1. RFC 8954
Internet Engineering Task Force (IETF)                     M. Sahni, Ed.
Request for Comments: 8954                            Palo Alto Networks
Updates: 6960                                              November 2020
Category: Standards Track                                               
ISSN: 2070-1721

       Online Certificate Status Protocol (OCSP) Nonce Extension


   This document specifies the updated format of the Nonce extension in
   the Online Certificate Status Protocol (OCSP) request and response
   messages.  OCSP is used to check the status of a certificate, and the
   Nonce extension is used to cryptographically bind an OCSP response
   message to a particular OCSP request message.  This document updates
   RFC 6960.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Table of Contents

   1.  Introduction
     1.1.  Terminology
   2.  OCSP Extensions
     2.1.  Nonce Extension
   3.  Security Considerations
     3.1.  Replay Attack
     3.2.  Nonce Collision
   4.  IANA Considerations
   5.  Changes to Appendix B of RFC 6960
     5.1.  Changes to Appendix B.1 OCSP in ASN.1 - 1998 Syntax
     5.2.  Changes to Appendix B.2 OCSP in ASN.1 - 2008 Syntax
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Author's Address

1.  Introduction

   This document updates the usage and format of the Nonce extension in
   OCSP request and response messages.  This extension was previously
   defined in Section 4.4.1 of [RFC6960].  [RFC6960] does not mention
   any minimum or maximum length of the nonce in the Nonce extension.
   Lacking limits on the length of the nonce in the Nonce extension,
   OCSP responders that follow [RFC6960] may be vulnerable to various
   attacks, like Denial-of-Service attacks [RFC4732] or chosen-prefix
   attacks (to get a desired signature), and possible evasions using the
   Nonce extension data.  This document specifies a lower limit of 1 and
   an upper limit of 32 for the length of the nonce in the Nonce
   extension.  This document updates [RFC6960].

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  OCSP Extensions

   The message formats for OCSP requests and responses are defined in
   [RFC6960].  [RFC6960] also defines the standard extensions for OCSP
   messages based on the extension model employed in X.509 version 3
   certificates (see [RFC5280]).  This document only specifies the new
   format for the Nonce extension and does not change the specifications
   of any of the other standard extensions defined in [RFC6960].

2.1.  Nonce Extension

   This section replaces the entirety of Section 4.4.1 of [RFC6960],
   which describes the OCSP Nonce extension.

   The nonce cryptographically binds a request and a response to prevent
   replay attacks.  The nonce is included as one of the
   requestExtensions in requests; in responses, it would be included as
   one of the responseExtensions.  In both the request and the response,
   the nonce will be identified by the object identifier id-pkix-ocsp-
   nonce, while the extnValue is the value of the nonce.  If the Nonce
   extension is present, then the length of the nonce MUST be at least 1
   octet and can be up to 32 octets.

   A server MUST reject any OCSP request that has a nonce in the Nonce
   extension with a length of either 0 octets or more than 32 octets
   with the malformedRequest OCSPResponseStatus, as described in
   Section 4.2.1 of [RFC6960].

   The value of the nonce MUST be generated using a cryptographically
   strong pseudorandom number generator (see [RFC4086]).  The minimum
   nonce length of 1 octet is defined to provide backward compatibility
   with older clients that follow [RFC6960].  Newer OCSP clients that
   support this document MUST use a length of 32 octets for the nonce in
   the Nonce extension.  OCSP responders MUST accept lengths of at least
   16 octets and MAY choose to ignore the Nonce extension for requests
   where the length of the nonce is less than 16 octets.

      id-pkix-ocsp           OBJECT IDENTIFIER ::= { id-ad-ocsp }
      id-pkix-ocsp-nonce     OBJECT IDENTIFIER ::= { id-pkix-ocsp 2 }

      Nonce ::= OCTET STRING(SIZE(1..32))

3.  Security Considerations

   The security considerations of OCSP, in general, are described in
   [RFC6960].  During the interval in which the previous OCSP response
   for a certificate is not expired but the responder has a changed
   status for that certificate, a copy of that OCSP response can be used
   to indicate that the status of the certificate is still valid.
   Including a client's nonce value in the OCSP response makes sure that
   the response is the latest response from the server and not an old

3.1.  Replay Attack

   The Nonce extension is used to avoid replay attacks.  Since the OCSP
   responder may choose not to send the Nonce extension in the OCSP
   response even if the client has sent the Nonce extension in the
   request [RFC5019], an on-path attacker can intercept the OCSP request
   and respond with an earlier response from the server without the
   Nonce extension.  This can be mitigated by configuring the server to
   use a short time interval between the thisUpdate and nextUpdate
   fields in the OCSP response.

3.2.  Nonce Collision

   If the value of the nonce used by a client in the OCSP request is
   predictable, then an attacker may prefetch responses with the
   predicted nonce and can replay them, thus defeating the purpose of
   using the nonce.  Therefore, the value of the Nonce extension in the
   OCSP request MUST contain cryptographically strong randomness and
   MUST be freshly generated at the time of the creation of the OCSP
   request.  Also, if the length of the nonce is too small (e.g., 1
   octet), then an on-path attacker can prefetch responses with all the
   possible values of the nonce and replay a matching nonce.

4.  IANA Considerations

   This document has no IANA actions.

5.  Changes to Appendix B of RFC 6960

   This section updates the ASN.1 definitions of the OCSP Nonce
   extension in Appendices B.1 and B.2 of [RFC6960].  Appendix B.1
   defines OCSP using ASN.1 - 1998 Syntax; Appendix B.2 defines OCSP
   using ASN.1 - 2008 Syntax.

5.1.  Changes to Appendix B.1 OCSP in ASN.1 - 1998 Syntax

   OLD Syntax:

   The definition of OCSP Nonce extension is not provided in
   Appendix B.1 of [RFC6960] for the ASN.1 - 1998 Syntax.

   NEW Syntax:

       Nonce ::= OCTET STRING(SIZE(1..32))

5.2.  Changes to Appendix B.2 OCSP in ASN.1 - 2008 Syntax

   OLD Syntax:

           BY id-pkix-ocsp-nonce }

   NEW Syntax:

       re-ocsp-nonce EXTENSION ::= { SYNTAX OCTET STRING(SIZE(1..32))
           IDENTIFIED BY id-pkix-ocsp-nonce }

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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, DOI 10.17487/RFC5280, May 2008,

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

6.2.  Informative References

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,

   [RFC4732]  Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
              Denial-of-Service Considerations", RFC 4732,
              DOI 10.17487/RFC4732, December 2006,

   [RFC5019]  Deacon, A. and R. Hurst, "The Lightweight Online
              Certificate Status Protocol (OCSP) Profile for High-Volume
              Environments", RFC 5019, DOI 10.17487/RFC5019, September
              2007, <https://www.rfc-editor.org/info/rfc5019>.

Author's Address

   Mohit Sahni (editor)
   Palo Alto Networks
   3000 Tannery Way
   Santa Clara, CA 95054
   United States of America

   Email: msahni@paloaltonetworks.com
  1. RFC 8954