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RFC 9266 PROPOSED STANDARD SECURITY · TLS · AUTHENTICATION

TLS 1.3 Channel Binding: Channel Bindings for TLS 1.3

A way to tie a login to the exact TLS 1.3 connection it runs over, using exported keying material.

On rfc-editor.org ↗
PUBLISHED
July 2022
AUTHORS
S. Whited
STREAM
IETF
BUILDS ON
RFC 5801 — GS2, RFC 5802 — SCRAM, RFC 5929 — TLS channel bindings, RFC 7677 — SCRAM-SHA-256
BINDING TYPE NAME
tls-exporter
EXPORTER LABEL
EXPORTER-Channel-Binding
CONTEXT VALUE
Zero-length string
OUTPUT LENGTH
32 bytes
BINDING IS SECRET
No
DOI
10.17487/RFC9266
LINEAGE · FROM THE RFC INDEXOpen full graph →
'10RFC 5801GS2'10RFC 5802SCRAM'10RFC 5929TLS channel bindings'15RFC 7677SCRAM-SHA-256'22RFC 9266TLS 1.3 Channel Binding

RFC text

Contents
Original text · RFC Editor View on rfc-editor.org ↗ 
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

Abstract

   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
   7677.

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
   https://www.rfc-editor.org/info/rfc9266.

Copyright Notice

   Copyright (c) 2022 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
     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",
   "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.

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
   information.

   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"
   registry:

   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 <sam@samwhited.com>

   Owner/Change controller name and email address:  IESG

   Expert reviewer name and contact information:  IETF KITTEN WG
      <kitten@ietf.org> or IETF TLS WG <tls@ietf.org>

   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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
              <https://www.rfc-editor.org/info/rfc5056>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <https://www.rfc-editor.org/info/rfc5705>.

   [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, <https://www.rfc-editor.org/info/rfc5801>.

   [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,
              <https://www.rfc-editor.org/info/rfc5802>.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
              <https://www.rfc-editor.org/info/rfc5929>.

   [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,
              <https://www.rfc-editor.org/info/rfc7677>.

   [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,
              <https://www.rfc-editor.org/info/rfc8446>.

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,
              <https://www.rfc-editor.org/info/rfc5246>.

   [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,
              <https://www.rfc-editor.org/info/rfc7627>.

   [TRIPLE-HANDSHAKE]
              Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
              A., and P. Strub, "Triple Handshakes Considered Harmful:
              Breaking and Fixing Authentication over TLS", March 2014,
              <https://www.mitls.org/pages/attacks/3SHAKE>.

Author's Address

   Sam Whited
   Atlanta, GA
   United States of America
   Email: sam@samwhited.com
   URI:   https://blog.samwhited.com/

ISO Plain Language

On this page
PROBLEMThe older tls-unique channel binding was found to be open to the triple handshake vulnerability, and it is not defined for TLS 1.3, so TLS 1.3 had no channel binding at all.
MECHANISMA new binding type, tls-exporter, takes 32 bytes of exported keying material from the TLS connection, using the label EXPORTER-Channel-Binding and an empty context value.
WHY IT MATTERStls-exporter becomes the default channel binding over TLS 1.3 and later for SCRAM and the GS2 mechanism family, and the mandatory-to-implement one whenever channel binding is implemented for TLS 1.3.

§1Why TLS 1.3 Needed a New Channel Binding

The tls-unique channel binding type defined in RFC 5929 was found to be susceptible to the triple handshake vulnerability without the extended master secret extension defined in RFC 7627. 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 analysed as part of the core protocol work, so the specification of channel bindings for TLS 1.3 was deferred. Appendix C.5 of RFC 8446 notes the lack of channel bindings for TLS 1.3. This document defines such channel bindings and fills that gap.

It also updates RFC 5929 by adding an additional unique channel binding type, tls-exporter, that replaces some usage of tls-unique.

A channel binding is what ties an authentication exchange, such as a password login, to the encrypted connection underneath it. The two ends of the connection can then show that they belong together, which stops a password captured on one connection from being replayed on another.

TLS 1.3 was released without a way to tie a login to the connection it runs over, so this RFC adds one. It is called tls-exporter, and it works by asking the TLS library for a value that only this connection can produce.

Illustrative, not from the RFC: where the binding value is produced and used
TLS ClientTLS Server
  1. TLS handshakeTLS 1.3 always produces unique master secrets.
  2. Export 32 bytesLabel EXPORTER-Channel-Binding, empty context value.
  3. Export the same 32 bytesBoth ends derive the same binding for this connection.
  4. Authentication exchange with the bindingSCRAM or a GS2 mechanism includes the binding value.

§2The tls-exporter Channel Binding

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 RFC 5705 and Section 7.5 of RFC 8446, by supplying the following inputs.

  • Label: the ASCII string EXPORTER-Channel-Binding with no terminating NUL.
  • Context value: a zero-length string.
  • Length: 32 bytes.

This channel binding mechanism is defined only when the TLS handshake results in unique master secrets. That is true of TLS versions prior to 1.3 when the extended master secret extension of RFC 7627 is in use, and it is always true for TLS 1.3.

The value that comes out is not secret. The IANA registration records the channel-binding type as unique, the channel type as TLS, and that channel binding is secret: no. The derived data MUST NOT be used for any purpose other than channel bindings as described in RFC 5056, and in particular implementations MUST NOT use channel binding as a secret key to protect privileged information.

The binding is 32 bytes produced by the TLS library's exporter, which takes a label, an optional context and a length. The label here is the fixed string EXPORTER-Channel-Binding, the context is empty, and the result is the same on both ends of the connection and different on any other connection.

What goes into the tls-exporter binding
  • LabelThe ASCII string EXPORTER-Channel-Binding, with no terminating NUL.
  • Context valueA zero-length string.
  • Length32 bytes.
  • Not a secretThe binding value must not be used to protect privileged information.

§3Using tls-exporter with SCRAM and GSS-API

The specifications for Salted Challenge Response Authentication Mechanism (SCRAM) (RFC 5802, RFC 7677) and for Generic Security Service Application Program Interface mechanisms over Simple Authentication and Security Layer (SASL) (RFC 5801) define tls-unique as the default channel binding to use over TLS. As tls-unique is not defined for TLS 1.3 and later, this document updates RFC 5801, RFC 5802 and RFC 7677 to use tls-exporter as the default channel binding over TLS 1.3 and later. The document does not change the default channel binding for SCRAM mechanisms over TLS 1.2, which is still tls-unique.

The same update makes 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.

SCRAM and the GS2 mechanism family, which cover most password and Kerberos logins over TLS, used to have one default way of binding to the connection. From TLS 1.3 onwards that default is tls-exporter. The rule for TLS 1.2 and older is unchanged.

The two channel binding types for TLS
Aspecttls-uniquetls-exporter
Defined for TLS 1.3NoYes
Default for SCRAM over TLS 1.2YesNo
Default for SCRAM over TLS 1.3 and laterNoYes
Value usedFrom the handshake32 bytes of exported keying material

§4Security 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 RFC 5056. In particular, implementations MUST NOT use channel binding as a secret key to protect privileged information. The security considerations sections of RFC 5056, RFC 5705 and RFC 8446 apply.

The definition of channel bindings in RFC 5056 defines a unique channel binding as one that is unique to the channel endpoints and unique over time, that is, a value unique to a specific instance of the lower-layer security protocol. With TLS as the lower-layer protocol, this corresponds to uniquely identifying the specific TLS connection. However, a stronger form of uniqueness is possible, which would also uniquely identify the upper-layer application or authentication protocol consuming the binding. The distinction matters only when there are multiple instances of an authentication protocol, or multiple distinct authentication protocols, running over the same lower-layer protocol, which is rare. In the usual case, where a client establishes one TLS connection, runs a single upper-layer protocol over it and then closes both, the stronger uniqueness is trivially achieved.

The channel binding type defined here provides only the weaker form of uniqueness. It does not protect against cross-protocol attacks for multiple authentication protocols running over the same instance of the lower-layer protocol, and it does not protect against replay attacks that replay a message from one instance of an authentication protocol in a different instance of the same protocol over the same lower-layer connection. Both properties are highly desirable when performing formal analysis of upper-layer protocols, because without them such analysis is essentially impossible. In some cases one or both properties are already provided by specific upper-layer protocols, but that depends on the mechanism in question, and formal analysis requires the property in a generic manner across all potential upper-layer protocols.

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

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. 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.

The binding value is not a secret, so it must not be used to protect anything. It also only identifies the TLS connection, not the login that runs over it, so one connection should carry one authentication exchange and then be closed. Renegotiating a TLS connection makes the binding undefined, so TLS 1.3 is the safe choice.

§5IANA Registrations

IANA has registered tls-exporter in the Channel-Binding Types registry. The channel-binding unique prefix is tls-exporter, the channel-binding type is unique, the channel type is TLS, the published specification is RFC 9266, and channel binding is secret: no. The description is the EKM value obtained from the current TLS connection, the intended usage is COMMON, and the change controller is the IESG. The registry entry also points readers at the published specification for advice on the applicability of this channel binding type.

IANA has also added a registration in the TLS Exporter Labels registry, under the Transport Layer Security parameters registry: the value EXPORTER-Channel-Binding, with DTLS-OK set to Y and Recommended set to Y, referenced to RFC 9266.

The RFC registers the name tls-exporter for the new binding type and the label EXPORTER-Channel-Binding in the list of TLS exporter labels.

§6References

[RFC5056]RFC 5056Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, November 2007.November 2007[RFC5705]RFC 5705Rescorla, E., "Keying Material Exporters for Transport Layer Security (TLS)", RFC 5705, March 2010.March 2010[RFC5801]RFC 5801Josefsson, 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, July 2010.July 2010[RFC5802]RFC 5802Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams, "Salted Challenge Response Authentication Mechanism (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010.July 2010[RFC5929]RFC 5929Altman, J., Williams, N., and L. Zhu, "Channel Bindings for TLS", RFC 5929, July 2010.July 2010[RFC7627]RFC 7627Bhargavan, K., Ed., et al., "Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension", RFC 7627, September 2015.September 2015[RFC7677]RFC 7677Hansen, T., "SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms", RFC 7677, November 2015.November 2015[RFC8446]RFC 8446Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, August 2018.August 2018