





Internet Engineering Task Force (IETF)                     A. Popov, Ed.
Request for Comments: 8472                                   M. Nystroem
Category: Standards Track                                Microsoft Corp.
ISSN: 2070-1721                                               D. Balfanz
                                                             Google Inc.
                                                            October 2018


              Transport Layer Security (TLS) Extension for
                   Token Binding Protocol Negotiation

Abstract

   This document specifies a Transport Layer Security (TLS) extension
   for the negotiation of Token Binding protocol version and key
   parameters.  Negotiation of Token Binding in TLS 1.3 and later
   versions is beyond the scope of this document.

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/rfc8472.

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
   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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Token Binding Negotiation ClientHello Extension . . . . . . .   2
   3.  Token Binding Negotiation ServerHello Extension . . . . . . .   3
   4.  Negotiating Token Binding Protocol Version and Key Parameters   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     6.1.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .   6
     6.2.  Triple Handshake Vulnerability in TLS 1.2 and Older TLS
           Versions  . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In order to use the Token Binding protocol [RFC8471], the client and
   server need to agree on the Token Binding protocol version and the
   parameters (signature algorithm and length) of the Token Binding key.
   This document specifies a new TLS [RFC5246] extension to accomplish
   this negotiation without introducing additional network round trips
   in TLS 1.2 and earlier versions.  [TOKENBIND-TLS13] addresses Token
   Binding in TLS 1.3.  The negotiation of the Token Binding protocol
   and key parameters in combination with TLS 1.3 and later versions is
   beyond the scope of this document.  (Note: This document deals with
   TLS 1.2 and therefore refers to RFC 5246 (which has been obsoleted by
   RFC 8446).  [TOKENBIND-TLS13] addresses Token Binding in TLS 1.3).

1.1.  Requirements Language

   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.  Token Binding Negotiation ClientHello Extension

   The client uses the "token_binding" TLS extension to indicate the
   highest supported Token Binding protocol version and key parameters.

   enum {
       token_binding(24), (65535)
   } ExtensionType;



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   The "extension_data" field of this extension contains a
   "TokenBindingParameters" value.

   struct {
       uint8 major;
       uint8 minor;
   } TB_ProtocolVersion;

   enum {
       rsa2048_pkcs1.5(0), rsa2048_pss(1), ecdsap256(2), (255)
   } TokenBindingKeyParameters;

   struct {
       TB_ProtocolVersion token_binding_version;
       TokenBindingKeyParameters key_parameters_list<1..2^8-1>
   } TokenBindingParameters;

   "token_binding_version" indicates the version of the Token Binding
   protocol the client wishes to use during this connection.  If the
   client supports multiple Token Binding protocol versions, it SHOULD
   indicate the latest supported version (the one with the highest
   TB_ProtocolVersion.major and TB_ProtocolVersion.minor) in
   TokenBindingParameters.token_binding_version.  For example, if the
   client supports versions {1, 0} and {0, 13} of the Token Binding
   protocol, it SHOULD indicate version {1, 0}. Please note that the
   server MAY select any lower protocol version; see Section 3
   ("Token Binding Negotiation ServerHello Extension") for more details.
   If the client does not support the Token Binding protocol version
   selected by the server, then the connection proceeds without Token
   Binding.  [RFC8471] describes version {1, 0} of the protocol.

   Please note that the representation of the Token Binding protocol
   version using two octets ("major" and "minor") is for human
   convenience only and carries no protocol significance.

   "key_parameters_list" contains the list of identifiers of the Token
   Binding key parameters supported by the client, in descending order
   of preference.  [RFC8471] establishes an IANA registry for Token
   Binding key parameters identifiers.

3.  Token Binding Negotiation ServerHello Extension

   The server uses the "token_binding" TLS extension to indicate support
   for the Token Binding protocol and to select the protocol version and
   key parameters.






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   The server that supports Token Binding and receives a ClientHello
   message containing the "token_binding" extension will include the
   "token_binding" extension in the ServerHello if all of the following
   conditions are satisfied:

   1.  The server supports the Token Binding protocol version offered by
       the client, or a lower version.

   2.  The server finds acceptable Token Binding key parameters in the
       client's list.

   3.  The server is also negotiating the extended master secret
       [RFC7627] and renegotiation indication [RFC5746] TLS extensions.
       This requirement applies when TLS 1.2 or an older TLS version is
       used (see Section 6 ("Security Considerations") for more
       details).

   The server will ignore any key parameters that it does not recognize.
   The "extension_data" field of the "token_binding" extension is
   structured the same as described above for the client
   "extension_data".

   "token_binding_version" contains the lower of:

   o  the Token Binding protocol version offered by the client in the
      "token_binding" extension, and

   o  the highest Token Binding protocol version supported by the
      server.

   "key_parameters_list" contains exactly one Token Binding key
   parameters identifier selected by the server from the client's list.

4.  Negotiating Token Binding Protocol Version and Key Parameters

   It is expected that a server will have a list of Token Binding key
   parameters identifiers that it supports, in preference order.  The
   server MUST only select an identifier that the client offered.  The
   server SHOULD select the most highly preferred key parameters
   identifier it supports, which is also advertised by the client.  In
   the event that the server supports none of the key parameters that
   the client advertises, then the server MUST NOT include the
   "token_binding" extension in the ServerHello.








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   The client receiving the "token_binding" extension MUST terminate the
   handshake with a fatal "unsupported_extension" alert if any of the
   following conditions are true:

   1.  The client did not include the "token_binding" extension in the
       ClientHello.

   2.  "token_binding_version" is higher than the Token Binding protocol
       version advertised by the client.

   3.  "key_parameters_list" includes more than one Token Binding key
       parameters identifier.

   4.  "key_parameters_list" includes an identifier that was not
       advertised by the client.

   5.  TLS 1.2 or an older TLS version is used, but the extended master
       secret [RFC7627] and TLS renegotiation indication [RFC5746]
       extensions are not negotiated (see Section 6
       ("Security Considerations") for more details).

   If the "token_binding" extension is included in the ServerHello and
   the client supports the Token Binding protocol version selected by
   the server, it means that the version and key parameters have been
   negotiated between the client and the server and SHALL be definitive
   for the TLS connection.  TLS 1.2 and earlier versions support
   renegotiation, which allows the client and server to renegotiate the
   Token Binding protocol version and key parameters on the same
   connection.  The client MUST use the negotiated key parameters in the
   "provided_token_binding" as described in [RFC8471].

   If the client does not support the Token Binding protocol version
   selected by the server, then the connection proceeds without Token
   Binding.  There is no requirement for the client to support any Token
   Binding versions other than the one advertised in the client's
   "token_binding" extension.

   Client and server applications can choose to handle failure to
   negotiate Token Binding in a variety of ways: continue using the
   connection as usual, shorten the lifetime of tokens issued during
   this connection, require stronger authentication, terminate the
   connection, etc.

   The Token Binding protocol version and key parameters are negotiated
   for each TLS connection, which means that the client and server
   include their "token_binding" extensions in both the full TLS
   handshake that establishes a new TLS session and the subsequent
   abbreviated TLS handshakes that resume the TLS session.



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5.  IANA Considerations

   This document updates the "TLS ExtensionType Values" registry.  The
   registration for the "token_binding" TLS extension is as follows:

      Value: 24

      Extension name: token_binding

      Recommended: Yes

      Reference: This document

   This document uses the "Token Binding Key Parameters" registry
   created by [RFC8471].  This document creates no new registrations in
   the registry.

6.  Security Considerations

6.1.  Downgrade Attacks

   The Token Binding protocol version and key parameters are negotiated
   via the "token_binding" extension within the TLS handshake.  TLS
   detects handshake message modification by active attackers;
   therefore, it is not possible for an attacker to remove or modify the
   "token_binding" extension without breaking the TLS handshake.  The
   signature algorithm and key length used in the Token Binding of type
   "provided_token_binding" MUST match the parameters negotiated via the
   "token_binding" extension.

6.2.  Triple Handshake Vulnerability in TLS 1.2 and Older TLS Versions

   The Token Binding protocol relies on the TLS exporters [RFC5705] to
   associate a TLS connection with a Token Binding.  The triple
   handshake attack [TRIPLE-HS] is a known vulnerability in TLS 1.2 and
   older TLS versions; it allows an attacker to synchronize keying
   material between TLS connections.  The attacker can then successfully
   replay bound tokens.  For this reason, the Token Binding protocol
   MUST NOT be negotiated with these TLS versions, unless the extended
   master secret [RFC7627] and renegotiation indication [RFC5746] TLS
   extensions have also been negotiated.










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7.  References

7.1.  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>.

   [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>.

   [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>.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <https://www.rfc-editor.org/info/rfc5746>.

   [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>.

   [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>.

   [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-editor.org/info/rfc8471>.

7.2.  Informative References

   [TOKENBIND-TLS13]
              Harper, N., "Token Binding for Transport Layer Security
              (TLS) Version 1.3 Connections", Work in Progress,
              draft-ietf-tokbind-tls13-01, May 2018.







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   [TRIPLE-HS]
              Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
              A., and P. Strub, "Triple Handshakes and Cookie Cutters:
              Breaking and Fixing Authentication over TLS", IEEE
              Symposium on Security and Privacy, DOI 10.1109/SP.2014.14,
              May 2014.

Acknowledgements

   This document incorporates comments and suggestions offered by Eric
   Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony
   Nadalin, Michael B. Jones, Bill Cox, Nick Harper, Brian Campbell,
   Benjamin Kaduk, Alexey Melnikov, and others.

   This document was produced under the chairmanship of John Bradley and
   Leif Johansson.  The area directors included Eric Rescorla, Kathleen
   Moriarty, and Stephen Farrell.

Authors' Addresses

   Andrei Popov (editor)
   Microsoft Corp.
   United States of America

   Email: andreipo@microsoft.com


   Magnus Nystroem
   Microsoft Corp.
   United States of America

   Email: mnystrom@microsoft.com


   Dirk Balfanz
   Google Inc.
   United States of America

   Email: balfanz@google.com












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