Internet DRAFT - draft-short-pkinit-freshness
draft-short-pkinit-freshness
Kitten Working Group M. Short, Ed.
Internet-Draft S. Moore
Intended status: Standards Track P. Miller
Expires: April 30, 2015 Microsoft Corporation
October 27, 2014
Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
Freshness Extension
draft-short-pkinit-freshness-00
Abstract
This document describes how to extend Public Key Cryptography for
Initial Authentication in Kerberos (PKINIT) extension [RFC4556] to
exchange an opaque data blob which a KDC can validate to ensure that
the client is currently in possession of the private key during a
PKInit AS exchange.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Kerberos message flow using KRB_AS_REQ without pre-
authentication . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Message Exchanges . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Generation of KRB_ERROR Message . . . . . . . . . . . . . 4
2.2. Generation of KRB_AS_REQ Message . . . . . . . . . . . . 4
2.3. Receipt of KRB_AS_REQ Message . . . . . . . . . . . . . . 4
3. PreAuthentication Data Types . . . . . . . . . . . . . . . . 4
4. PA-PK-AS-KDCTOKEN . . . . . . . . . . . . . . . . . . . . . . 5
5. Extended PKAuthenticator . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
Kerberos PKINIT [RFC4556] defines two schemes to use asymmetric
cryptography in a Kerberos preauthenticator. One uses Diffie-Hellman
key exchange and the other depends on public key encryption. The
public key encryption scheme is less commonly used for two reasons:
o Elliptic Curve Cryptography (ECC) Support [RFC5349] only supports
Elliptic Curve Diffie-Hellman (ECDH) key agreement.
o Requires certificates with an encryption key which is not deployed
on many existing smart cards.
In the Diffie-Hellman exchange, the client uses its private key only
to sign the AuthPack specified in Section 3.2.1 of [RFC4556] which is
performed before any traffic is sent to the KDC. Thus a client can
generate requests with future times in the PKAuthenticator, and then
send those requests at the future times. Unless the time is outside
the validity period of the client's certificate, the KDC will
validate it and return a TGT the client can use without possessing
the private key.
As a result, a client performing PKINIT with the Diffie-Hellman key
exchange does not prove current possession of the private key being
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used for authentication. It proves only prior use of that key.
Ensuring that the client has current possession of the private key
requires that the signed PKAuthenticator data include information
that the client could not have predicted in advance.
1.1. Kerberos message flow using KRB_AS_REQ without pre-authentication
Today some password-based AS exchanges [RFC4120] depend on the client
sending a KRB_AS_REQ without pre-authentication to trigger the KDC to
provide the Kerberos client with information needed to complete an AS
exchange such as the supported encryption types and salt value (see
message flow below):
KDC Client Application Server
<---- AS-REQ without pre-authentication
KRB-ERROR ---->
<---- AS-REQ
AS-REP ---->
<---- TGS-REQ
TGS-REP ---->
Figure 1
We can use this mechanism in PKInit for KDCs to provide data which
the client returns as part of the KRB_AS_REQ to ensure that the
PA_PK_AS_REQ [RFC4556] was not pregenerated.
1.2. Requirements Language
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 RFC 2119 [RFC2119].
2. Message Exchanges
This following summarizes the message flow with extensions to
[RFC4120] and [RFC4556] required to support a KDC provided freshness
token during the initial request for a ticket:
1. The client generates a KRB_AS_REQ with the OPT-HARDWARE-AUTH
option specified in Section 2.9.3 [RFC4120] to the KDC.
2. The KDC generates a KRB_ERROR as specified in Section 3.1.3 of
[RFC4120] providing a freshness token.
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3. The client receives the error as specified in Section 3.1.4 of
[RFC4120] and includes the freshness token as part of the
KRB_AS_REQ as specified in [RFC4120] and [RFC4556].
4. The KDC receives and validates the KRB_AS_REQ as specified in
Section 3.2.2 [RFC4556] then additionally validates the freshness
token.
5. The KDC and client continue as specified in [RFC4120] and
[RFC4556].
2.1. Generation of KRB_ERROR Message
The KDC will indicate support by adding to the METHOD-DATA object the
PA-PK-AS-KDCTOKEN with padata-type is PA_PK_AS_KDCTOKEN.
2.2. Generation of KRB_AS_REQ Message
After the client receives the KRB-ERROR message, when generating the
PKInit AS-REQ it extracts PA-PK-AS-KDCTOKEN as an opaque data blob.
When generating the PKAuthenticator, the PA-PK-AS-KDCTOKEN SHALL be
added as an opaque blob in the kdcToken field so it becomes part of
the signed data in the KRB_AS_REQ.
2.3. Receipt of KRB_AS_REQ Message
After validating the PA_PK_AS_REQ message normally, the KDC will
validate the PA-PK-AS-KDCTOKEN in an implementation specific way. If
the freshness token is not valid, the KDC MUST return
KDC_ERR_PREAUTH_FAILED with PA-PK-AS-KDCTOKEN. Since the freshness
tokens are validated by KDCs in the same realm, standardizing the
contents of the freshness token is not a concern for
interoperability.
3. PreAuthentication Data Types
The following are the new PreAuthentication data types:
+----------------------+-------------------+
| Padata and Data Type | Padata-type Value |
+----------------------+-------------------+
| PA_PK_AS_KDCTOKEN | TBD |
+----------------------+-------------------+
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4. PA-PK-AS-KDCTOKEN
The PA-PK-AS-KDCTOKEN structure specifies an freshness token. Its
structure is defined using ASN.1 notation. The syntax is as follows:
PA-PK-AS-KDCTOKEN ::= OCTET STRING
5. Extended PKAuthenticator
The PKAuthenticator structure specified in Section 3.2.1 [RFC4556] is
extended to include a new kdcToken as follows:
PKAuthenticator ::= SEQUENCE {
cusec [0] INTEGER (0..999999),
ctime [1] KerberosTime,
-- cusec and ctime are used as in [RFC4120], for
-- replay prevention.
nonce [2] INTEGER (0..4294967295),
-- Chosen randomly; this nonce does not need to
-- match with the nonce in the KDC-REQ-BODY.
paChecksum [3] OCTET STRING OPTIONAL,
-- MUST be present.
-- Contains the SHA1 checksum, performed over
-- KDC-REQ-BODY.
...,
kdcToken [4] PA-PK-AS-KDCTOKEN OPTIONAL,
-- MUST be present if sent by KDC
...
}
6. Acknowledgements
Nathan Ide and Magnus Nystrom reviewed the document and provided
suggestions for improvements.
7. IANA Considerations
IANA is requested to assign numbers for PA_PK_AS_KDCTOKEN listed in
the Kerberos Parameters registry Pre-authentication and Typed Data as
follows:
+------+-------------------+------------+
| Type | Value | Reference |
+------+-------------------+------------+
| TBD | PA_PK_AS_KDCTOKEN | [This RFC] |
+------+-------------------+------------+
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8. Security Considerations
The freshness token SHOULD include either signing or sealing data
from the KDC to prevent tampering. Kerberos error messages are not
integrity protected unless authenticated using Kerberos FAST
[RFC6113].
The freshness token SHOULD include signing, encrypting or sealing
data from the KDC to determine authenticity. Even if FAST is
required to provide integrity protection, a different KDC would not
be able to validate freshness tokens without some kind of shared
database.
Since the client treats the KDC provided data blob as opaque,
changing the contents will not impact existing clients. Thus
extensions to the freshness token do not impact client
interoperability.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial
Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.
[RFC5349] Zhu, L., Jaganathan, K., and K. Lauter, "Elliptic Curve
Cryptography (ECC) Support for Public Key Cryptography for
Initial Authentication in Kerberos (PKINIT)", RFC 5349,
September 2008.
9.2. Informative References
[RFC6113] Hartman, S. and L. Zhu, "A Generalized Framework for
Kerberos Pre-Authentication", RFC 6113, April 2011.
Authors' Addresses
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Michiko Short (editor)
Microsoft Corporation
USA
Email: michikos@microsoft.com
Seth Moore
Microsoft Corporation
USA
Email: sethmo@microsoft.com
Paul Miller
Microsoft Corporation
USA
Email: paumil@microsoft.com
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