Internet-Draft SASL SCRAM July 2022
Melnikov Expires 12 January 2023 [Page]
Network Working Group
5802, 7677 (if approved)
Intended Status:
Standards Track
A. Melnikov, Ed.
Isode Ltd

Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms


This document updates requirements on implementations of various Salted Challenge Response Authentication Mechanism (SCRAM) Simple Authentication and Security Layer (SASL) mechanisms based on more modern security practices.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on 12 January 2023.

Table of Contents

1. Introduction

The intent of this document is to serve as an implementor's roadmap for implementing various Salted Challenge Response Authentication Mechanism (SCRAM) [RFC5802] SASL [RFC4422] mechanisms.

[RFC5802] defined the generic SCRAM framework and described instantiation of a SCRAM mechanism using SHA-1 hash function: SCRAM-SHA-1 (and SCRAM-SHA-1-PLUS). [RFC7677] described another instantiation using SHA-256 hash function (SCRAM-SHA-256 and SCRAM-SHA-256-PLUS) and also clarified conditions for using the mandatory-to-implement "tls-unique" channel binding with TLS 1.2. [tls-1.3-channel-binding] defines the "tls-exporter" channel binding that is to be used when a SCRAM mechanism is used over TLS 1.3 [RFC8446] or later.

[I-D.melnikov-scram-sha-512] and [I-D.melnikov-scram-sha3-512] define further instantiations of SCRAM using SHA-512 and SHA3-512 hash functions respectively.

[I-D.melnikov-scram-2fa] defines an extension to SCRAM for two factor authentication. It is applicable to all instantiations of SCRAM.

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

3. Implementation Recommendations

[tls-1.3-channel-binding] document updated [RFC5802] and [RFC7677] to use the "tls-exporter" channel binding as the mandatory to implement (instead of "tls-unique") when a SCRAM mechanism is used over TLS 1.3 [RFC8446] or later.

[[Discuss if rough consensus can be reached on this in the KITTEN WG.]] All SCRAM implementations SHOULD support [I-D.melnikov-scram-2fa] to allow for two factor authentication with SCRAM.

[[Possibly narrow down choices to only one of these. Discuss in the KITTEN WG.]] Unless required for backward compatibility, server and client implementations MUST support SCRAM-SHA-512-PLUS/SCRAM-SHA-512 [I-D.melnikov-scram-sha-512] and/or SCRAM-SHA3-512-PLUS/SCRAM-SHA3-512 [I-D.melnikov-scram-sha3-512] instead of SCRAM-SHA-1-PLUS/SCRAM-SHA-1 [RFC5802].

[RFC5803] describes how SCRAM hashes can be stored in LDAP. It is compatible with all versions of SCRAM described in this document, including SCRAM-SHA-256, SCRAM-SHA-512 and SCRAM-SHA3-512.

4. Security Considerations

The security considerations from [RFC5802] still apply.

To be secure, SCRAM-*-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. When using SCRAM over TLS 1.3 [RFC8446], the "tls-exporter" channel binding [tls-1.3-channel-binding] is the default (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 RFC XXXX as an extra reference for the following SASL SCRAM mechanisms listed in the "SASL SCRAM Family Mechanisms" registry: SCRAM-SHA-1, SCRAM-SHA-1-PLUS, SCRAM-SHA-256 and SCRAM-SHA-256-PLUS.

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, , <>.
Melnikov, A., "Lightweight Directory Access Protocol (LDAP) Schema for Storing Salted Challenge Response Authentication Mechanism (SCRAM) Secrets", RFC 5803, DOI 10.17487/RFC5803, , <>.
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, , <>.
Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms", RFC 7677, DOI 10.17487/RFC7677, , <>.
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, , <>.
Melnikov, A., "Extensions to Salted Challenge Response (SCRAM) for 2 factor authentication", Work in Progress, Internet-Draft, draft-melnikov-scram-2fa-03, , <>.
Melnikov, A., "SCRAM-SHA-512 and SCRAM-SHA-512-PLUS Simple Authentication and Security Layer (SASL) Mechanisms", Work in Progress, Internet-Draft, draft-melnikov-scram-sha-512-02, , <>.
Melnikov, A., "SCRAM-SHA3-512 and SCRAM-SHA3-512-PLUS Simple Authentication and Security Layer (SASL) Mechanisms", Work in Progress, Internet-Draft, draft-melnikov-scram-sha3-512-02, , <>.

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