Network Working Group                                       S. Santesson
Internet-Draft                                              3xA Security
Intended status: Proposed Standard                       P. Hallam-Baker
Updates: 2560 (once approved)                      Default Deny Security
Expires: September 9, 2010                                 March 8, 2010

                         OCSP Algorithm Agility
                     draft-ietf-pkix-ocspagility-08

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Abstract

   The OSCP specification defined in RFC 2560 [RFC2560] requires server
   responses to be signed but does not specify a mechanism for selecting
   the signature algorithm to be used. This may lead to avoidable
   interoperability failures in contexts where multiple signature
   algorithms are in use. This document specifies rules for server
   signature algorithm selection and an extension that allows a client
   to advise a server that specific signature algorithms are supported.



Table of Contents

   1  Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
      1.1  Requirements Language . . . . . . . . . . . . . . . . . . . 3
   2  OCSP Algorithm Agility Requirements  . . . . . . . . . . . . . . 3
   3  Updates to Mandatory and Optional Cryptographic Algorithms . . . 4
   4  Client Indication of Preferred Signature Algorithms  . . . . . . 5
   5  Responder Signature Algorithm Selection  . . . . . . . . . . . . 6
      5.1  Dynamic Response  . . . . . . . . . . . . . . . . . . . . . 6
      5.2  Static Response . . . . . . . . . . . . . . . . . . . . . . 6
   6  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 7
   7  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 7
   8  Security Considerations  . . . . . . . . . . . . . . . . . . . . 7
      8.1  Use of insecure algorithms  . . . . . . . . . . . . . . . . 7
      8.2  Man in the Middle Downgrade Attack  . . . . . . . . . . . . 8
      8.3. Denial of Service Attack  . . . . . . . . . . . . . . . . . 8
   9  References . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
      9.1  Normative References  . . . . . . . . . . . . . . . . . . . 9
      9.2  Informative References  . . . . . . . . . . . . . . . . . . 9
   Appendix A - ASN.1 Modules  . . . . . . . . . . . . . . . . . . .  10
      A.1  ASN.1 Module  . . . . . . . . . . . . . . . . . . . . . .  10
      A.2  1988 ASN.1 Module . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12
















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1  Introduction

1.1  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  OCSP Algorithm Agility Requirements

   OCSP RFC 2560 [RFC2560] defines a protocol for obtaining certificate
   status information from an online service. An OCSP Responder may or
   may not be issued an OCSP Responder certificate by the CA that issued
   the certificate whose status is being queried.  An OCSP Responder may
   provide pre-signed OCSP responses or may sign responses when queried.

   RFC 2560 [RFC2560] specifies a means for an OCSP responder to
   indicate the signature and digest algorithms used in a response but
   not how those algorithms are specified.  The only algorithm
   requirements established by that protocol specification are that the
   OCSP client SHALL support the DSA sig-alg-oid specified in section
   7.2.2 of [RFC2459] and SHOULD be capable of processing RSA signatures
   as specified in section 7.2.1 of [RFC2459].  The only requirement
   placed on responders by RFC 2560 is that they SHALL support the SHA1
   hashing algorithm.

   Since algorithms other than the mandatory to implement algorithms are
   Allowed, and since a client currently has no mechanism to indicate
   it's algorithm preferences, there is always a risk that a server
   choosing a non-mandatory algorithm, will generate a response that the
   client may not support.

   While an OCSP Responder may apply rules for algorithm selection,
   e.g., using the signature algorithm employed by the CA for signing
   CRLs and certificates, such rules may fail in common situations:

   o  The algorithm used to sign the CRLs and certificates may not be
      consistent with key pair being used by the OCSP Responder to sign
      responses.

   o  A request for an unknown certificate provides no basis for a
      responder to select from among multiple algorithm options.

   The last criterion cannot be resolved through the information
   available from in-band signaling using the RFC 2560 [RFC2560]
   protocol, without modifying the protocol.




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   In addition, an OCSP Responder may wish to employ different signature
   algorithms than the one used by the CA to sign certificates and CRLs
   for several reasons:

   o  The responder may employ an algorithm for certificate status
      response that is less computationally demanding than for signing
      the certificate itself.

   o  An implementation may wish to guard against the possibility of a
      compromise resulting from a signature algorithm compromise by
      employing two separate signature algorithms.


   This document describes:

   o  A mechanism that allows a client to indicate the set of preferred
      signature algorithms.

   o  Rules for signature algorithm selection that maximizes the
      probability of successful operation in the case that no supported
      preferred algorithm(s) are specified.


3  Updates to Mandatory and Optional Cryptographic Algorithms

   Section 4.3  "Mandatory and Optional Cryptographic Algorithms" of RFC
   2560 [RFC2560] is updated as follows:

   OLD: Clients that request OCSP services SHALL be capable of
        processing responses signed used DSA keys identified by the DSA
        sig-alg-oid specified in section 7.2.2 of [RFC2459]. Clients
        SHOULD also be capable of processing RSA signatures as specified
        in section 7.2.1 of [RFC2459]. OCSP responders SHALL support the
        SHA1 hashing algorithm.

   NEW: Clients that request OCSP services SHALL be capable of
        processing responses signed using RSA with SHA-1 (identified by
        sha1WithRSAEncryption OID specified in [RFC3279]) and RSA with
        SHA-256 (identified by sha256WithRSAEncryption OID specified in
        [RFC4055]). Clients SHOULD also be capable of processing
        responses signed using DSA keys (identified by the id-dsa-with-
        sha1 OID specified in [RFC3279]). Clients MAY support other
        algorithms.








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4  Client Indication of Preferred Signature Algorithms

   A client MAY declare a preferred set of algorithms in a request by
   including a preferred signature algorithms extension in
   requestExtensions of the OCSPRequest [RFC2560].


     id-pkix-ocsp-pref-sig-algs OBJECT IDENTIFIER ::= { id-pkix-ocsp 8 }

     PreferredSignatureAlgorithms ::= SEQUENCE OF
                                      PreferredSignatureAlgorithm

     PreferredSignatureAlgorithm ::= SEQUENCE {
        sigIdentifier   AlgorithmIdentifier,
        certIdentifier  AlgorithmIdentifier OPTIONAL
        }

   The syntax of AlgorithmIdentifier is defined in section 4.1.1.2 of
   RFC 5280 [RFC5280]

   sigIdentifier specifies the signature algorithm the client prefers,
   e.g. algorithm=ecdsa-with-sha256. Parameters are absent for most
   common signature algorithms.

   certIdentifier specifies the subject public key algorithm identifier
   the client prefers in the server's certificate used to validate the
   OCSP response. e.g. algorithm=id-ecPublicKey and parameters=
   secp256r1.

   certIdentifier is OPTIONAL and provides means to specify parameters
   necessary to distinguish among different usages of a particular
   algorithm, e.g. it may be used by the client to specify what curve it
   supports for a given elliptic curve algorithm.

   The client MUST support each of the specified preferred signature
   algorithms and the client MUST specify the algorithms in the order of
   preference.

   The server SHOULD use one of the preferred signature algorithms for
   signing OCSP responses to the requesting client.











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5  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 facilitate interoperability.

5.1  Dynamic Response

   A responder MAY maximize the potential for ensuring interoperability
   by selecting a supported signature algorithm using the following
   order of precedence, as long as the selected algorithm meets all
   security requirements of the OCSP responder, where the first method
   has the highest precedence:

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

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

   3.  Select the signing algorithm used to sign the OCSPRequest

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

   5.  Select a mandatory or recommended 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 that meets the responder's
   criteria for cryptographic algorithm strength.

5.2  Static Response

   For purposes of efficiency, an OCSP responder is permitted to
   generate static responses in advance of a request. The case may not
   permit the responder to make use of the client request data during
   the response generation, however the responder SHOULD still use the
   client request data during the selection of the pre-generated
   response to be returned.  Responders MAY use the historical client
   requests as part of the input to the decisions of what different
   algorithms should be used to sign the pre-generated responses.






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6  Acknowledgements

   The authors acknowledges Santosh Chokhani for the helpful comments
   made on earlier drafts, Sean Turner for proposing the syntax for
   algorithm identifiers, Jim Schaad for providing and testing the ASN.1
   module in Annex A and Stephen Kent for valuable review and input.


7  IANA Considerations

   This document requires no actions by IANA.


8  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
   trustworthy.

8.1  Use of insecure algorithms

   It is not always possible for a responder to generate a response that
   the client is expected to understand and that meets contemporary
   standards for cryptographic security.  In such cases an OCSP
   responder operator MUST balance the risk of employing a compromised
   security solution and the cost of mandating an upgrade, including the
   risk that the alternative chosen by end users will offer even less
   security or no security.

   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 algorithm that is either not supported or not



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   considered acceptably secure.

8.2  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 since the OCSP Responder MUST NOT sign OCSP
   Responses using weak algorithms even if requested by the client.  In
   addition, the client can reject OCSP responses that do not meet its
   own criteria for acceptable cryptographic security no matter what
   mechanism is used to determine the signing algorithm of the response.


8.3. Denial of Service Attack

   Algorithm agility mechanisms defined in this document introduces a
   slightly increased attack surface for Denial of Service attacks where
   the client request is altered to require algorithms that are not
   supported by the server, alternatively does not match pre-generated
   responses.






























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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.

   [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.

   [RFC3279]  W. Polk, R. Housley, L. Bassham, "Algorithms and
              Identifiers for the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 3279, April 2002.


   [RFC4055]  J. Schaad, B. Kaliski, R. Housley, "Additional Algorithms
              and Identifiers for RSA Cryptography for use in the
              Internet X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 4055, June
              2005.

   [RFC5280]  D. Cooper, S. Santesson, S. Farrell, S. Boeyen, R. Housley
              and W. Polk, "Internet X.509 Public Key Infrastructure
              Certificate and Certificate Revocation List (CRL)
              Profile", RFC 5280, May 2008.

   [NEWASN]   P. Hoffman, J. Schaad, "New ASN.1 Modules for PKIX",
              draft-ietf-pkix-new-asn1, August 2009.


9.2  Informative References

   [RFC2459]  R. Housley, W. Ford, W. Polk, D. Solo, "Internet X.509
              Public Key Infrastructure - Certificate and CRL Profile",
              January 1999














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Appendix A - ASN.1 Modules

A.1  ASN.1 Module

 OCSP-AGILITY-2009 { iso(1) identified-organization(3) dod(6)
     internet(1)  security(5) mechanisms(5) pkix(7) id-mod(0)
     id-mod-ocsp-agility-2009-93(66) }

 DEFINITIONS EXPLICIT TAGS ::=
 BEGIN

   EXPORTS ALL;   -- export all items from this module
   IMPORTS

   id-pkix-ocsp
     FROM OCSP-2009  -- From OCSP [RFC2560]
      { iso(1) identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) id-mod(0) id-mod-ocsp-02(48) }

   AlgorithmIdentifier{}, SIGNATURE-ALGORITHM
     FROM AlgorithmInformation-2009 -- From [NEWASN]
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-algorithmInformation-02(58) }

   EXTENSION
     FROM PKIX-CommonTypes-2009 -- From [NEWASN]
      { iso(1) identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) id-mod(0) id-mod-pkixCommon-02(57)} ;

    --  Add re-preferred-signature-algorithms to the set of extensions
    --  for TBSRequest.requestExtensions

   re-preferred-signature-algorithms EXTENSION ::= {
      SYNTAX PreferredSignatureAlgorithm
      IDENTIFIED BY id-pkix-ocsp-pref-sig-algs  }

   id-pkix-ocsp-pref-sig-algs OBJECT IDENTIFIER ::= { id-pkix-ocsp 8 }

   PreferredSignatureAlgorithms ::= SEQUENCE OF
                                      PreferredSignatureAlgorithm

   PreferredSignatureAlgorithm ::= SEQUENCE {
     sigIdentifier  AlgorithmIdentifier{SIGNATURE-ALGORITHM, {...}},
     certIdentifier AlgorithmIdentifier{SIGNATURE-ALGORITHM, {...}}
                      OPTIONAL  }

 END



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A.2  1988 ASN.1 Module

 OCSP-AGILITY-88 { iso(1) identified-organization(3) dod(6) internet(1)
     security(5) mechanisms(5) pkix(7) id-mod(0)
     id-mod-ocsp-agility-2009-88(67) }

 DEFINITIONS EXPLICIT TAGS ::=
 BEGIN

   -- EXPORTS ALL;
   IMPORTS

   id-pkix-ocsp
   FROM OCSP  {iso(1) identified-organization(3) dod(6) internet(1)
     security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-ocsp(14)}

   AlgorithmIdentifier
   FROM PKIX1Explicit88 { iso(1) identified-organization(3) dod(6)
     internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
     id-pkix1-explicit(18) };

     id-pkix-ocsp-pref-sig-algs OBJECT IDENTIFIER ::= { id-pkix-ocsp 8 }

     PreferredSignatureAlgorithms ::= SEQUENCE OF
                                      PreferredSignatureAlgorithm

     PreferredSignatureAlgorithm ::= SEQUENCE {
        sigIdentifier   AlgorithmIdentifier,
        certIdentifier  AlgorithmIdentifier OPTIONAL
        }

 END



















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Author's Address

   Phillip Hallam-Baker
   Default Deny Security

   Email: phill@hallambaker.com


   Stefan Santesson
   3xA Security AB
   Sweden

   Email: sts@aaa-sec.com






































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