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Security Kitten Requests for Comment This document registers: the SASL mechanisms SCRAM-SHA-256 and SCRAM-SHA-256-PLUS, provdes guidance for secure implentation of the original SCRAM-SHA-1-PLUS mechanism, and updates the SCRAM registration procedures of RFC 5802.

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 . After publication of , it was discovered that Transport Layer Security (TLS) does not have the expected properties for the tls-unique channel binding to be secure . 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. Note: this paragraph may be removed before publication. This document was written because requires that new SASL mechanisms in the SCRAM family be subject to IETF review. This document is being discussed in the KITTEN working group (see the kitten@ietf.org mailing list). It was pursued further because of a desire for its use within a document being discussed in the HTTP-AUTH working group (see the httpauth@ietf.org mailing list).

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 .

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 , except that the hash function for HMAC() and H() uses SHA-256 instead of SHA-1 . 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. The GSS-API mechanism OID for SCRAM-SHA-256 is TBD1 (see ). 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. n,,n=user,r=rOprNGfwEbeRWgbNEkqO r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0, s=W22ZaJ0SNY7soEsUEjb6gQ==,i=4096 c=biws,r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0, p=dHzbZapWIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ= v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=

The security considerations from still apply. To be secure, SCRAM-SHA-256-PLUS and SCRAM-SHA-1-PLUS MUST either be used over a TLS channel that has had negotiated, or session resumption MUST NOT have been used. See and 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 . 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 an 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 iteration count is stable). Therefore the recommendation of this specification is that the 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.

The IANA registry for SCRAM-* (the SCRAM family of SASL mechanisms) in the SASL Mechanism registry () is 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 Published specification (optional, recommended): RFCXXXX 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" 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.

A new IANA registry is to be 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. 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 Published specification (optional, recommended): RFCXXXX Minimum iteration-count: The minimum 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" 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. [RFC Editor: This note should be removed before publication.] Note to IANA and the RFC Editor: The above string "TBD-BY-IANA" is NOT to be filled in with an OID within THIS document, but is to be placed as is within 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 . The existing entries for SASL SCRAM-SHA-1 and SCRAM-SHA-1-PLUS are to be moved from the existing SASL Mechanism registry to the SASL SCRAM Family Mechanism registry. When doing so, the following values are to be added: Minimum iteration-count: 4096 OID: 1.3.6.1.5.5.14 (from ) The following new SASL SCRAM mechanisms are added to the SASL SCRAM Family Mechanism registry: IANA has added the following entries to the SASL SCRAM Family Mechanism registry established by RFCXXXX: 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 of RFCXXXX Published specification (optional, recommended): RFCXXXX Minimum iteration-count: 4096 OID: TBD1 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 of RFCXXXX Published specification (optional, recommended): RFCXXXX Minimum iteration-count: 4096 OID: "TBD1" 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: [This note may be removed on publication.] IANA needs to assign the GSS-API mechanism OID TBD1 listed above from the iso.org.dod.internet.security.mechanisms prefix (see the "SMI Security for Mechanism Codes" registry).

This document benefited from discussions on the KITTEN WG mailing list. The author would like to specially thank Russ Albery, 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.

This section should be removed before publication as an RFC.

Added text to the Abstract, Introduction, and Security Considerations sections regarding tls-session-hash negotiation.

Changed from Informational document to Standards Track. Beefed up the Security Considerations section. At the request of IANA, reworked the IANA Considerations section.

Removed !!!!!!!!!!!!!!!! comments requesting discussion after discussion on kitten mailing list.

Added Security Considerations section. Added Minimum iteration-count and associated OID fields to registration forms and reworked the IANA Considerations section.