Common Authentication Technology Next Generation (kitten) F. Schmaus Internet-Draft Friedrich-Alexander-Universität Erlangen-Nürnberg Intended status: Standards Track T. Molitor Expires: 12 November 2026 Monal Instant Messenger C. Egger Chalmers University of Technology 11 May 2026 The Hashed Token SASL Mechanism draft-ietf-kitten-sasl-ht-01 Abstract This document specifies the family of Hashed Token SASL mechanisms, which enable a proof-of-possession-based authentication scheme and are meant to quickly re-authenticate a previous session. The Hashed Token SASL mechanism's authentication sequence consists of only one round-trip. The usage of short-lived, exclusively ephemeral hashed tokens is achieving the single round-trip property. The SASL mechanism specified herein further provides hash agility, mutual authentication, support for channel binding, and the capability to exchange authenticated key/value pairs. About This Document This note is to be removed before publishing as an RFC. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-kitten-sasl-ht/. Source for this draft and an issue tracker can be found at https://github.com/flowdalic/xeps/tree/master/draft-ietf-kitten-sasl- ht. 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 https://datatracker.ietf.org/drafts/current/. Schmaus, et al. Expires 12 November 2026 [Page 1] Internet-Draft The Hashed Token SASL Mechanism May 2026 Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 12 November 2026. Copyright Notice Copyright (c) 2026 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 (https://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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Conventions and Terminology . . . . . . . . . . . . . . . 4 1.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 4 2. The HT Family of Mechanisms . . . . . . . . . . . . . . . . . 4 3. The HT Authentication Exchange . . . . . . . . . . . . . . . 5 3.1. Initiator First Message . . . . . . . . . . . . . . . . . 6 3.2. Initiator Authentication . . . . . . . . . . . . . . . . 7 3.3. Final Responder Message . . . . . . . . . . . . . . . . . 7 3.3.1. Success Response . . . . . . . . . . . . . . . . . . 8 3.3.2. Failure Response . . . . . . . . . . . . . . . . . . 8 4. Compliance with SASL Mechanism Requirements . . . . . . . . . 9 5. Requirements for the Application-Protocol Extension . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . 10 8.2. Informative References . . . . . . . . . . . . . . . . . 12 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 Schmaus, et al. Expires 12 November 2026 [Page 2] Internet-Draft The Hashed Token SASL Mechanism May 2026 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, 3) support for channel binding, and 4) the optional exchange of authenticated key/value pairs. The ability to include arbitrary key/value pairs allows the initiator and responder to negotiate session parameters or exchange context- specific data concurrently with the authentication exchange, with cryptographic guarantees regarding their integrity and authenticity. An example use case for these key/value pairs is transmitting a downgrade protection hash of the initially offered SASL mechanisms and channel-binding types (see [XEP-0474]). Clients should 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 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 request a new SASL-HT token (see Section 5 (Section 5)). Examples of such an application- protocol-specific extension based on HT are [XEP-0397] and [XEP-0484]. 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. Schmaus, et al. Expires 12 November 2026 [Page 3] Internet-Draft The Hashed Token SASL Mechanism May 2026 1.1. Conventions and Terminology 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 capitals, as shown here. These words may also appear in this document in lower case as plain English words, absent their normative meanings. 1.2. Applicability Because this mechanism transports information that an attacker should not control, the HT mechanism *MUST* only be used over channels protected by Transport Layer Security (TLS, see [RFC8446]) or over similar integrity-protected and authenticated channels. Also, the application-protocol-specific extension that requests a new SASL-HT token *SHOULD* only be used over similarly protected channels. The family of HT mechanisms is not applicable for proxy authentication since they cannot carry an authorization identity string (authzid). 2. The HT Family of Mechanisms Each mechanism in this family differs by choice of the hash algorithm and the selection of the channel binding [RFC5929] type. An HT mechanism name is a string beginning with "HT-" followed by the capitalized name of the used hash, followed by "-", and suffixed by one of 'ENDP', 'UNIQ', 'EXPR' or 'NONE'. Hence, each HT mechanism has a name of the following form: HT-- Where is the capitalized "Hash Name String" of the IANA "Named Information Hash Algorithm Registry" [iana-hash-alg] as specified in [RFC6920], and is one of 'ENDP', 'UNIQ', 'EXPR' or 'NONE' 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]. Schmaus, et al. Expires 12 November 2026 [Page 4] Internet-Draft The Hashed Token SASL Mechanism May 2026 In the special case of 'NONE' no channel binding will be used. In this case, cb-data is to be an empty string. +=========+======================+ | cb-type | Channel Binding Type | +=========+======================+ | ENDP | tls-server-end-point | +---------+----------------------+ | UNIQ | tls-unique | +---------+----------------------+ | EXPR | tls-exporter | +---------+----------------------+ | NONE | _No_ Channel Binding | +---------+----------------------+ Table 1: Mapping of cb-type to Channel Binding Types The following table lists some examples of HT SASL mechanisms registered by this document. +==================+===================+======================+ | Mechanism Name | HT Hash Algorithm | Channel-binding | | | | unique prefix | +==================+===================+======================+ | HT-SHA-512-ENDP | SHA-512 | tls-server-end-point | +------------------+-------------------+----------------------+ | HT-SHA-512-UNIQ | SHA-512 | tls-unique | +------------------+-------------------+----------------------+ | HT-SHA3-512-ENDP | SHA3-512 | tls-server-end-point | +------------------+-------------------+----------------------+ | HT-SHA-256-UNIQ | SHA-256 | tls-unique | +------------------+-------------------+----------------------+ | HT-SHA-256-NONE | SHA-256 | N/A | +------------------+-------------------+----------------------+ Table 2: Examples of HT SASL mechanisms 3. The HT Authentication Exchange The mechanism consists of a simple exchange of precisely two messages between the initiator and responder. Both messages allow the inclusion of arbitrary key/value pairs. The following syntax specifications use the Augmented Backus-Naur form (ABNF) notation as specified in [RFC5234]. Schmaus, et al. Expires 12 November 2026 [Page 5] Internet-Draft The Hashed Token SASL Mechanism May 2026 3.1. Initiator First Message The HT mechanism starts with the initiator-msg, which is sent by the initiator to the responder. The following lists the ABNF grammar for SASL-HT in general and initiator-msg in particular: initiator-msg = authcid NUL extra-initiator-values NUL initiator-hashed-token authcid = 1*SAFE ;; MUST accept up to 255 octets extra-initiator-values = key-value-pairs key-value-pairs = [ key-value-pair *( "," key-value-pair ) ] key-value-pair = 1*key-value-char "=" 1*key-value-char initiator-hashed-token = 1*OCTET key-value-char = ALPHA / DIGIT / "/" / "+" / "-" / "_" 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 The initiator's first message starts with the authentication identity (authcid, see[RFC4422]) as UTF-8 [RFC3629] encoded string and its terminating null octet followed by an optional set of comma-separated key/value pairs (extra-initiator-values). This, in turn, is followed by another null octet and the initiator-hashed-token. The extra-initiator-values allow the initiator to pass arbitrary key/ value pairs to the responder during the initial exchange. Because these key/value pairs are appended to the initiator-hmac-message before the HMAC calculation, their integrity and authenticity are guaranteed by the resulting initiator-hashed-token. If no extra values are being sent, the extra-initiator-values field remains empty, resulting in two consecutive null octets between the authcid and the initiator-hashed-token. The value of the initiator-hashed-token is defined as follows: Schmaus, et al. Expires 12 November 2026 [Page 6] Internet-Draft The Hashed Token SASL Mechanism May 2026 initiator-hashed-token := HMAC(token, initiator-hmac-message) initiator-hmac-message := "Initiator" || cb-data || extra-initiator-values HMAC() is the function defined in [RFC2104] with H being the selected HT hash algorithm, 'cb-data' represents the data provided by the selected channel binding type, and 'token' are the UTF-8 encoded octets of the SASL-HT token string, which acts as a shared secret between initiator and responder. The initiator-msg *MAY* be included in TLS 1.3 0-RTT early data, as specified in [RFC8446]. If this is the case, then the initiating entity *MUST NOT* contain any further application protocol payload in the early data besides the HT initiator-msg and potentially required framing of the SASL profile. The responder *MUST* abort the SASL authentication if the early data contains an additional application- protocol payload. SASL-HT allows exploiting TLS 1.3 early data for "0.5 Round Trip Time (RTT)" re-authentication of the application protocol's session. Using TLS early data requires extra care when implementing: The early data should only contain the SASL-HT payload, i.e., the initiator-msg, and not an application-protocol- specific payload. The reason for this is that another entity could replay the early data. Therefore, the early data needs must represent an idempotent operation. On the other hand, if the responding entity can verify the early data, it can send an additional application-protocol-specific payload together with the "re-authentication successful" response to the initiating entity. 3.2. Initiator Authentication Upon receiving the initiator-msg, the responder calculates the value of initiator-hashed-token and compares it with the received value found in the initiator-msg. If both values are equal, then the initiator has been successfully authenticated. Otherwise, if both values are not equal, then authentication *MUST* fail. 3.3. Final Responder Message After the responder authenticated the initiator, the responder continues the SASL authentication by sending the responder-msg to the initiator. The ABNF for responder-msg is: Schmaus, et al. Expires 12 November 2026 [Page 7] Internet-Draft The Hashed Token SASL Mechanism May 2026 responder-msg = success-response / failure-response success-response = NUL extra-responder-values NUL responder-hashed-token extra-responder-values = key-value-pairs responder-hashed-token = 1*OCTET failure-response = %x01 failure-description 3.3.1. Success Response A success response starts with an octet whose value is set to zero (null), followed by an optional set of comma-separated key/value pairs (extra-responder-values). This is followed by another null octet and the octet string of the result of the HMAC function (responder-hashed-token). responder-hashed-token := HMAC(token, responder-hmac-message) responder-hmac-message := "Responder" || cb-data || extra-responder-values Similar to the initiator's message, the responder can use extra- responder-values to return arbitrary data. Because these values are incorporated into the responder-hmac-message prior to calculating the HMAC, the initiating entity can mutually authenticate the responder while simultaneously verifying the integrity of the provided key/ value pairs. The initiating entity *MUST* verify the responder-msg to achieve mutual authentication. 3.3.2. Failure Response A failure response starts with an octet whose value is set to one (0x01), followed by an octet string describing the reason for the failure (failure-description). failure-description = "unknown-user" / "invalid-token" / "other-error"/ failure-description-ext failure-description-ext = 1*SAFE ;; additional custom failure reasons Unrecognized failure descriptions should be treated as "other-error". The responder may substitute the actual failure cause with "other- error" to prevent information disclosure. Schmaus, et al. Expires 12 November 2026 [Page 8] Internet-Draft The Hashed Token SASL Mechanism May 2026 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-SHA3-512-ENDP", "HT- SHA3-512-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 the form of a Unicode string. The returned token *MUST* have been newly generated by a cryptographically secure random number generator, and it MUST contain at least 128 bits of entropy. It is *RECOMMENDED* that the protocol allows the requestor to signal the name of the SASL mechanism that the requestor intends to use with the token. If a token is used with a mechanism different from the one signaled upon requesting the token, then the authentication *MUST* fail. This requirement 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. It is *RECOMMENDED* that the protocol defines a way for a client to request rotation or revocation of a token. 6. Security Considerations The HT mechanism *MUST* be used over a TLS channel that has the session hash extension [RFC7627] negotiated. It is *RECOMMENDED* that implementations periodically require a full authentication using a strong SASL mechanism that does not use the SASL-HT token. Schmaus, et al. Expires 12 November 2026 [Page 9] Internet-Draft The Hashed Token SASL Mechanism May 2026 A cryptographically secure random generator must generate the SASL-HT token. See [RFC4086] for more information about Randomness Requirements for Security. In addition, a comparison of the initiator's HMAC with the responder's calculated HMAC *SHOULD* be performed via constant-time comparison functions to protect against timing attacks. The tokens used with HT mechanisms *SHOULD* have a limited lifetime, e.g., based on usage count or time elapsed since issuance. Due to the additional security properties afforded by channel binding, it is *RECOMMENDED* that clients use HT mechanisms with channel binding whenever possible. 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-ietf-kitten-sasl-ht Published specification (optional, recommended): draft-ietf- kitten-sasl-ht-XX (TODO) Person & email address to contact for further information: IETF SASL WG kitten@ietf.org (mailto:kitten@ietf.org) Intended usage: COMMON Owner/Change controller: IESG iesg@ietf.org (mailto:iesg@ietf.org) Note: Members of this family MUST be explicitly registered using the "IETF Review" [RFC8126] registration procedure. Reviews MUST be requested on the Kitten WG mailing list kitten@ietf.org (mailto:kitten@ietf.org) (or a successor designated by the responsible Security AD). 8. References 8.1. Normative References Schmaus, et al. Expires 12 November 2026 [Page 10] Internet-Draft The Hashed Token SASL Mechanism May 2026 [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, . [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, . [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005, . [RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple Authentication and Security Layer (SASL)", RFC 4422, DOI 10.17487/RFC4422, June 2006, . [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007, . [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, . [RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010, . [RFC6920] Farrell, S., Kutscher, D., Dannewitz, C., Ohlman, B., Keranen, A., and P. Hallam-Baker, "Naming Things with Hashes", RFC 6920, DOI 10.17487/RFC6920, April 2013, . [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, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . Schmaus, et al. Expires 12 November 2026 [Page 11] Internet-Draft The Hashed Token SASL Mechanism May 2026 [RFC9266] Whited, S., "Channel Bindings for TLS 1.3", RFC 9266, DOI 10.17487/RFC9266, July 2022, . [iana-hash-alg] Williams, N., "IANA Named Information Hash Algorithm Registry", 2010, . [iana-cbt] Williams, N., "IANA Channel-Binding Types", 2010, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 8.2. Informative References [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, . [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, . [XEP-0397] Schmaus, F., "XEP-0397: Instant Stream Resumption", 3 November 2018, . [XEP-0474] Molitor, T., "XEP-0474: SASL SCRAM Downgrade Protection", 25 October 2025, . [XEP-0484] Wild, M., "XEP-0484: Fast Authentication Streamlining Tokens", 30 June 2024, . Schmaus, et al. Expires 12 November 2026 [Page 12] Internet-Draft The Hashed Token SASL Mechanism May 2026 Acknowledgments This document benefited from discussions on the KITTEN WG mailing list. The authors especially thank Thijs Alkemade, Sam Whited, and Alexey Melnikov for their comments. Furthermore, we would like to thank Alexander Würstlein, who devised the idea to pin the token to a SASL mechanism for increased security. And last but not least, thanks to Matthew Wild for working on the -NONE variant of SASL-HT. Authors' Addresses Florian Schmaus Friedrich-Alexander-Universität Erlangen-Nürnberg Germany Email: flow@cs.fau.de Thilo Molitor Monal Instant Messenger Germany Email: thilo+ietf@eightysoft.de Christoph Egger Chalmers University of Technology Sweden Email: christoph.egger@chalmers.se Schmaus, et al. Expires 12 November 2026 [Page 13]