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.
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Hansen Standards Track [Page 1]
RFC 7677 SASL SCRAM-SHA-256/SCRAM-SHA-256-PLUS November 2015
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.
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.
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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.
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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RFC 7677 SASL SCRAM-SHA-256/SCRAM-SHA-256-PLUS November 2015
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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RFC 7677 SASL SCRAM-SHA-256/SCRAM-SHA-256-PLUS November 2015
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:
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:
RFC 7677 SASL SCRAM-SHA-256/SCRAM-SHA-256-PLUS November 2015
[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>.
[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>.
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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RFC 7677 SASL SCRAM-SHA-256/SCRAM-SHA-256-PLUS November 2015
Author's Address
Tony Hansen AT&T Laboratories 200 Laurel Ave. South Middletown, NJ 07748 United States