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Internet Engineering Task Force                          P. Hallam-Baker
Internet-Draft                                              VeriSign Inc
Intended status: Informational                          November 1, 2007
Expires: May 4, 2008

                         OCSP Algorithm Agility

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   The behavior of an OCSP server is specified for cases in which the
   OCSP server is capable of supporting more than one signature

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 3
   2.  OCSP Algorithm Agility Requirements . . . . . . . . . . . . . . 3
   3.  Client Indication of Preferred Signature Algorithms . . . . . . 4
   4.  Responder Signature Algorithm Selection . . . . . . . . . . . . 4
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
     7.1.  Use of insecure algorithms  . . . . . . . . . . . . . . . . 6
     7.2.  Man in the Middle Downgrade Attack  . . . . . . . . . . . . 6
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6
   Intellectual Property and Copyright Statements  . . . . . . . . . . 7

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1.  Introduction

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  OCSP Algorithm Agility Requirements

   OCSP RFC 2560 [RFC2560] defines a protocol for obtaining certificate
   status information from an online service.  A particular OCSP
   protocol may or may not be provided by the CA that issued the
   certificate whose status is being queried and may or may provide a
   realtime indication of the certificate status or a time delayed
   status indication.

   RFC 2560 [RFC2560] requires the implementation but not the use of a
   set of mandatory cryptographic algorithms.  The OCSP protocol
   provides a means for an OCSP responder to indicate the signature and
   digest algorithms used in a response but not how that algorithm is to
   be chosen or how the client might influence the choice of signature

   In practice this approach has proved insufficient to ensure
   interoperability of implementations, in particular when a transition
   from the use of one cryptographic algorithm to another is in
   progress.  The mandatory cryptographic algorithms are the only ones
   that a responder can expect a client to implement according to the
   specification but this is a clearly unacceptable choice if a
   different signing algorithm has been chosen for the certificate whose
   status is being querried precisely because the mandatory algorithm is
   not acceptably secure.

   Although security concerns may lead to the use of a different signing
   algorithm in a certificate there is little security advantage to be
   used from using a stronger signing algorithm in an OCSP response than
   in the certificate whose status is being reported.  A possible
   exception being in the case where a certificate issuer determines
   that a certificate signing algorithm has become regarded as
   unacceptably secure during the period of validity for a certificate
   and thus requires a means of revoking the certificates that does not
   depend on the compromised algorithm.

   While use of a non-mandated signing algorithm is often motivated by
   security concerns, this is not always the case: performance, key and
   signature value sizes are also possible motivations.

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   In most cases an OCSP reponder may assume that the client is capable
   of verifying any signature algorithm that is used in either the
   certificate whose status is being verified or the certificate of the
   OCSP responder.

   There are however circumstances in which it may be desirable to use a
   different form of signature or demonstration of authenticity for the
   OCSP response.  In a wireless application, the use of a signature
   algorithm that provides a compact signature value such as DSA or ECC
   might be desirable.

   The introduction of delegated certificate path discovery and
   validation (e.g.  XKMS, SCVP) gives rise to further exceptions.  A
   certificate path discovery service may be required to establish a
   certificate path for a public key algorithm that the discovery
   service itself does not support.  In many embedded applications a
   relying party application may use an OCSP service to verify the
   current status of a certificate that has previously (but not
   necessarily contemporaneously) been subject to path validation by
   another component in the system.

3.  Client Indication of Preferred Signature Algorithms

   A client MAY declare a preferred set of algorithms algorithms in a
   request using the preferred signature algorithm extension.

   id-pkix-ocsp-preferred-signature-algorithms OBJECT IDENTIFIER ::= {
   id-pkix-ocsp x } PreferredSignatureAlgorithms ::= SEQUENCE {
   Algorithms SEQUENCE OF AlgorithmIdentifier }

   If a set of preferred signature algorithms is declared the client
   MUST support each of the specified algorithms.

   If a set of preferred algorithms is declared the OCSP responder
   SHOULD use one of the specified signing algorithms.

4.  Responder Signature Algorithm Selection

   RFC 2560 [RFC2560] does not specify a mechanism for deciding the
   signature algorithm to be used in an OCSP response.  As previously
   noted this does not provide a sufficient degree of certainty as to
   the algorithm selected to guarantee interoperation.

   A responder MAY maximize the potential for ensuring interoperability
   by selecting the OCSP signature algorithm using the following order
   of precedence where the first method has the highest precedence:

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   1.  Using an algorithm specified as a preferred signing algorithm in
       the client request.

   2.  Using the signing algorithm used to sign the CertID specified in
       the query.

   3.  Using the signing algorithm used to sign a CRL issued by the
       certificate issuer providing status information for the
       certificate specified by CertID.

   4.  Using a signature algorithm that has been advertised as being the
       default signature algorithm for the signing service using an out
       of band mechanism

   5.  Using a mandatory signing algorithm specified for the version of
       the OCSP protocol in use.

   A responder SHOULD always apply the lowest numbered selection
   mechanism that is known, supported and meets the responder's criteria
   for cryptographic algorithm strength.

5.  Acknowledgements

   The author acknowleges the helpful comments made on earlier drafts of
   this work by Santosh Chokhani and Stefan Santesson

6.  IANA Considerations

   Do we register ASN.1 OIDs with IANA?  If not, why not?

7.  Security Considerations

   The mechanism used to choose the response signing algorithm MUST be
   considered to be sufficiently secure against cryptanalytic attack for
   the intended application.

   In most applications it is sufficient for the signing algorithm to be
   at least as secure as the signing algorithm used to sign the original
   certificate whose status is being queried.  This criteria may not
   hold in long term archival applications however in which the status
   of a certificate is being queried for a date in the distant past,
   long after the signing algorithm has ceased being considered

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7.1.  Use of insecure algorithms

   The security of the signing algorithm used by the responder MUST take
   precedence over all other considerations.  A responder MUST NOT
   generate a signature for a signing mechanism that is considered
   unacceptably insecure regardless of the other circumstances.

   In archival applications it is quite possible that an OCSP responder
   might be asked to report the validity of a certificate on a date in
   the distant past.  Such a certificate might employ a signing method
   that is no longer considered acceptably secure.  In such
   circumstances the responder MUST NOT generate a signature for a
   signing mechanism that is considered unacceptably insecure.

   A client MUST accept any signing algorithm in a response that it
   specified as a preferred signing algorithm in the request.  It
   follows therefore that a client MUST NOT specify as a preferred
   signing algorithm any signing algorithm that is either not supported
   or not considered acceptably secure.

7.2.  Man in the Middle Downgrade Attack

   The mechanism to support client indication of preferred signature
   algorithms is not protected against a man in the middle downgrade
   attack.  This constraint is not considered to be a significant
   security concern as the client MUST NOT accept any signing algorithm
   that does not meet its own criteria for acceptable cryptographic
   security no matter what mechanism is used to determine the signing
   algorithm of the response.

8.  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.

Author's Address

   Phillip Hallam-Baker
   VeriSign Inc

   Email: pbaker@verisign.com

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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

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