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Common Authentication Technology Next Generation              F. Schmaus
Internet-Draft                                                  C. Egger
Intended status: Experimental           University of Erlangen-Nuremberg
Expires: November 7, 2019                                    May 6, 2019

                    The Hashed Token SASL Mechanism


   This document specifies the family of Hashed Token SASL mechanisms
   which enable a proof-of-possession-based authentication scheme and
   are meant to be used for quick re-authentication of 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 herin further provides
   hash agility, mutual authentication and is secured by channel

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 November 7, 2019.

Copyright Notice

   Copyright (c) 2019 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
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   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   3
     1.2.  Applicability . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The HT Family of Mechanisms . . . . . . . . . . . . . . . . .   4
   3.  The HT Authentication Exchange  . . . . . . . . . . . . . . .   5
     3.1.  Initiator First Message . . . . . . . . . . . . . . . . .   5
     3.2.  Initiator Authentication  . . . . . . . . . . . . . . . .   6
     3.3.  Final Responder Message . . . . . . . . . . . . . . . . .   6
   4.  Compliance with SASL Mechanism Requirements . . . . . . . . .   6
   5.  Requirements for the Application-Protocol Extension . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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:

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     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).  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

1.1.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

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 Transport Layer Security (TLS, see [RFC5246]),
   or over similar integrity-protected and authenticated channels.
   Also, the application protcol specific extension which requests a new
   SASL-HT token SHOULD only be used over similarly protected channels.

   Also, when TLS is used, the client MUST successfully validate the
   server's certificate ([RFC5280], [RFC6125]).

   The family of HT mechanisms is not applicable for proxy
   authentication since they can not carry an authorization identity
   string (authzid).

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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:


   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' or 'UNIQ'
   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.  Valid channel
   binding types are defined in the IANA "Channel-Binding Types"
   registry [iana-cbt] as specified in [RFC5056].

                    | cb-type | Channel Binding Type |
                    |   ENDP  | tls-server-end-point |
                    |   UNIQ  |      tls-unique      |

                Mapping of cb-type to Channel Binding Types

   The following table lists the HT SASL mechanisms registered by this

   |  Mechanism Name  |     HT Hash      |    Channel-binding unique   |
   |                  |    Algorithm     |            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         |

                        Defined HT SASL mechanisms

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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].

3.1.  Initiator First Message

   The HT mechanism starts with the initiator-msg, send by the initiator
   to the responder.  The following lists the ABNF grammar for the

        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

   The initiator first message starts with the authentication identity
   (authcid, see[RFC4422]) as UTF-8 [RFC3629] encoded string.  It is
   followed by initiator-hashed-token separated by as single null octet.

   The value of the initiator-hashed-token is defined as follows:

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

   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 include any further application protocol payload in
   the early data besides the HT initiator-msg and potential required

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   framing of the SASL profile.  The responder MUST abort the SASL
   authentication if the early data contains additional application
   protocol payload.

      TODO: It should be possible to exploit TLS 1.3 early data for
      "0.5" RTT resumption of the application protocol's session.  That
      is, on resumption the initiating entity MUST NOT send any
      application protocol payload together with first flight data,
      besides the HT initiator-msg.  But if the responding entity is
      able to verify the TLS 1.3 early data, then it can send additional
      application protocol payload right away together with the
      "resumption successful" response to the initiating entity.

      TODO: Add note why HMAC() is always involved, even if HMAC() is
      usually not required when modern hash algorithms are used.

3.2.  Initiator Authentication

   Upon receiving the initiator-msg, the responder calculates itself 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.

   If the responder was able to authenticate the initiator, then the
   used token MUST be revoked immediately.

3.3.  Final Responder Message

   After the initiator was authenticated the responder continues the
   SASL authentication by sending the responder-msg to the initiator.

   The ABNF for responder-msg is:

                          responder-msg = 1*OCTET

   The responder-msg value is defined as follows:

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

   The initiating entity MUST verify the responder-msg to achieve mutual

4.  Compliance with SASL Mechanism Requirements

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

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   1.  "HT-SHA-256-ENDP", "HT-SHA-256-UNIQ", "HT-SHA-3-512-ENDP" and

   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

   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.

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7.  IANA Considerations

   IANA has added 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 FIXME 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).

8.  References

8.1.  Normative References

              Williams, N., "IANA Channel-Binding Types", 2010,

              Williams, N., "IANA Named Information Hash Algorithm
              Registry", 2010, <https://www.iana.org/assignments/named-

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [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,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

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   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

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,

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/info/rfc6120>.

              Schmaus, F., "XEP-0397: Instant Stream Resumption", 2018,

Appendix A.  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

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Authors' Addresses

   Florian Schmaus
   University of Erlangen-Nuremberg

   Email: schmaus@cs.fau.de

   Christoph Egger
   University of Erlangen-Nuremberg

   Email: egger@cs.fau.de

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