with a failure, and the token MUST be constructed no to "commit" to the new K_TOKEN or the new K_MAC unless the MAC verifies.

Appendix B.  Example of DSKPP-PRF Realizations

B.1.  Introduction

This example appendix defines DSKPP-PRF in terms of AES [FIPS197‑AES] (National Institute of Standards and Technology, “Specification for the Advanced Encryption Standard (AES),” November 2001.) and HMAC [RFC2104] (Krawzcyk, H., Bellare, M., and R. Canetti, “HMAC: Keyed-Hashing for Message Authentication,” February 1997.).

B.2.  DSKPP-PRF-AES

B.2.1.  Identification

For cryptographic modules supporting this realization of DSKPP-PRF, the following URI MAY be used to identify this algorithm in DSKPP:

urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-aes

When this URI is used to identify the encryption algorithm to use, the method for encryption of R_C values described in Section 5.2 (Encryption of Pseudorandom Nonces Sent from the DSKPP Client (Applicable to Four-Pass and Two-Pass DSKPP)) MUST be used.

B.2.2.  Definition

DSKPP-PRF-AES (k, s, dsLen)

Input:

k

Encryption key to use

s

Octet string consisting of randomizing material. The length of the string s is sLen.

dsLen

Desired length of the output

Output:

DS

A pseudorandom string, dsLen-octets long

Steps:

1.

Let bLen be the output block size of AES in octets:
bLen = (AES output block length in octets)
(normally, bLen = 16)

2.

If dsLen > (2**32 - 1) * bLen, output "derived data too long" and stop

3.

Let n be the number of bLen-octet blocks in the output data, rounding up, and let j be the number of octets in the last block:
n = ROUND( dsLen / bLen)
j = dsLen - (n - 1) * bLen

4.

For each block of the pseudorandom string DS, apply the function F defined below to the key k, the string s and the block index to compute the block:
B1 = F (k, s, 1) ,
B2 = F (k, s, 2) ,
...
Bn = F (k, s, n)

The function F is defined in terms of the OMAC1 construction from [FSE2003] (Iwata, T. and K. Kurosawa, “OMAC: One-Key CBC MAC. In Fast Software Encryption,” 2003.), using AES as the block cipher:

F (k, s, i) = OMAC1-AES (k, INT (i) || s)

where INT (i) is a four-octet encoding of the integer i, most significant octet first, and the output length of OMAC1 is set to bLen.

Concatenate the blocks and extract the first dsLen octets to product the desired data string DS:

DS = B1 || B2 || ... || Bn<0..j-1>

Output the derived data DS.

B.2.3.  Example

If we assume that dsLen = 16, then:

n = 16 / 16 = 1

j = 16 - (1 - 1) * 16 = 16

DS = B1 = F (k, s, 1) = OMAC1-AES (k, INT (1) || s)

B.3.  DSKPP-PRF-SHA256

B.3.1.  Identification

For cryptographic modules supporting this realization of DSKPP-PRF, the following URI MAY be used to identify this algorithm in DSKPP:

urn:ietf:params:xml:schema:keyprov:protocol#dskpp-prf-sha256

When this URI is used to identify the encryption algorithm to use, the method for encryption of R_C values described in Section 5.2 (Encryption of Pseudorandom Nonces Sent from the DSKPP Client (Applicable to Four-Pass and Two-Pass DSKPP)) MUST be used.

B.3.2.  Definition

DSKPP-PRF-SHA256 (k, s, dsLen)

Input:

k

Encryption key to use

s

Octet string consisting of randomizing material. The length of the string s is sLen.

dsLen

Desired length of the output

Output:

DS

A pseudorandom string, dsLen-octets long

Steps:

1.

Let bLen be the output size of SHA-256 in octets of [FIPS180‑SHA] (National Institute of Standards and Technology, “Secure Hash Standard,” February 2004.) (no truncation is done on the HMAC output):
bLen = 32
(normally, bLen = 16)

2.

If dsLen > (2**32 - 1) * bLen, output "derived data too long" and stop

3.

Let n be the number of bLen-octet blocks in the output data, rounding up, and let j be the number of octets in the last block:
n = ROUND( dsLen / bLen)
j = dsLen - (n - 1) * bLen

4.

For each block of the pseudorandom string DS, apply the function F defined below to the key k, the string s and the block index to compute the block:
B1 = F (k, s, 1) ,
B2 = F (k, s, 2) ,
...
Bn = F (k, s, n)

The function F is defined in terms of the HMAC construction from [RFC2104] (Krawzcyk, H., Bellare, M., and R. Canetti, “HMAC: Keyed-Hashing for Message Authentication,” February 1997.), using SHA-256 as the digest algorithm:

F (k, s, i) = HMAC-SHA256 (k, INT (i) || s)

where INT (i) is a four-octet encoding of the integer i, most significant octet first, and the output length of HMAC is set to bLen.

Concatenate the blocks and extract the first dsLen octets to product the desired data string DS:

DS = B1 || B2 || ... || Bn<0..j-1>

Output the derived data DS.

B.3.3.  Example

If we assume that sLen = 256 (two 128-octet long values) and dsLen = 16, then:

n = ROUND ( 16 / 32 ) = 1

j = 16 - (1 - 1) * 32 = 16

B1 = F (k, s, 1) = HMAC-SHA256 (k, INT (1) || s)

DS = B1<0 ... 15>

That is, the result will be the first 16 octets of the HMAC output.

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