Internet-Draft SASL SCRAM-SHA-512/SCRAM-SHA-512-PLUS October 2021
Melnikov Expires 22 April 2022 [Page]
Network Working Group
Intended Status:
Standards Track
A. Melnikov, Ed.
Isode Ltd

SCRAM-SHA-512 and SCRAM-SHA-512-PLUS Simple Authentication and Security Layer (SASL) Mechanisms


This document registers the Simple Authentication and Security Layer (SASL) mechanisms SCRAM-SHA-512 and SCRAM-SHA-512-PLUS.

Status of This Memo

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This Internet-Draft will expire on 22 April 2022.

Table of Contents

1. Introduction

This document registers the SASL [RFC4422] mechanisms SCRAM-SHA-512 and SCRAM-SHA-512-PLUS. SHA-512 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.

2. Key Word Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all


The SCRAM-SHA-512 and SCRAM-SHA-512-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-512 instead of SHA-1 [RFC6234].

For the SCRAM-SHA-512 and SCRAM-SHA-512-PLUS SASL mechanisms, the hash iteration-count announced by a server SHOULD be at least 10000.

The GSS-API mechanism OID for SCRAM-SHA-512 is<TBD> (see Section 5).

This is a simple example of a SCRAM-SHA-512 authentication exchange when the client doesn't support channel bindings. The username 'user' and password 'pencil' are being used.

[[TBD: Add an example]]

4. Security Considerations

The security considerations from [RFC5802] still apply.

To be secure, SCRAM-SHA-512-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. When using SCRAM over TLS 1.2 [RFC5246], the "tls-unique" channel binding is still the default channel binding to use (see Section 6.1 of [RFC5802]), assuming the above conditions are satisfied. As "tls-unique" channel binding is not defined for TLS 1.3 [RFC8446], when using SCRAM over TLS 1.3, the "tls-exporter" channel binding [tls-1.3-channel-binding] MUST be the default channel binding (in the sense specified in Section 6.1 of [RFC5802]) to use.

See [RFC4270] and [RFC6194] for reasons to move from SHA-1 to a strong security mechanism like SHA-512.

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 10000 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 10000, but careful consideration ought to be given to using a significantly higher value, particularly where mobile use is less important.

5. IANA Considerations

IANA is requested to add the following new SASL SCRAM mechanisms to the "SASL SCRAM Family Mechanisms" registry:

6. References

6.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple Authentication and Security Layer (SASL)", RFC 4422, DOI 10.17487/RFC4422, , <>.
Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, , <>.
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, , <>.
Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, , <>.
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, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <>.
Whited, S., "Channel Bindings for TLS 1.3", Work in Progress, Internet-Draft, draft-ietf-kitten-tls-channel-bindings-for-tls13-11, , <>.

6.2. Informative References

Hoffman, P. and B. Schneier, "Attacks on Cryptographic Hashes in Internet Protocols", RFC 4270, DOI 10.17487/RFC4270, , <>.
Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, DOI 10.17487/RFC5226, , <>.
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, , <>.


This document is based on RFC 7677 by Tony Hansen.

Thank you to Ludovic Bocquet for comments and corrections.

Author's Address

Alexey Melnikov (editor)
Isode Ltd
14 Castle Mews
TW12 2NP
United Kingdom