Internet Engineering Task Force (IETF)                         S. Whited
Request for Comments: 9266                                     July 2022
Updates: 5801, 5802, 5929, 7677
Category: Standards Track
ISSN: 2070-1721

                      Channel Bindings for TLS 1.3


   This document defines a channel binding type, tls-exporter, that is
   compatible with TLS 1.3 in accordance with RFC 5056, "On the Use of
   Channel Bindings to Secure Channels".  Furthermore, it updates the
   default channel binding to the new binding for versions of TLS
   greater than 1.2.  This document updates RFCs 5801, 5802, 5929, and

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Table of Contents

   1.  Introduction
     1.1.  Conventions and Terminology
   2.  The 'tls-exporter' Channel Binding Type
   3.  TLS 1.3 with SCRAM or GSS-API over SASL
   4.  Security Considerations
     4.1.  Uniqueness of Channel Bindings
     4.2.  Use with Legacy TLS
   5.  IANA Considerations
     5.1.  Registration of Channel Binding Type
     5.2.  Registration of Channel Binding TLS Exporter Label
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Author's Address

1.  Introduction

   The "tls-unique" channel binding type defined in [RFC5929] was found
   to be susceptible to the "triple handshake vulnerability"
   [TRIPLE-HANDSHAKE] without the extended master secret extension
   defined in [RFC7627].  While TLS 1.3 uses a complete transcript hash
   akin to the extended master secret procedures, the safety of channel
   bindings with TLS 1.3 was not analyzed as part of the core protocol
   work, so the specification of channel bindings for TLS 1.3 was
   deferred.  Appendix C.5 of [RFC8446] notes the lack of channel
   bindings for TLS 1.3; this document defines such channel bindings and
   fills that gap.  Furthermore, this document updates [RFC5929] by
   adding an additional unique channel binding type, "tls-exporter",
   that replaces some usage of "tls-unique".

1.1.  Conventions and Terminology

   Throughout this document, the acronym "EKM" is used to refer to
   "Exported Keying Material" as defined in [RFC5705].

   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.

2.  The 'tls-exporter' Channel Binding Type

   Channel binding mechanisms are not useful until TLS implementations
   expose the required data.  To facilitate this, "tls-exporter" uses
   Exported Keying Material (EKM), which is already widely exposed by
   TLS implementations.  The EKM is obtained using the keying material
   exporters for TLS, as defined in [RFC5705] and Section 7.5 of
   [RFC8446], by supplying the following inputs:

   Label:  The ASCII string "EXPORTER-Channel-Binding" with no
      terminating NUL.

   Context value:  Zero-length string.

   Length:  32 bytes.

   This channel binding mechanism is defined only when the TLS handshake
   results in unique master secrets.  This is true of TLS versions prior
   to 1.3 when the extended master secret extension of [RFC7627] is in
   use, and it is always true for TLS 1.3 (see Appendix D of [RFC8446]).

3.  TLS 1.3 with SCRAM or GSS-API over SASL

   The specifications for Salted Challenge Response Authentication
   Mechanism (SCRAM) [RFC5802] [RFC7677] and Generic Security Service
   Application Program Interface (GSS-API) over Simple Authentication
   and Security Layer (SASL) [RFC5801] define "tls-unique" as the
   default channel binding to use over TLS.  As "tls-unique" is not
   defined for TLS 1.3 (and greater), this document updates [RFC5801],
   [RFC5802], and [RFC7677] to use "tls-exporter" as the default channel
   binding over TLS 1.3 (and greater).  Note that this document does not
   change the default channel binding for SCRAM mechanisms over TLS 1.2
   [RFC5246], which is still "tls-unique" (also note that RFC 5246 has
   been obsoleted by RFC 8446).

   Additionally, this document updates the aforementioned documents to
   make "tls-exporter" the mandatory-to-implement channel binding if any
   channel bindings are implemented for TLS 1.3.  Implementations that
   support channel binding over TLS 1.3 MUST implement "tls-exporter".

4.  Security Considerations

   The channel binding type defined in this document is constructed so
   that disclosure of the channel binding data does not leak secret
   information about the TLS channel and does not affect the security of
   the TLS channel.

   The derived data MUST NOT be used for any purpose other than channel
   bindings as described in [RFC5056].  In particular, implementations
   MUST NOT use channel binding as a secret key to protect privileged

   The Security Considerations sections of [RFC5056], [RFC5705], and
   [RFC8446] apply to this document.

4.1.  Uniqueness of Channel Bindings

   The definition of channel bindings in [RFC5056] defines the concept
   of a "unique" channel binding as being one that is unique to the
   channel endpoints and unique over time, that is, a value that is
   unique to a specific instance of the lower-layer security protocol.
   When TLS is the lower-layer security protocol, as for the channel
   binding type defined in this document, this concept of uniqueness
   corresponds to uniquely identifying the specific TLS connection.

   However, a stronger form of uniqueness is possible, which would
   entail uniquely identifying not just the lower-layer protocol but
   also the upper-layer application or authentication protocol that is
   consuming the channel binding.  The distinction is relevant only when
   there are multiple instances of an authentication protocol, or
   multiple distinct authentication protocols, that run atop the same
   lower-layer protocol.  Such a situation is rare; most consumers of
   channel bindings establish an instance of the lower-layer secure
   protocol, run a single application or authentication protocol as the
   upper-layer protocol, then terminate both upper and lower-layer
   protocols.  In this situation, the stronger form of uniqueness is
   trivially achieved, given that the channel binding value is unique in
   the sense of [RFC5056].

   The channel binding type defined by this document provides only the
   weaker type of uniqueness, as per [RFC5056]; it does not achieve the
   stronger uniqueness per the upper-layer protocol instance described
   above.  This stronger form of uniqueness would be useful in that it
   provides protection against cross-protocol attacks for the multiple
   authentication protocols running over the same instance of the lower-
   layer protocol, and it provides protection against replay attacks
   that seek to replay a message from one instance of an authentication
   protocol in a different instance of the same authentication protocol,
   again running over the same instance of the lower-layer protocol.
   Both of these properties are highly desirable when performing formal
   analysis of upper-layer protocols; if these properties are not
   provided, such formal analysis is essentially impossible.  In some
   cases, one or both of these properties may already be provided by
   specific upper-layer protocols, but that is dependent on the
   mechanism(s) in question, and formal analysis requires that the
   property is provided in a generic manner across all potential upper-
   layer protocols that exist or might exist in the future.

   Accordingly, applications that make use of the channel binding type
   defined in this document MUST NOT use the channel binding for more
   than one authentication mechanism instance on a given TLS connection.
   Such applications MUST immediately close the TLS connection after the
   conclusion of the upper-layer protocol.

4.2.  Use with Legacy TLS

   While it is possible to use this channel binding mechanism with TLS
   versions below 1.3, extra precaution must be taken to ensure that the
   chosen cipher suites always result in unique master secrets.  For
   more information, see [RFC7627] and the Security Considerations
   section of [RFC5705] (TLS 1.3 always provides unique master secrets,
   as discussed in Appendix D of [RFC8446]).

   When TLS renegotiation is enabled on a connection, the "tls-exporter"
   channel binding type is not defined for that connection, and
   implementations MUST NOT support it.

   In general, users wishing to take advantage of channel binding should
   upgrade to TLS 1.3 or later.

5.  IANA Considerations

5.1.  Registration of Channel Binding Type

   IANA has registered tls-exporter in the "Channel-Binding Types"

   Channel-binding unique prefix:  tls-exporter

   Channel-binding type:  unique

   Channel type:  TLS [RFC8446]

   Published specification:  RFC 9266

   Channel-binding is secret:  no

   Description:  The EKM value obtained from the current TLS connection.

   Intended usage:  COMMON

   Person and email address to contact for further information:  Sam
      Whited <>

   Owner/Change controller name and email address:  IESG

   Expert reviewer name and contact information:  IETF KITTEN WG
      <> or IETF TLS WG <>

   Note:  See the published specification for advice on the
      applicability of this channel binding type.

5.2.  Registration of Channel Binding TLS Exporter Label

   IANA has added the following registration in the "TLS Exporter
   Labels" registry under the "Transport Layer Security (TLS)
   Parameters" registry:

   Value:  EXPORTER-Channel-Binding

   DTLS-OK:  Y

   Recommended:  Y

   Reference:  RFC 9266

6.  References

6.1.  Normative References

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

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <>.

   [RFC5801]  Josefsson, S. and N. Williams, "Using Generic Security
              Service Application Program Interface (GSS-API) Mechanisms
              in Simple Authentication and Security Layer (SASL): The
              GS2 Mechanism Family", RFC 5801, DOI 10.17487/RFC5801,
              July 2010, <>.

   [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,

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

   [RFC7677]  Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple
              Authentication and Security Layer (SASL) Mechanisms",
              RFC 7677, DOI 10.17487/RFC7677, November 2015,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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

6.2.  Informative References

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

   [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,

              Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
              A., and P. Strub, "Triple Handshakes Considered Harmful:
              Breaking and Fixing Authentication over TLS", March 2014,

Author's Address

   Sam Whited
   Atlanta, GA
   United States of America