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 <[email protected]>

  Owner/Change controller name and email address:  IESG

  Expert reviewer name and contact information:  IETF KITTEN WG
     <[email protected]> or IETF TLS WG <[email protected]>

  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: [email protected]
  URI:   https://blog.samwhited.com/

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