Voucher Profile for Bootstrapping Protocols
draft-ietf-anima-voucher-03

Abstract

This document defines a strategy to securely assign a pledge to an owner, using an artifact signed, directly or indirectly, by the pledge's manufacturer. This artifact is known as a "voucher".

The voucher artifact is a YANG-defined JSON document that has been signed using a PKCS#7 structure. The voucher artifact is generated by the pledge's manufacture or delegate (i.e. the MASA).

This document only defines the voucher artifact, leaving it to other documents to describe specialized protocols for accessing it.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on December 9, 2017.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. Terminology
• 3. Requirements Language
• 4. Survey of Voucher Types
• 5. Voucher
• 5.1. Tree Diagram
• 5.2. Examples
• 5.3. YANG Module
• 6. Design Considerations
• 6.1. Renewals instead of Revocations
• 6.2. Voucher Per Pledge
• 7. Security Considerations
• 7.1. Clock Sensitivity
• 7.2. Protect Voucher PKI in HSM
• 7.3. Test Domain Certificate Validity when Signing
• 8. IANA Considerations
• 8.1. The IETF XML Registry
• 8.2. The YANG Module Names Registry
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Acknowledgements
• Authors' Addresses

1. Introduction

This document defines a strategy to securely assign a pledge to an owner, using an artifact signed, directly or indirectly, by the pledge's manufacturer or delegate (i.e. the MASA). This artifact is known as the voucher.

The voucher artifact is a JSON document, conforming to a data model described by YANG [RFC7950], that has been signed using a PKCS#7 structure.

A voucher may be useful in several contexts, but the driving motivation herein is to support secure bootstrapping mechanisms. Assigning ownership is important to bootstrapping mechanisms so that the pledge can authenticate the network that's trying to take control of it.

The lifetimes of vouchers may vary. In some bootstrapping protocols the vouchers may be ephemeral, whereas in others the vouchers may be potentially long-lived. In order to support the second category of vouchers, this document recommends using short-life vouchers with programatic renewal, enabling the MASA to communicate the ongoing validity of vouchers.

This document only defines the voucher artifact, leaving it to other documents to describe specialized protocols for accessing it. Some bootstrapping protocols using the voucher artifact defined in this draft include: [I-D.ietf-netconf-zerotouch], [I-D.ietf-6tisch-dtsecurity-secure-join], and [I-D.ietf-anima-bootstrapping-keyinfra]).

2. Terminology

Imprint:

The process where a device obtains the cryptographic key material to identify and trust future interactions with a network. This term is taken from Konrad Lorenz's work in biology with new ducklings: "during a critical period, the duckling would assume that anything that looks like a mother duck is in fact their mother." An equivalent for a device is to obtain the fingerprint of the network's root certification authority certificate. A device that imprints on an attacker suffers a similar fate to a duckling that imprints on a hungry wolf. Securely imprinting is a primary focus of this document.[imprinting]. The analogy to Lorenz's work was first noted in [Stajano99theresurrecting].

Domain:

The set of entities or infrastructure under common administrative control. The goal of the bootstrapping protocol is to enable a Pledge to discover and join a Domain.

Join Registrar (and Coordinator):

A representative of the domain that is configured, perhaps autonomically, to decide whether a new device is allowed to join the domain. The administrator of the domain interfaces with a Join Registrar (and Coordinator) to control this process. i Typically a Join Registrar is "inside" its domain. For simplicity this document often refers to this as just "Registrar".

MASA:

The Manufacturer Authorized Signing Authority (MASA) service that signs vouchers. In some bootstrapping protocols, the MASA may have Internet presence and be integral to the bootstrapping process, whereas in other protocols the MASA may be an offline service that has no active role in the bootstrapping process.

Pledge:

The prospective device attempting to find and securely join a domain. When shipped it only trusts authorized representatives of the manufacturer.

Registrar

See Join Registrar

TOFU:

Trust on First Use. This is where a Pledge device makes no security decisions but rather simply trusts the first Domain entity it is contacted by. Used similarly to [RFC7435]. This is also known as the "resurrecting duckling" model.

Voucher:

A signed statement from the MASA service that indicates to a Pledge the cryptographic identity of the Domain it should trust.

3. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the sections below are to be interpreted as described in RFC 2119 [RFC2119].

4. Survey of Voucher Types

A voucher is a cryptographically protected statement to the Pledge device authorizing a zero-touch "imprint" on the Join Registrar of the domain. The specific information a voucher provides is influenced by the bootstrapping use case.

The voucher can impart the following information to the Join Registrar and Pledge:

Assertion Basis:

Indicates the method that protects the imprint (this is distinct from the voucher signature that protects the voucher itself). This might include manufacturer asserted ownership verification, assured logging operations or reliance on Pledge endpoint behavior such as secure root of trust of measurement. The Join Registrar might use this information. Only some methods are normatively defined in this document. Other methods are left for future work.

Authentication of Join Registrar:

Indicates how the Pledge can authenticate the Join Registrar. This might include an indication of the private PKIX trust anchor used by the Registrar, or an indication of a public PKIX trust anchor and additional CN-ID or DNS-ID information to complete authentication. Symmetric key or other methods are left for future work.

Anti-Replay Protections:

Time or nonce based information to constrain the voucher to time periods or bootstrap attempts.

A number of bootstrapping scenarios can be met using differing combinations of this information. All scenarios address the primary threat of a Man-in-The-Middle Registrar gaining control over the Pledge device. The following combinations are "types" of vouchers:

             |Assertion   |Registrar ID    | Validity    |
Voucher      |Log-|Veri-  |Trust  |CN-ID or| RTC | Nonce |
Name         | ged|  fied |Anchor |DNS-ID  |     |       |
---------------------------------------------------------|
Audit        |  X |       | X     |        |     | X     |
-------------|----|-------|-------|--------|-----|-------|
Nonceless    |  X |       | X     |        | X   |       |
Audit        |    |       |       |        |     |       |
-------------|----|-------|-------|--------|-----|-------|
Owner Audit  |  X |   X   | X     |        | X   | X     |
-------------|----|-------|-------|--------|-----|-------|
Owner ID     |    |   X   | X     |  X     | X   |       |
-------------|----|-------|----------------|-----|-------|
Bearer       |  X |       |   wildcard     | optional    |
out-of-scope |    |       |                |             |
-------------|----|-------|----------------|-------------|

NOTE: All voucher types include a 'Pledge ID serial number'
      (Not shown for space reasons)
              

Audit Voucher:

An Audit Voucher is named after the logging assertion mechanisms that the Registrar then "audits" to enforce local policy. The Registrar mitigates a MiTM Registrar by auditing that an unknown MiTM registrar does not appear in the log entries. This does not direct prevent the MiTM but provides a response mechanism that ensures the MiTM is unsuccessful. This advantage is that actual ownership knowledge is not required on the MASA service.

Nonceless Audit Voucher:

An Audit Voucher without a validity period statement. Fundamentally the same as an Audit Voucher except that it can be issued in advance to support network partitions or to provide a permanent voucher for remote deployments.

Ownership Audit Voucher:

An Audit Voucher where the MASA service has verified the Registrar as the authorized owner. The MASA service mitigates a MiTM Registrar by refusing to generate Audit Voucher's for unauthorized Registrars. The Registrar uses audit techniques to supplement the MASA. This provides an ideal sharing of policy decisions and enforcement between the vendor and the owner.

Ownership ID Voucher:

An Ownership ID Voucher is named after inclusion of the Pledge's CN-ID or DNS-ID within the voucher. The MASA service mitigates a MiTM Registrar by identifying the specific Registrar (via WebPKI) authorized to own the Pledge.

Bearer Voucher:

A Bearer Voucher is named after the inclusion of a Registrar ID wildcard. Because the Registrar identity is not indicated this voucher type must be treated as a secret and protected from exposure as any 'bearer' of the voucher can claim the Pledge device. Publishing a nonceless bearer voucher effectively turns the specified Pledge into a "TOFU" device with minimal mitigation against MiTM Registrars. Bearer vouchers are out-of-scope.

5. Voucher

The voucher's purpose is to securely assign a pledge to an owner. The voucher informs the pledge which entity it should consider to be its owner.

The voucher is signed a PKCS#7 SignedData structure, as specified by Section 9.1 of [RFC2315], encoded using ASN.1 distinguished encoding rules (DER), as specified in ITU-T X.690.

The PKCS#7 structure MUST contain JSON-encoded content conforming to the YANG module specified in Section 5.3.

The PKCS#7 structure MUST also contain a 'signerInfo' structure, as described in Section 9.1 of [RFC2315], containing the signature generated over the content using the MASA's private key.

The PKCS#7 structure SHOULD also contain all of the certificates leading up to and including the MASA's trust anchor certificate known to the pledges.

The PKCS#7 structure MAY also contain revocation objects for any intermediate CAs between the voucher-issuer and the trust anchor known to the pledge.

5.1. Tree Diagram

The following tree diagram [I-D.bjorklund-netmod-yang-tree-diagrams] illustrates a high-level view of a voucher document. Each field in the voucher is fully described by the YANG module provided in Section 5.3. Please review this YANG module for a detailed description of the voucher format.

module: ietf-voucher
  +--ro voucher
     +--ro created-on                       yang:date-and-time
     +--ro expires-on?                      yang:date-and-time
     +--ro assertion                        enumeration
     +--ro serial-number                    string
     +--ro idevid-issuer?                   binary
     +--ro pinned-domain-cert*              binary
     +--ro domain-cert-revocation-checks?   boolean
     +--ro nonce?                           binary
     +--ro last-renewal-date?               yang:date-and-time

5.2. Examples

This section provides a couple Voucher examples for illustration purposes.

The following example illustrates an ephemeral voucher (uses a nonce) encoded in JSON. As is expected with a dynamically-generated voucher, only a single pledge (device-identifier) is specified. The MASA generated this voucher using the 'logged' assertion type, knowing that it would be suitable for the pledge making the request.

{
  "ietf-voucher:voucher": {
    "created-on": "2016-10-07T19:31:42Z",
    "assertion": "logged",
    "serial-number": "JADA123456789",
    "serial-number-issuer": "some binary identifier",
    "domain-cert-trusted-ca": "base64-encoded X.509 DER",
    "domain-cert-identifier": {
      "subject": "base64-encoded Subject DER"
    },
    "nonce": "base64-encoded octet string"
  }
}

The following illustrates a long-lived voucher (no nonce), encoded in XML. This particular voucher applies to more than one pledge (unique-id), which might relate to, for instance, they were all issued as part of the same purchase order. This voucher includes both a trust anchor certificate (trusted-ca-certificate) as well as some additional information (cn-id and dns-id) that can be used to identify a specific domain certificate issued, perhaps indirectly, by the trust anchor CA.

{
  "ietf-voucher:voucher": {
    "created-on": "2016-10-07T19:31:42Z",
    "expires-on": "2016-10-21T19:31:42Z",
    "assertion": "verified",
    "serial-number": "JADA123456789",
    "serial-number-issuer": "some binary identifier",
    "domain-cert-trusted-ca": "base64-encoded X.509 DER",
    "domain-cert-identifier": {
      "subject": "base64-encoded Subject DER"
    },
    "assert-revocations-on-PKIX-certs": "false",
    "last-renewal-date": "2017-10-07T19:31:42Z"
  }
}

5.3. YANG Module

<CODE BEGINS> file "ietf-voucher@2017-06-07.yang"

module ietf-voucher {
  yang-version 1.1;

  namespace
    "urn:ietf:params:xml:ns:yang:ietf-voucher";
  prefix "vch";

  import ietf-yang-types {
    prefix yang;
    reference "RFC 6991: Common YANG Data Types";
  }

  import ietf-restconf {
    prefix rc;
    description
      "This import statement is only present to access
       the yang-data extension defined in RFC 8040.";
    reference "RFC 8040: RESTCONF Protocol";
  }

  organization
   "IETF ANIMA Working Group";

  contact
   "WG Web:   <http://tools.ietf.org/wg/anima/>
    WG List:  <mailto:anima@ietf.org>
    Author:   Kent Watsen
              <mailto:kwatsen@juniper.net>
    Author:   Max Pritikin
              <mailto:pritikin@cisco.com>
    Author:   Michael Richardson
              <mailto:mcr+ietf@sandelman.ca>";

  description
   "This module defines the format for a voucher, which is produced by
    a pledge's manufacturer or delegate (MASA) to securely assign one
    or more pledges to an 'owner', so that the pledges may establish a
    secure connection to the owner's network infrastructure.

    The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
    'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in
    the module text are to be interpreted as described in RFC 2119.";

  revision "2017-06-07" {
    description
     "Initial version";
    reference
     "RFC XXXX: Voucher Profile for Bootstrapping Protocols";
  }

  rc:yang-data voucher-artifact {
    uses voucher-grouping;
  }

  grouping voucher-grouping {
    description
      "Grouping only exists for pyang tree output...";

    container voucher {
      config false;
      description
        "A voucher that can be used to assign one or more
         pledges to an owner.";

      leaf created-on {
        type yang:date-and-time;
        mandatory true;
        description
          "A value indicating the date this voucher was created.  This
           node is optional because its primary purpose is for human
           consumption.  However, when present, pledges that have
           reliable clocks SHOULD ensure that this created-on value
           is not greater than the current time.";
      }

      leaf expires-on {
        type yang:date-and-time;
        must "not(../nonce)";
        description
          "A value indicating when this voucher expires.  The node is
           optional as not all pledges support expirations, such as
           pledges lacking a reliable clock.

           If this field exists, then the the pledges MUST ensure that
           the expires-on time has not yet passed. A pledge without
           an accurate clock cannot meet this requirement.

           The expires-on value MUST NOT exceed the expiration date
           of any of the listed 'pinned-domain-cert' certificates.";

      }

      leaf assertion {
        type enumeration {
          enum verified {
            description
              "Indicates that the ownership has been positively
               verified by the MASA (e.g., through sales channel
               integration).";
          }
          enum logged {
            description
              "Indicates that this ownership assignment has been
               logged into a database maintained by the MASA, after
               first verifying that there has not been a previous
               claim in the database for the same pledge (voucher
               transparency).";
          }
        }
        mandatory true;
        description
          "The assertion is a statement from the MASA regarding how
           the owner was verified.   This statement enables pledges
           to support more detailed policy checks.  Pledges MUST
           ensure that the assertion provided is acceptable before
           processing the voucher.";
      }

      leaf serial-number {
        type string;
        mandatory true;
        description
          "The serial number of the hardware.  When processing a
           voucher, a pledge MUST ensure that its serial number
           matches this value.  If no match occurs, then the
           pledge MUST NOT process this voucher.";
      }

      leaf idevid-issuer {
        type binary;
        description
          "The RFC5280 4.2.1.1 Authority Key Identifier OCTET STRING
           from the pledge's IDevID certificate.  Optional since some
           serial-numbers are already unique within the scope of a
           MASA.  Inclusion of the statistically unique key identifier
           ensures statistically unique identification of the hardware.
           When processing a voucher, a pledge MUST ensure that its
           IDevID Authority Key Identifier matches this value.  If no
           match occurs, then the pledge MUST NOT process this voucher.
          
           When issuing a voucher, the MASA MUST ensure that this field
           is populated for serial numbers that are not otherwise unique
           within the scope of the MASA.";
      }

      leaf-list pinned-domain-cert {
        type binary;
        min-elements 1;
        description
          "An X.509 v3 certificate structure as specified by RFC 5280,
           Section 4 encoded using the ASN.1 distinguished encoding
           rules (DER), as specified in ITU-T X.690.

           This certificate is used by a pledge to trust a public key
           infrastructure, in order to verify a domain certificate
           supplied to the pledge separately by the bootstrapping
           protocol.  The domain certificate MUST have this certificate
           somewhere in its chain of certificates.  This certificate
           MAY be an end-entity certificate, including a self-signed
           entity.";
        reference
          "RFC 5280:
             Internet X.509 Public Key Infrastructure Certificate
             and Certificate Revocation List (CRL) Profile.
           ITU-T X.690:
              Information technology – ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER),
              Canonical Encoding Rules (CER) and Distinguished
              Encoding Rules (DER).";
      }

      leaf domain-cert-revocation-checks {
        type boolean;
        must "../expires-on";
        description
          "A processing instruction to the pledge that it MUST verify
           the revocation status for the domain certificate.  This 
           instruction is only available for vouchers that expire. If
           this field is not set, then normal PKIX behaviour applies
           to validation of the domain certificate.";
      }

      leaf nonce {
        type binary {
          length "8..32";
        }
        must "not(../expires-on)";
        description
          "A value that can be used by a pledge in some bootstrapping
           protocols to enable anti-replay protection.  This node is
           optional because it is not used by all bootstrapping
           protocols.

           When present, the pledge MUST compare the provided nonce
           value with another value that the pledge randomly generated
           and sent to a bootstrap server in an earlier bootstrapping
           message.  If the values do not match, then the pledge MUST
           NOT process this voucher.";
      }

      leaf last-renewal-date {
        type yang:date-and-time;
        must "../expires-on";
        description
          "The date that the MASA projects to be the last date it
           will renew a voucher on. This field is merely informative, it
           is not processed by pledges.

           Circumstances may occur after a voucher is generated that 
           may alter a voucher's validity period.  For instance, a
           vendor may associate validity periods with support contracts,
           which may be terminated or extended over time.";
      }

    } // end voucher
  } // end voucher-grouping
}

<CODE ENDS>

6. Design Considerations

6.1. Renewals instead of Revocations

The lifetimes of vouchers may vary. In some bootstrapping protocols, the vouchers may be created and consumed immediately whereas, in other bootstrapping solutions, there may be a significant delay between when a voucher is created and when it is consumed. In cases when there is a delay, there is a need for the pledge to ensure that the assertions made when the voucher was created are still valid when it is consumed.

A revocation artifact is generally used to verify the continued validity of an assertion such as a PKIX certificate, web token, or a "voucher". With this approach, a potentially long-lived assertion is paired with a reasonably fresh revocation status check to ensure that the assertion is still valid. However, this approach increases solution complexity, as it introduces the need for additional protocols and code paths to distribute and process the revocations.

Addressing the short-comings of revocations, this document recommends instead the use of lightweight renewals of short-lived non-revocable vouchers. That is, rather than issue a long-lived voucher, the expectation is for the MASA to instead issue a short-lived voucher along with a promise (reflected in the 'last-renewal-date' field) to re-issue the voucher again when needed. Importantly, while issuing the initial voucher may incur heavyweight verification checks (are you who you say you are? does the pledge actually belong to you?), re-issuing the voucher should be a lightweight process, as it ostensibly only updates the voucher's validatity period. With this approach, there is only the one artifact, and only one code path is needed to process it, without any possibility for a pledge to choose to skip the revocation status check because, for instance, the OCSP Responder is not reachable.

While this document recommends issuing short-lived vouchers, the voucher artifact does not restrict the ability to create a long-lived vouchers, if required, however no revocation method is described.

Note that a voucher may be signed by a chain of intermediate CAs leading up to the trust anchor certificate known by the pledge. Even though the voucher itself is not revocable, it may still be revoked, per se, if one of the intermediate CA certificates is revoked.

6.2. Voucher Per Pledge

The solution described herein originally enabled a single voucher to apply to many pledges, using lists of regular expressions to represent ranges of serial numbers. However, it was determined that blocking the renewal of a voucher that applied to many devices would be excessive when only the ownership for a single pledge needed to be blocked. Thus, the voucher format now only supports a single serial-number to be listed.

7. Security Considerations

7.1. Clock Sensitivity

An attacker could use an expired voucher to gain control over a device that has no understand of time.

To defend against this there are three things: devices are required to verify that the expires-on field has not yet passed. Devices without access to time can use nonces to get ephermal vouchers. Thirdly, vouchers without expiration times may be used, which will appear in the audit log, informing the security decision.

This document defines artifacts containing time values for voucher expirations, which require an accurate clock in order to be processed correctly. Vendors planning on issuing vouchers with expiration values must ensure devices have an accurate clock when shipped from manufacturing facilities, and take steps to prevent clock tampering. If it is not possible to ensure clock accuracy then vouchers with expirations should not be issued.

7.2. Protect Voucher PKI in HSM

A voucher is signed by a CA, that may itself be signed by a chain of CAs leading to a trust anchor known to a pledge. Revocation checking of the intermediate certificates may be difficult in some scenarios. The voucher format supports the existing PKIX revocation information distribution within the limits of the current PKI technology (a PKCS7 structure can contain revocation objects as well), but pledges MAY accept vouchers without checking X.509 certificate revocation (when 'domain-cert-revocation-checks' is false). Without revocation checking, a compromized MASA keychain could be used to issue vouchers ad infinitum without recourse. For this reason, MASA implementations wanting to support such deployments SHOULD ensure that all the CA private keys used for signing the vouchers are protected by hardware security modules (HSMs).

7.3. Test Domain Certificate Validity when Signing

If a domain certificate is compromised, then any outstanding vouchers for that domain could be used by the attacker. The domain administrator is clearly expected to initiate revocation of any domain identity certificates (as is normal in PKI solutions).

Similarly they are expected to contact the MASA to indicate that an outstanding (presumably short lifetime) voucher should be blocked from automated renewal. Protocols for voucher distribution are RECOMMENDED to check for revocation of any domain identity certificates before automated renewal of vouchers.

8. IANA Considerations

8.1. The IETF XML Registry

This document registers a URIs in the IETF XML registry [RFC3688]. Following the format in [RFC3688], the following registration is requested:

   URI: urn:ietf:params:xml:ns:yang:ietf-voucher
   Registrant Contact: The ANIMA WG of the IETF.
   XML: N/A, the requested URI is an XML namespace.

8.2. The YANG Module Names Registry

This document registers a YANG module in the YANG Module Names registry [RFC6020]. Following the format defined in [RFC6020], the the following registration is requested:

   name:         ietf-voucher
   namespace:    urn:ietf:params:xml:ns:yang:ietf-voucher
   prefix:       vch
   reference:    RFC XXXX

9. References

9.1. Normative References

9.2. Informative References

Appendix A. Acknowledgements

The authors would like to thank for following for lively discussions on list and in the halls (ordered by last name):

Authors' Addresses