| Kitten | T. Hansen |
| Internet-Draft | AT&T Laboratories |
| Updates: 5802 (if approved) | July 20, 2015 |
| Intended status: Standards Track | |
| Expires: January 21, 2016 |
SCRAM-SHA-256 and SCRAM-SHA-256-PLUS SASL Mechanisms
draft-hansen-scram-sha256-03
This document registers the SASL mechanisms SCRAM-SHA-256 and SCRAM-SHA-256-PLUS. It also updates the SCRAM registration procedures of RFC 5802.
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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].
Note: this paragraph may be removed before publication.
This document was written because [RFC5802] 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 [RFC2119].
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.
The GSS-API mechanism OID for SCRAM-SHA-256 is TBD1 (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.
The security considerations from [RFC5802] still apply.
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 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 ([RFC4422]) is updated as follows. The email address for reviews has been updated, and the note at the end changed.
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.
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:
The following new SASL SCRAM mechanisms are added to the SASL SCRAM Family Mechanism registry:
[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, Simon Josefsson, Pearl Liang, Alexey Melnikov, Peter Saint-Andre, Martin Thompson and Nico Williams for their comments on this topic.
| [RFC4422] | Melnikov, A. and K. Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, DOI 10.17487/RFC4422, June 2006. |
| [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. |
| [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. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC4270] | Hoffman, P. and B. Schneier, "Attacks on Cryptographic Hashes in Internet Protocols", RFC 4270, DOI 10.17487/RFC4270, November 2005. |
| [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. |
| [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. |
This section should be removed before publication as an RFC.
Changed from Informational document to Standards Track.
Beefed up the Security Considerations [security] section.
At the request of IANA, reworked the IANA Considerations [iana] section.
Removed !!!!!!!!!!!!!!!! comments requesting discussion after discussion on kitten mailing list.
Added Security Considerations [security] section.
Added Minimum iteration-count and associated OID fields to registration forms and reworked the IANA Considerations [iana] section.