SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms
A short RFC that adds SHA-256 versions of the SCRAM password login and tightens the rules for using channel binding safely.
Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms", RFC 7677, DOI 10.17487/RFC7677, November 2015, <https://www.rfc-editor.org/info/rfc7677>.
@misc{rfc7677,
series = {Request for Comments},
number = 7677,
howpublished = {RFC 7677},
publisher = {RFC Editor},
doi = {10.17487/RFC7677},
url = {https://www.rfc-editor.org/info/rfc7677},
author = {T. Hansen},
title = {{SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms}},
pagetotal = 8,
year = 2015,
month = nov,
} RFC text
Contents
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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.
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.
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.)
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])
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>.
[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.
Author's Address
Tony Hansen
AT&T Laboratories
200 Laurel Ave. South
Middletown, NJ 07748
United States
Email: tony+scramsha256@maillennium.att.com
ISO Plain Language
On this page
§1Why a SHA-256 Version of SCRAM
This document registers two SASL mechanisms, SCRAM-SHA-256 and SCRAM-SHA-256-PLUS. SASL is the Simple Authentication and Security Layer, the framework a protocol uses when it needs to check who is at the other end before letting a conversation start. SHA-256 has stronger security properties than SHA-1, and the RFC expects SCRAM mechanisms based on it to have greater predicted longevity than the ones based on SHA-1.
The registration form for the SCRAM family of algorithms is also updated from RFC 5802.
There was a second reason for the document. After RFC 5802 was published, it was discovered that Transport Layer Security (TLS), described in RFC 5246, does not have the expected properties for the "tls-unique" channel binding to be secure. Channel binding is the mechanism that ties an authentication exchange to the encrypted channel it happens over, so that a stolen password cannot be replayed over a different channel. The RFC therefore contains normative text that applies to both the original SCRAM-SHA-1-PLUS mechanism and the newly introduced SCRAM-SHA-256-PLUS mechanism.
The login method was sound, but the hash function underneath it was showing its age, so this RFC registers a stronger version. It also closes a gap that turned up after the original login method was published: the way some servers tied a login to its encrypted connection was not as safe as everyone assumed.
§3What SCRAM-SHA-256 Changes
The RFC does not redefine SCRAM. The two new mechanisms are defined in the same way that SCRAM-SHA-1 and SCRAM-SHA-1-PLUS are defined in RFC 5802, except that the hash function for HMAC() and H() uses SHA-256 instead of SHA-1, as specified in RFC 6234. The -PLUS variant is the one that uses channel binding; the plain variant is there for clients that do not support it.
One concrete parameter is set. The hash iteration-count announced by a server SHOULD be at least 4096. The GSS-API mechanism OID for SCRAM-SHA-256 is 1.3.6.1.5.5.18.
It is the same login with the same steps. Only the hashing function inside is stronger, and the server is asked to repeat that hashing at least 4096 times before it answers.
§3A SCRAM-SHA-256 Exchange
The RFC gives a simple SCRAM-SHA-256 authentication exchange for a client that does not support channel bindings, using the username user and the password pencil.
The client opens with the username and a random nonce, in the form n,,n=user,r=rOprNGfwEbeRWgbNEkqO. The server answers with its own extended nonce, a salt and the iteration count: r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0, s=W22ZaJ0SNY7soEsUEjb6gQ==, i=4096. The client then sends a channel-binding flag, the combined nonce and a proof: c=biws, r=..., p=dHzbZapWIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=. The server finishes with its own verification value: v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=.
Across the four messages the password itself never appears. What travels is a nonce, a salt, an iteration count and two values computed from the password.
You and the server trade four short notes. Each note proves a bit more that you both know the secret, but the secret itself is never written on any of them.
- n,,n=user,r=rOprNGfwEbeRWgbNEkqOThe client sends the username and a nonce.
- r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0, s=W22ZaJ0SNY7soEsUEjb6gQ==, i=4096The server returns the combined nonce, the salt and the iteration count.
- c=biws, r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0, p=dHzbZapWIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=The client sends its proof.
- v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=The server confirms the proof.
C: n,,n=user,r=rOprNGfwEbeRWgbNEkqO
S: r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0,
s=W22ZaJ0SNY7soEsUEjb6gQ==,i=4096
C: c=biws,r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hNlF$k0,
p=dHzbZapWIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=
S: v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=§4Security Considerations
The security considerations from RFC 5802 still apply. On top of that, the RFC sets one rule for the -PLUS mechanisms: to be secure, SCRAM-SHA-256-PLUS and SCRAM-SHA-1-PLUS MUST be used over a TLS channel that has had the session hash extension of RFC 7627 negotiated, or session resumption MUST NOT have been used. The RFC points at RFC 4270 and RFC 6194 for reasons to move from SHA-1 to a strong security mechanism like SHA-256.
The strength of the mechanism depends in part on the hash iteration-count, written i in RFC 5802. As a rule of thumb the iteration-count should be such that a modern machine will take 0.1 seconds to perform the complete algorithm. The RFC notes that this is unlikely to be practical on mobile devices and other relatively low-performance systems: at the time of writing the rule of thumb gives around 15,000 iterations, while a hash iteration-count of 4096 takes around 0.5 seconds on current mobile handsets.
The computational cost can be avoided by caching the ClientKey, assuming the Salt and the hash iteration-count are stable. The recommendation of the specification is therefore 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.
The more times the server repeats the hashing, the harder it is for a thief to guess the password, but the slower things get on a phone. So the RFC sets a floor of 4096 and asks for more where the devices are not phones.
§5Registering the SCRAM Family
Section 5.1 updates the registration of the SCRAM family in the SASL mechanism registry of RFC 4422. The email address for reviews is updated and the note at the end changes. Members of the family MUST be explicitly registered using the "IETF Review" procedure of RFC 5226, and reviews MUST be requested on the KITTEN mailing list. The intended usage is COMMON and the change controller is the IESG. A note is added for future designers: each new SASL SCRAM mechanism MUST be explicitly registered with IANA within the SASL SCRAM Family Mechanisms registry.
Section 5.2 adds a new IANA registry named "SASL SCRAM Family Mechanisms", which gives every member of the family two new fields: a minimum iteration-count and an associated OID. The existing entries for SASL SCRAM-SHA-1 and SASL SCRAM-SHA-1-PLUS move from the existing SASL mechanism registry into the new one, with a minimum iteration-count of 4096 and the OID 1.3.6.1.5.5.14 that came from RFC 5802.
Two new mechanisms are added to the new registry. SCRAM-SHA-256 and SCRAM-SHA-256-PLUS both get a minimum iteration-count of 4096 and the OID 1.3.6.1.5.5.18. Only one OID is needed for a SCRAM-NAME and SCRAM-NAME-PLUS pair, and the same OID is given to both entries.
IANA keeps the list of every official login method. This RFC adds a proper shelf for the SCRAM family, moves the older entries onto it with their numbers, and puts the two new ones there too with a floor on how many times the hashing must be repeated.