Skip to main content

Token Binding for Transport Layer Security (TLS) Version 1.3 Connections

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft whose latest revision state is "Expired".
Author Nick Harper
Last updated 2017-11-21
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd (None)
IESG IESG state I-D Exists
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD (None)
Send notices to (None)
Network Working Group                                          N. Harper
Internet-Draft                                               Google Inc.
Updates: TBNEGO (if approved)                          November 21, 2017
Intended status: Standards Track
Expires: May 25, 2018

Token Binding for Transport Layer Security (TLS) Version 1.3 Connections


   Negotiation of the Token Binding protocol is only defined for
   Transport Layer Security (TLS) versions 1.2 and earlier.  Token
   Binding users may wish to use it with TLS 1.3; this document defines
   a backwards compatible way to negotiate Token Binding on TLS 1.3

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

   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 May 25, 2018.

Copyright Notice

   Copyright (c) 2017 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
   ( 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 Simplified BSD License text as described in Section 4.e of

Harper                    Expires May 25, 2018                  [Page 1]
Internet-Draft            TLS 1.3 Token Binding            November 2017

   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   Negotiating Token Binding using a TLS [I-D.ietf-tls-tls13] extension
   as described in [I-D.ietf-tokbind-negotiation] is fairly
   straightforward, but is restricted to TLS 1.2 and earlier.  Only one
   minor change is needed to use this extension to negotiate Token
   Binding on connections using TLS 1.3 and later.  Instead of the
   server putting the "token_binding" extension in the ServerHello like
   in TLS 1.2, in TLS 1.3 the server puts it in EncryptedExtensions

   This document also non-normatively provides a clarification for the
   definition of the TokenBinding.signature field from
   [I-D.ietf-tokbind-protocol], since TLS 1.3 defines an alternate (but
   API-compatible) exporter mechanism to the one in [RFC5705] used in

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Token Binding TLS Extension

   In TLS 1.3, the "token_binding" TLS extension may be present only in
   ClientHello and EncryptedExtensions handshake messages.  The format
   of the "token_binding" TLS extension remains the same as defined in

   A client puts the "token_binding" TLS extension in its ClientHello to
   indicate its support for the Token Binding protocol.  The client
   should follow the same rules for when to send this extension and the
   contents of its data as in section 2 of
   [I-D.ietf-tokbind-negotiation].  Since the "token_binding" extension
   remains unchanged from TLS 1.2 to TLS 1.3 in the ClientHello, a
   client sending the "token_binding" extension in a TLS 1.3 ClientHello
   is backwards compatible with a server that only supports TLS 1.2.

   A server puts the "token_binding" TLS extension in the
   EncryptedExtensions message following its ServerHello to indicate
   support for the Token Binding protocol and to select protocol version
   and key parameters.  The server includes the extension following the
   same rules as section 3 of [I-D.ietf-tokbind-negotiation], with the
   following changes:

Harper                    Expires May 25, 2018                  [Page 2]
Internet-Draft            TLS 1.3 Token Binding            November 2017

   o  The "token_binding" TLS extension is in EncryptedExtensions
      instead of ServerHello.

   o  The server MUST NOT include both the "token_binding" extension and
      the "early_data" extension on the same connection.

3.  Interaction with 0-RTT Data

   [I-D.ietf-tls-tls13] requires that extensions define their
   interaction with 0-RTT.  The "token_binding" extension MUST NOT be
   used with 0-RTT unless otherwise specified in another draft.  A
   client MAY include both "early_data" and "token_binding" extensions
   in its ClientHello - this indicates that the client is willing to
   resume a connection and send early data (without Token Binding), or
   negotiate Token Binding on the connection and have early data

4.  Clarification of TokenBinding.signature

   This non-normative section provides a clarification on the definition
   of the TokenBinding.signature field when used on a TLS 1.3

   [I-D.ietf-tokbind-protocol] defines the TokenBinding.signature field
   in terms of an exported keying material (EKM) value as defined in
   [RFC5705].  [I-D.ietf-tls-tls13] provides an equivalent interface in
   section 7.5.  For clarity, using the terminology from
   [I-D.ietf-tls-tls13], the EKM used in section 3.3 of
   [I-D.ietf-tokbind-protocol] in TLS 1.3 is the exporter value (section
   7.5 of [I-D.ietf-tls-tls13]) computed with the following parameters:

   o  Secret: exporter_master_secret.

   o  label: The ASCII string "EXPORTER-Token-Binding" with no
      terminating NUL.

   o  context_value: No context value is supplied.

   o  key_length: 32 bytes.

   These are the same input values as specified in section 3.3 of

5.  Security Considerations

   The consideration regarding downgrade attacks in
   [I-D.ietf-tokbind-negotiation] still apply here: The parameters
   negotiated in the "token_binding" extension are protected by the TLS

Harper                    Expires May 25, 2018                  [Page 3]
Internet-Draft            TLS 1.3 Token Binding            November 2017

   handshake.  An active network attacker cannot modify or remove the
   "token_binding" extension without also breaking the TLS connection.

   This extension cannot be used with 0-RTT data, so the concerns in
   [I-D.ietf-tls-tls13] about replay do not apply here.

6.  References

6.1.  Normative References

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
              July 2017.

              Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
              "Transport Layer Security (TLS) Extension for Token
              Binding Protocol Negotiation", draft-ietf-tokbind-
              negotiation-10 (work in progress), October 2017.

   [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-

6.2.  Informative References

              Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
              Hodges, "The Token Binding Protocol Version 1.0", draft-
              ietf-tokbind-protocol-16 (work in progress), October 2017.

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

Author's Address

   Nick Harper
   Google Inc.


Harper                    Expires May 25, 2018                  [Page 4]