





Network Working Group                                             L. Zhu
Request for Comments: 4557                                 K. Jaganathan
Category: Standards Track                          Microsoft Corporation
                                                             N. Williams
                                                        Sun Microsystems
                                                               June 2006


         Online Certificate Status Protocol (OCSP) Support for
                      Public Key Cryptography for
              Initial Authentication in Kerberos (PKINIT)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document defines a mechanism to enable in-band transmission of
   Online Certificate Status Protocol (OCSP) responses in the Kerberos
   network authentication protocol.  These responses are used to verify
   the validity of the certificates used in Public Key Cryptography for
   Initial Authentication in Kerberos (PKINIT), which is the Kerberos
   Version 5 extension that provides for the use of public key
   cryptography.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   3. Message Definition ..............................................2
   4. Security Considerations .........................................3
   5. Acknowledgements ................................................4
   6. References ......................................................4
      6.1. Normative References .......................................4
      6.2. Informative References .....................................4







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1.  Introduction

   Online Certificate Status Protocol (OCSP) [RFC2560] enables
   applications to obtain timely information regarding the revocation
   status of a certificate.  Because OCSP responses are well bounded and
   small in size, constrained clients may wish to use OCSP to check the
   validity of the certificates for Kerberos Key Distribution Center
   (KDC) in order to avoid transmission of large Certificate Revocation
   Lists (CRLs) and therefore save bandwidth on constrained networks
   [OCSP-PROFILE].

   This document defines a pre-authentication type [RFC4120], where the
   client and the KDC MAY piggyback OCSP responses for certificates used
   in authentication exchanges, as defined in [RFC4556].

   By using this OPTIONAL extension, PKINIT clients and the KDC can
   maximize the reuse of cached OCSP responses.

2.  Conventions Used in This Document

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in [RFC2119].

3.  Message Definition

   A pre-authentication type identifier is defined for this mechanism:

              PA-PK-OCSP-RESPONSE              18

   The corresponding padata-value field [RFC4120] contains the DER [X60]
   encoding of the following ASN.1 type:

          PKOcspData ::= SEQUENCE OF OcspResponse
                         -- If more than one OcspResponse is
                         -- included, the first OcspResponse
                         -- MUST contain the OCSP response
                         -- for the signer's certificate.
                         -- The signer refers to the client for
                         -- AS-REQ, and the KDC for the AS-REP,
                         -- respectively.

          OcspResponse ::= OCTET STRING
                         -- Contains a complete OCSP response,
                         -- as defined in [RFC2560].

   The client MAY send OCSP responses for certificates used in PA-PK-
   AS-REQ [RFC4556] via a PA-PK-OCSP-RESPONSE.



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   The KDC that receives a PA-PK-OCSP-RESPONSE SHOULD send a PA-PK-
   OCSP-RESPONSE containing OCSP responses for certificates used in the
   KDC's PA-PK-AS-REP.  The client can request a PA-PK-OCSP-RESPONSE by
   using a PKOcspData containing an empty sequence.

   The KDC MAY send a PA-PK-OCSP-RESPONSE when it does not receive a
   PA-PK-OCSP-RESPONSE from the client.

   The PA-PK-OCSP-RESPONSE sent by the KDC contains OCSP responses for
   certificates used in PA-PK-AS-REP [RFC4556].

   Note the lack of integrity protection for the empty or missing OCSP
   response; lack of an expected OCSP response from the KDC for the
   KDC's certificates SHOULD be treated as an error by the client,
   unless it is configured otherwise.

   When using OCSP, the response is signed by the OCSP server, which is
   trusted by the receiver.  Depending on local policy, further
   verification of the validity of the OCSP servers may be needed

   The client and the KDC SHOULD ignore invalid OCSP responses received
   via this mechanism, and they MAY implement CRL processing logic as a
   fall-back position, if the OCSP responses received via this mechanism
   alone are not sufficient for the verification of certificate
   validity.  The client and/or the KDC MAY ignore a valid OCSP response
   and perform its own revocation status verification independently.

4.  Security Considerations

   The pre-authentication data in this document do not actually
   authenticate any principals, but are designed to be used in
   conjunction with PKINIT.

   There is no binding between PA-PK-OCSP-RESPONSE pre-authentication
   data and PKINIT pre-authentication data other than a given OCSP
   response corresponding to a certificate used in a PKINIT pre-
   authentication data element.  Attacks involving removal or
   replacement of PA-PK-OCSP-RESPONSE pre-authentication data elements
   are, at worst, downgrade attacks, where a PKINIT client or KDC would
   proceed without use of CRLs or OCSP for certificate validation, or
   denial-of-service attacks, where a PKINIT client or KDC that cannot
   validate the other's certificate without an accompanying OCSP
   response might reject the AS exchange or might have to download very
   large CRLs in order to continue.  Kerberos V does not protect against
   denial-of-service attacks; therefore, the denial-of-service aspect of
   these attacks is acceptable.





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   If a PKINIT client or KDC cannot validate certificates without the
   aid of a valid PA-PK-OCSP-RESPONSE, then it SHOULD fail the AS
   exchange, possibly according to local configuration.

5.  Acknowledgements

   This document was based on conversations among the authors, Jeffrey
   Altman, Sam Hartman, Martin Rex, and other members of the Kerberos
   working group.

6.  References

6.1.  Normative References

   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2560]      Myers, M., Ankney, R., Malpani, A., Galperin, S., and
                  C. Adams, "X.509 Internet Public Key Infrastructure
                  Online Certificate Status Protocol - OCSP", RFC 2560,
                  June 1999.

   [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.

   [X690]         ASN.1 encoding rules: Specification of Basic Encoding
                  Rules (BER), Canonical Encoding Rules (CER) and
                  Distinguished Encoding Rules (DER), ITU-T
                  Recommendation X.690 (1997) | ISO/IEC International
                  Standard 8825-1:1998.

6.2.  Informative References

   [OCSP-PROFILE] Deacon, A. and R. Hurst, "Lightweight OCSP Profile for
                  High Volume Environments", Work in Progress, May 2006.











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Authors' Addresses

   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   EMail: lzhu@microsoft.com


   Karthik Jaganathan
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   EMail: karthikj@microsoft.com


   Nicolas Williams
   Sun Microsystems
   5300 Riata Trace Ct
   Austin, TX  78727
   US

   EMail: Nicolas.Williams@sun.com
























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