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Internet Engineering Task Force (IETF) S. Santesson Request for Comments: 6277 3xA Security Updates: 2560 P. Hallam-Baker Category: Standards Track Default Deny Security ISSN: 2070-1721 June 2011

        Online Certificate Status Protocol Algorithm Agility

Abstract

 The Online Certificate Status Protocol (OCSP) 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.

Status of This Memo

 This is an Internet Standards Track document.

 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6277.

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.

 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Santesson & Hallam-Baker Standards Track [Page 1] RFC 6277 OCSP Algorithm Agility June 2011

Table of Contents

 1. Introduction ....................................................2
    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 .............4
 5. Responder Signature Algorithm Selection .........................5
    5.1. Dynamic Response ...........................................5
    5.2. Static Response ............................................6
 6. Acknowledgements ................................................6
 7. Security Considerations .........................................6
    7.1. Use of Insecure Algorithms .................................6
    7.2. Man-in-the-Middle Downgrade Attack .........................7
    7.3. Denial-of-Service Attack ...................................7
 8. References ......................................................7
    8.1. Normative References .......................................7
    8.2. Informative References .....................................8
 Appendix A.  ASN.1 Modules .........................................9
    A.1. ASN.1 Module ...............................................9
    A.2. 1988 ASN.1 Module .........................................10

1. Introduction

 The Online Certificate Status Protocol (OCSP) [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 certification authority (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 Digital Signature Algorithm (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.

 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.

Santesson & Hallam-Baker Standards Track [Page 2] RFC 6277 OCSP Algorithm Agility June 2011

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

 Since algorithms other than those that are mandatory to implement are
 allowed and since a client currently has no mechanism to indicate its
 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
 certificate revocation lists (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 the 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.

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

 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.

Santesson & Hallam-Baker Standards Track [Page 3] RFC 6277 OCSP Algorithm Agility June 2011

 o  Rules for signature algorithm selection that maximize 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.

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,
      pubKeyAlgIdentifier  SMIMECapability OPTIONAL
      }

 The syntax of AlgorithmIdentifier is defined in Section 4.1.1.2 of
 RFC 5280 [RFC5280].  The syntax of SMIMECapability is defined in RFC
 5751 [RFC5751].

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

Santesson & Hallam-Baker Standards Track [Page 4] RFC 6277 OCSP Algorithm Agility June 2011

 pubKeyAlgIdentifier 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.

 pubKeyAlgIdentifier 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, from the most preferred to the least preferred.

 Section 5 of this document describes how a server selects an
 algorithm for signing OCSP responses to the requesting client.

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

 As long as the selected algorithm meets all security requirements of
 the OCSP responder, a responder MAY maximize the potential for
 ensuring interoperability by selecting a supported signature
 algorithm using the following order of precedence, 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 certificate
     revocation list (CRL) issued by the certificate issuer to provide
     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.

Santesson & Hallam-Baker Standards Track [Page 5] RFC 6277 OCSP Algorithm Agility June 2011

 A responder SHOULD always apply the lowest-numbered selection
 mechanism that results in the selection of a known and supported
 algorithm 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.

6. Acknowledgements

 The authors acknowledge 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 Appendix A, and Stephen Kent for valuable review and input.

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.  However, this
 criteria may not hold in long-term archival applications in which the
 status of a certificate is being queried for a date in the distant
 past, long after the signing algorithm has ceased to be considered
 trustworthy.

7.1. 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.

Santesson & Hallam-Baker Standards Track [Page 6] RFC 6277 OCSP Algorithm Agility June 2011

 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 using a
 signing mechanism that is not considered acceptably secure.

 A client MUST accept any signing algorithm in a response that it
 specified as a preferred signing algorithm in the request.
 Therefore, it follows that a client MUST NOT specify a preferred
 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 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.

7.3. Denial-of-Service Attack

 Algorithm agility mechanisms defined in this document introduce 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.  Denial-of-service considerations from RFC
 4732 [RFC4732] are relevant for this document.

8. References

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

Santesson & Hallam-Baker Standards Track [Page 7] RFC 6277 OCSP Algorithm Agility June 2011

 [RFC4055]  Schaad, J., Kaliski, B., and 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]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, May 2008.

 [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
            Mail Extensions (S/MIME) Version 3.2 Message
            Specification", RFC 5751, January 2010.

 [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
            Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
            June 2010.

8.2. Informative References

 [RFC2459]  Housley, R., Ford, W., Polk, W., and D. Solo, "Internet
            X.509 Public Key Infrastructure Certificate and CRL
            Profile", RFC 2459, January 1999.

 [RFC4732]  Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
            Denial-of-Service Considerations", RFC 4732, December
            2006.

Santesson & Hallam-Baker Standards Track [Page 8] RFC 6277 OCSP Algorithm Agility June 2011

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{}, SMIMECapability{}, SIGNATURE-ALGORITHM,
 PUBLIC-KEY
   FROM AlgorithmInformation-2009 -- From [RFC5912]
     { 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 [RFC5912]
    { 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 PreferredSignatureAlgorithms
    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, {...}},
  pubKeyAlgIdentifier SMIMECapability{PUBLIC-KEY, {...}} OPTIONAL  }

END

Santesson & Hallam-Baker Standards Track [Page 9] RFC 6277 OCSP Algorithm Agility June 2011

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 [RFC2560]
   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 -- From [RFC5280]
    { iso(1) identified-organization(3) dod(6) internet(1) security(5)
      mechanisms(5) pkix(7) id-mod(0) id-pkix1-explicit(18) };

 SMIMECapability
   FROM SecureMimeMessageV3dot1 -- From [RFC5751]
    { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
      smime(16) modules(0) msg-v3dot1(21) }

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

   PreferredSignatureAlgorithms ::= SEQUENCE OF
                                    PreferredSignatureAlgorithm

   PreferredSignatureAlgorithm ::= SEQUENCE {
      sigIdentifier        AlgorithmIdentifier,
      pubKeyAlgIdentifier  SMIMECapability OPTIONAL
      }

END

Santesson & Hallam-Baker Standards Track [Page 10] RFC 6277 OCSP Algorithm Agility June 2011

Authors' Addresses

 Stefan Santesson
 3xA Security AB
 Sweden

 Email: sts@aaa-sec.com

 Phillip Hallam-Baker
 Default Deny Security

 EMail: hallam@gmail.com

Santesson & Hallam-Baker Standards Track [Page 11]