SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms
RFC 7677

Internet Engineering Task Force (IETF)                         T. Hansen
Request for Comments: 7677                             AT&T Laboratories
Updates: 5802                                              November 2015
Category: Standards Track
ISSN: 2070-1721

                  SCRAM-SHA-256 and SCRAM-SHA-256-PLUS
       Simple Authentication and Security Layer (SASL) Mechanisms

Abstract

   This document registers the Simple Authentication and Security Layer
   (SASL) mechanisms SCRAM-SHA-256 and SCRAM-SHA-256-PLUS, provides
   guidance for secure implementation of the original SCRAM-SHA-1-PLUS
   mechanism, and updates the SCRAM registration procedures of RFC 5802.

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/rfc7677.

Copyright Notice

   Copyright (c) 2015 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
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   described in the Simplified BSD License.

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RFC 7677          SASL SCRAM-SHA-256/SCRAM-SHA-256-PLUS    November 2015

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Key Word Definitions  . . . . . . . . . . . . . . . . . . . .   2
   3.  SCRAM-SHA-256 and SCRAM-SHA-256-PLUS  . . . . . . . . . . . .   2
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
     5.1.  Updates to SCRAM-* Registration . . . . . . . . . . . . .   4
     5.2.  SASL-SCRAM Family Mechanisms Registration Procedure . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   This document registers the SASL mechanisms SCRAM-SHA-256 and SCRAM-
   SHA-256-PLUS.  SHA-256 has stronger security properties than SHA-1,
   and it is expected that SCRAM mechanisms based on it will have
   greater predicted longevity than the SCRAM mechanisms based on SHA-1.

   The registration form for the SCRAM family of algorithms is also
   updated from [RFC5802].

   After publication of [RFC5802], it was discovered that Transport
   Layer Security (TLS) [RFC5246] does not have the expected properties
   for the "tls-unique" channel binding to be secure [RFC7627].
   Therefore, this document contains normative text that applies to both
   the original SCRAM-SHA-1-PLUS and the newly introduced SCRAM-SHA-
   256-PLUS mechanism.

2.  Key Word Definitions

   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 [RFC2119].

3.  SCRAM-SHA-256 and SCRAM-SHA-256-PLUS

   The SCRAM-SHA-256 and SCRAM-SHA-256-PLUS SASL mechanisms are defined
   in the same way that SCRAM-SHA-1 and SCRAM-SHA-1-PLUS are defined in
   [RFC5802], except that the hash function for HMAC() and H() uses
   SHA-256 instead of SHA-1 [RFC6234].

   For the SCRAM-SHA-256 and SCRAM-SHA-256-PLUS SASL mechanisms, the
   hash iteration-count announced by a server SHOULD be at least 4096.

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RFC 7677          SASL SCRAM-SHA-256/SCRAM-SHA-256-PLUS    November 2015

   The GSS-API mechanism OID for SCRAM-SHA-256 is 1.3.6.1.5.5.18 (see
   Section 5).

   This is a simple example of a SCRAM-SHA-256 authentication exchange
   when the client doesn't support channel bindings.  The username
   'user' and password 'pencil' are being used.

   C: n,,n=user,r=rOprNGfwEbeRWgbNEkqO

   S: r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0,
      s=W22ZaJ0SNY7soEsUEjb6gQ==,i=4096

   C: c=biws,r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0,
      p=dHzbZapWIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=

   S: v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=

4.  Security Considerations

   The security considerations from [RFC5802] still apply.

   To be secure, either SCRAM-SHA-256-PLUS and SCRAM-SHA-1-PLUS MUST be
   used over a TLS channel that has had the session hash extension
   [RFC7627] negotiated, or session resumption MUST NOT have been used.

   See [RFC4270] and [RFC6194] for reasons to move from SHA-1 to a
   strong security mechanism like SHA-256.

   The strength of this mechanism is dependent in part on the hash
   iteration-count, as denoted by "i" in [RFC5802].  As a rule of thumb,
   the hash iteration-count should be such that a modern machine will
   take 0.1 seconds to perform the complete algorithm; however, this is
   unlikely to be practical on mobile devices and other relatively low-
   performance systems.  At the time this was written, the rule of thumb
   gives around 15,000 iterations required; however, a hash iteration-
   count of 4096 takes around 0.5 seconds on current mobile handsets.
   This computational cost can be avoided by caching the ClientKey
   (assuming the Salt and hash iteration-count is stable).  Therefore,
   the recommendation of this specification is that the hash iteration-
   count SHOULD be at least 4096, but careful consideration ought to be
   given to using a significantly higher value, particularly where
   mobile use is less important.

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5.  IANA Considerations

5.1.  Updates to SCRAM-* Registration

   The IANA registry for SCRAM-* (the SCRAM family of SASL mechanisms)
   in the SASL mechanism registry ([RFC4422]) has been updated as
   follows.  The email address for reviews has been updated, and the
   note at the end changed.

      To: iana@iana.org
      Subject: Registration of a new SASL family SCRAM

      SASL mechanism name (or prefix for the family): SCRAM-*
      Security considerations: Section 7 of [RFC5802]
      Published specification (optional, recommended): RFC 7677
      Person & email address to contact for further information:
         IETF KITTEN WG <kitten@ietf.org>
      Intended usage: COMMON
      Owner/Change controller: IESG <iesg@ietf.org>
      Note: Members of this family MUST be explicitly registered using
         the "IETF Review" [RFC5226] registration procedure.  Reviews
         MUST be requested on the KITTEN mailing list kitten@ietf.org
         (or a successor designated by the responsible Security AD).

      Note to future SCRAM-mechanism designers: each new SASL SCRAM
      mechanism MUST be explicitly registered with IANA within the SASL
      SCRAM Family Mechanisms registry.

5.2.  SASL-SCRAM Family Mechanisms Registration Procedure

   A new IANA registry has been added for members of the SCRAM family of
   SASL mechanisms, named "SASL SCRAM Family Mechanisms".  It adds two
   new fields to the existing SCRAM mechanism registry: Minimum
   iteration-count and Associated OID.  Below is the template for
   registration of a new SASL family SCRAM.  (Note that the string
   "TBD-BY-IANA" should be left as is, so that it may be filled in at
   registration time by IANA.)

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      To: iana@iana.org
      Subject: Registration of a new SASL SCRAM family mechanism

      SASL mechanism name (or prefix for the family): SCRAM-<NAME>
      Security considerations: Section 7 of [RFC5802]
      Published specification (optional, recommended): RFC 7677
      Minimum iteration-count: The minimum hash iteration-count that
         servers SHOULD announce
      Associated OID: TBD-BY-IANA
      Person & email address to contact for further information:
         IETF KITTEN WG <kitten@ietf.org>
      Intended usage: COMMON
      Owner/Change controller: IESG <iesg@ietf.org>

      Note: Members of this family MUST be explicitly registered using
      the "IETF Review" [RFC5226] registration procedure.  Reviews MUST
      be requested on the KITTEN mailing list kitten@ietf.org (or a
      successor designated by the responsible Security Area Director).

      Note: At publication of a new SASL SCRAM Family Mechanism, IANA
      SHOULD assign a GSS-API mechanism OID for this mechanism from the
      iso.org.dod.internet.security.mechanisms prefix (see the "SMI
      Security for Mechanism Codes" registry) and fill in the value for
      "TBD-BY-IANA" above.  Only one OID needs to be assigned for a
      SCRAM-<NAME> and SCRAM-<NAME>-PLUS pair.  The same OID should be
      assigned to both entries in the registry.

      Note to future SASL SCRAM mechanism designers: each new SASL SCRAM
      mechanism MUST be explicitly registered with IANA and MUST comply
      with the SCRAM-mechanism naming convention defined in Section 4 of
      [RFC5802].

   The existing entries for SASL SCRAM-SHA-1 and SCRAM-SHA-1-PLUS have
   been moved from the existing SASL mechanism registry to the "SASL
   SCRAM Family Mechanisms" registry.  At that time, the following
   values were added:

      Minimum iteration-count: 4096
      OID: 1.3.6.1.5.5.14 (from [RFC5802])

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RFC 7677          SASL SCRAM-SHA-256/SCRAM-SHA-256-PLUS    November 2015

   The following new SASL SCRAM mechanisms have been added to the "SASL
   SCRAM Family Mechanisms" registry:

      To: iana@iana.org
      Subject: Registration of a new SASL SCRAM Family mechanism
         SCRAM-SHA-256

      SASL mechanism name (or prefix for the family): SCRAM-SHA-256
      Security considerations: Section 4 of RFC 7677
      Published specification (optional, recommended): RFC 7677
      Minimum iteration-count: 4096
      OID: 1.3.6.1.5.5.18
      Person & email address to contact for further information:
         IETF KITTEN WG <kitten@ietf.org>
      Intended usage: COMMON
      Owner/Change controller: IESG <iesg@ietf.org>
      Note:

      To: iana@iana.org
      Subject: Registration of a new SASL SCRAM Family mechanism
         SCRAM-SHA-256-PLUS

      SASL mechanism name (or prefix for the family): SCRAM-SHA-256-PLUS
      Security considerations: Section 4 of RFC 7677
      Published specification (optional, recommended): RFC 7677
      Minimum iteration-count: 4096
      OID: 1.3.6.1.5.5.18
      Person & email address to contact for further information:
         IETF KITTEN WG <kitten@ietf.org>
      Intended usage: COMMON
      Owner/Change controller: IESG <iesg@ietf.org>
      Note:

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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4422]  Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
              Authentication and Security Layer (SASL)", RFC 4422,
              DOI 10.17487/RFC4422, June 2006,
              <http://www.rfc-editor.org/info/rfc4422>.

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   [RFC5802]  Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
              "Salted Challenge Response Authentication Mechanism
              (SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
              DOI 10.17487/RFC5802, July 2010,
              <http://www.rfc-editor.org/info/rfc5802>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <http://www.rfc-editor.org/info/rfc6234>.

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <http://www.rfc-editor.org/info/rfc7627>.

6.2.  Informative References

   [RFC4270]  Hoffman, P. and B. Schneier, "Attacks on Cryptographic
              Hashes in Internet Protocols", RFC 4270,
              DOI 10.17487/RFC4270, November 2005,
              <http://www.rfc-editor.org/info/rfc4270>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
              <http://www.rfc-editor.org/info/rfc6194>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

Acknowledgements

   This document benefited from discussions on the KITTEN WG mailing
   list.  The author would like to specially thank Russ Allbery, Dave
   Cridland, Shawn Emery, Stephen Farrell, Simon Josefsson, Pearl Liang,
   Alexey Melnikov, Peter Saint-Andre, Robert Sparks, Martin Thompson,
   and Nico Williams for their comments on this topic.

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Author's Address

   Tony Hansen
   AT&T Laboratories
   200 Laurel Ave. South
   Middletown, NJ  07748
   United States

   Email: tony+scramsha256@maillennium.att.com

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