Network Working Group                                          N. Harper
Internet-Draft                                               Google Inc.
Updates: RFC8472 (if approved)                          October 22, 2018
Intended status: Standards Track
Expires: April 25, 2019

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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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 25, 2019.

Copyright Notice

   Copyright (c) 2018 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
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   ( 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

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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 [RFC8446] extension as
   described in [RFC8472] 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 instead.

   This document also non-normatively provides a clarification for the
   definition of the TokenBinding.signature field from [RFC8471], since
   TLS 1.3 defines an alternate (but API-compatible) exporter mechanism
   to the one in [RFC5705] used in [RFC8471].

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 [RFC8472].  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 [RFC8472], with the following changes:

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   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

   [RFC8446] 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 rejected.

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

   The Token Binding protocol [RFC8471] defines the
   TokenBinding.signature field in terms of an exported keying material
   (EKM) value as defined in [RFC5705].  TLS 1.3 [RFC8446] provides an
   equivalent interface in section 7.5.  For clarity, using the
   terminology from [RFC8446], the EKM used in section 3.3 of [RFC8471]
   in TLS 1.3 is the exporter value (section 7.5 of [RFC8446]) 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 [RFC8472] still
   apply here: The parameters negotiated in the "token_binding"
   extension are protected by the TLS handshake.  An active network

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   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
   [RFC8446] about replay do not apply here.

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-

   [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,

   [RFC8472]  Popov, A., Ed., Nystroem, M., and D. Balfanz, "Transport
              Layer Security (TLS) Extension for Token Binding Protocol
              Negotiation", RFC 8472, DOI 10.17487/RFC8472, October
              2018, <>.

6.2.  Informative References

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

   [RFC8471]  Popov, A., Ed., Nystroem, M., Balfanz, D., and J. Hodges,
              "The Token Binding Protocol Version 1.0", RFC 8471,
              DOI 10.17487/RFC8471, October 2018, <https://www.rfc-

Author's Address

   Nick Harper
   Google Inc.


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