Network Working Group                                          N. Harper
Internet-Draft                                               Google Inc.
Updates: TBNEGO (if approved)                               May 21, 2018
Intended status: Standards Track
Expires: November 22, 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

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   This Internet-Draft will expire on November 22, 2018.

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   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of

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   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",
   "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.  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

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   same rules as section 3 of [I-D.ietf-tokbind-negotiation], with the
   following changes:

   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

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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
   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-tls13-28 (work in progress),
              March 2018.

              Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
              "Transport Layer Security (TLS) Extension for Token
              Binding Protocol Negotiation", draft-ietf-tokbind-
              negotiation-13 (work in progress), May 2018.

   [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-

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

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-18 (work in progress), May 2018.

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

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Author's Address

   Nick Harper
   Google Inc.


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