The Hashed Token SASL Mechanism

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Table of Contents

1. Introduction

This specification describes the family of Hashed Token (HT) Simple Authentication and Security Layer (SASL) [RFC4422] mechanisms, which enable a proof-of-possession-based authentication scheme. The HT mechanism is designed to be used with short-lived, exclusively ephemeral tokens, called SASL-HT tokens, and allow for quick, one round-trip, re-authentication of a previous session.¶

Further properties of the HT mechanism are 1) hash agility, 2) mutual authentication, and 3) being secured by channel binding.¶

Clients are supposed to request SASL-HT tokens from the server after being authenticated using a "strong" SASL mechanism like SCRAM [RFC5802]. Hence a typical sequence of actions using HT may look like the following:¶

A) Client authenticates using a strong mechanism (e.g., SCRAM)
B) Client requests secret SASL-HT token
C) Service returns SASL-HT token
   <normal client-server interaction here>
D) Connection between client and server gets interrupted,
   for example because of a WiFi ↔ GSM switch
E) Client resumes the previous session using HT and token from C)
F) Service revokes the successfully used SASL-HT token
   [goto B]

The HT mechanism requires an accompanying, application protocol specific, extension, which allows clients to requests a new SASL-HT token (see Section 5 (Section 5)). One example for such an application protocol specific extension based on HT is [XEP-0397]. This XMPP [RFC6120] extension protocol allows, amongst other things, B) and C),¶

Since the SASL-HT token is not salted, and only one hash iteration is used, the HT mechanism is not suitable to protect long-lived shared secrets (e.g. "passwords"). You may want to look at [RFC5802] for that.¶

2. The HT Family of Mechanisms

Each mechanism in this family differs by choice of the hash algorithm and the choice of the channel binding [RFC5929] type.¶

An HT mechanism name is a string beginning with "HT-" followed by the capitalised name of the used hash, followed by "-", and suffixed by one of 'ENDP' and 'UNIQ'.¶

Hence each HT mechanism has a name of the following form:¶

HT-<hash-alg>-<cb-type>

Where <hash-alg> is the capitalised "Hash Name String" of the IANA "Named Information Hash Algorithm Registry" [iana-hash-alg] as specified in [RFC6920], and <cb-type> is one of 'ENDP', 'UNIQ', or 'EXPR' denoting the channel binding type. In the case of 'ENDP', the tls-server-end-point channel binding type is used. In the case of 'UNIQ', the tls-unique channel binding type is used. In the case of 'EXPR', the tls-exporter [RFC9266] channel binding type is used. Valid channel binding types are defined in the IANA "Channel-Binding Types" registry [iana-cbt] as specified in [RFC5056].¶

The following table lists some examples of HT SASL mechanisms registered by this document.¶

3. The HT Authentication Exchange

The mechanism consists of a simple exchange of precisely two messages between the initiator and responder.¶

The following syntax specifications use the Augmented Backus-Naur form (ABNF) notation as specified in [RFC5234].¶

initiator-msg = authcid NUL initiator-hashed-token
authcid = 1*SAFE ; MUST accept up to 255 octets
initiator-hashed-token = 1*OCTET

NUL    = %0x00 ; The null octet
SAFE   = UTF1 / UTF2 / UTF3 / UTF4
         ;; any UTF-8 encoded Unicode character except NUL

UTF1   = %x01-7F ;; except NUL
UTF2   = %xC2-DF UTF0
UTF3   = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) /
         %xED %x80-9F UTF0 / %xEE-EF 2(UTF0)
UTF4   = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) /
         %xF4 %x80-8F 2(UTF0)
UTF0   = %x80-BF

initiator-hashed-token := HMAC(token, "Initiator" || cb-data)

responder-msg = 1*OCTET

responder-msg := HMAC(token, "Responder" || cb-data)

4. Compliance with SASL Mechanism Requirements

This section describes compliance with SASL mechanism requirements specified in Section 5 of [RFC4422].¶

1. "HT-SHA-256-ENDP", "HT-SHA-256-UNIQ", "HT-SHA-3-512-ENDP" and "HT-SHA-3-512-UNIQ".¶
2. Definition of server-challenges and client-responses: a) HT is a client-first mechanism. b) HT does send additional data with success (the responder-msg).¶
3. HT is not capable of transferring authorization identities from the client to the server.¶
4. HT does not offer any security layers (HT offers channel binding instead).¶
5. HT does not protect the authorization identity.¶

5. Requirements for the Application-Protocol Extension

It is REQUIRED that the application-protocol specific extension provides a mechanism to request a SASL-HT token in form of a Unicode string. The returned token MUST have been newly generated by a cryptographically secure random number generator and MUST contain at least 128 bit of entropy.¶

It is RECOMMENDED that the protocol allows the requestor to signal the name of the SASL mechanism which he intends to use with the token. If a token is used with a different mechanism than the one which was signalled upon requesting the token, then the authentication MUST fail. This allows pinning the token to a SASL mechanism, which increases the security because it makes it impossible for an attacker to downgrade the SASL mechanism.¶

6. Security Considerations

To be secure, the HT mechanism MUST be used over a TLS channel that has had the session hash extension [RFC7627] negotiated, or session resumption MUST NOT have been used.¶

It is RECOMMENDED that implementations periodically require a full authentication using a strong SASL mechanism which does not use the SASL-HT token.¶

It is of vital importance that the SASL-HT token is generated by a cryptographically secure random generator. See [RFC4086] for more information about Randomness Requirements for Security.¶

7. IANA Considerations

IANA is requested to add the following family of SASL mechanisms to the SASL Mechanism registry established by [RFC4422]:¶

• To: iana@iana.org¶

  Subject: Registration of a new SASL family HT¶

  SASL mechanism name (or prefix for the family): HT-*¶

  Security considerations: Section 6 of draft-schmaus-kitten-sasl-ht¶

  Published specification (optional, recommended): draft-schmaus-kitten-sasl-ht-XX (TODO)¶

  Person & email address to contact for further information: IETF SASL 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 WG mailing list kitten@ietf.org (or a successor designated by the responsible Security AD).¶

Acknowledgments

This document benefited from discussions on the KITTEN WG mailing list. The authors would like to especially thank Thijs Alkemade, Sam Whited and Alexey Melnikov for their comments on this topic. Furthermore, we would like to thank Alexander Wuerstlein, who came up with the idea to pin the token to a SASL mechanism for increased security.¶

Authors' Addresses