The Hashed Token SASL Mechanism
draft-schmaus-sasl-ht

Abstract

This document specifies a new SASL mechanism designed to authenticate using short-lived, exclusively emphermeral tokens.

Status of This Memo

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

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This Internet-Draft will expire on September 23, 2017.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Conventions and Terminology
• 1.2. Applicability
• 2. The HT-* Family of Mechanisms
• 3. The HT Mechanism
• 3.1. Compliance with SASL Mechanism Requirements
• 4. Security Considerations
• 5. IANA Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Appendix A. Acknowledgements
• Author's Address

1. Introduction

This section specifies the the family of Hashed Token (HT-) SASL mechanisms. This mechanism was designed to be used with short-lived tokens, used as shared secrets, for authentication. It provides hash agility, mutual authentication and is secured by channel binding. Since the 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.

1.1. Conventions and Terminology

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 RFC 2119 [RFC2119].

Additionally, the key words "MIGHT", "COULD", "MAY WISH TO", "WOULD PROBABLY", "SHOULD CONSIDER", and "MUST (BUT WE KNOW YOU WON'T)" in this document are to interpreted as described in RFC 6919 [RFC6919].

1.2. Applicability

Because this mechanism transports information that should not be controlled by an attacker, the HT-* mechanism MUST only be used over channels protected by TLS, or over similar integrity-protected and authenticated channels. In addition, when TLS is used, the client MUST successfully validate the server's certificate ([RFC5280],[RFC6125]).

2. The HT-* Family of Mechanisms

Each mechanism in this family differs by the choice of the hash algorithm and the choice of the channel binding [RFC5929] type. Each mechanism has a name of the form HT-[HA]-[CBT] where [HA] is the capitalized "Hash Name String" of the IANA "Named Information Hash Algorithm Registry" [iana-hash-alg] as specified in [RFC6920], and [CBT] is one of 'ENDP' or 'UNIQ' denoting the channel binding type. In case of 'ENDP', the tls-server-end-point channel binding type is used. In case of 'UNIQ', the tls-unique 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 a few examples of HT-* SASL mechanism names.

3. The HT Mechanism

The mechanism consists of a simple exchange of exactly two messages between the initiator and responder. It starts with the message from the initiator to the responder. This 'initiator-message' is defined as follows:

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

HMAC() is the function defined in [RFC2104] with H being the chosen hash algorithm, 'cb-data' represents the data provided by the channel binding type, and 'token' are the UTF-8 encoded bytes of the token String which acts as shared secret between initiator and responder. The initiator-message MUST NOT be included in TLS 1.3 0-RTT early data ([I-D.ietf-tls-tls13]).

This message is followed by a message from the responder to the initiator. This 'responder-message' is defined as follows:

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

The initiating entity MUST verify the responder-message to achieve mutual authentication.

3.1. 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 not send additional data with success.

3. HT is capable of transferring authorization identities from the client to the server. (TODO)
4. HT does not offer any security layers (HT offers channel binding instead).
5. HT does not protect the authorization identity.

4. Security Considerations

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

5. IANA Considerations

TODO

6. References

6.1. Normative References

6.2. Informative References

Appendix A. Acknowledgements

Thanks to Thijs Alkemade.

Author's Address