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 © 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

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.

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.

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.

Authors' Addresses

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

EMail:

          [email protected]
   
   Karthik Jaganathan
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

EMail:

          [email protected]
   
   Nicolas Williams
   Sun Microsystems
   5300 Riata Trace Ct
   Austin, TX  78727
   US
   
   EMail: [email protected]

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