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Versions: 00 draft-ietf-tokbind-protocol

Internet Engineering Task Force                            A. Popov, Ed.
Internet-Draft                                               M. Nystroem
Intended status: Standards Track                         Microsoft Corp.
Expires: April 16, 2015                                       D. Balfanz
                                                              A. Langley
                                                             Google Inc.
                                                        October 13, 2014


                 The Token Binding Protocol Version 1.0
                      draft-popov-token-binding-00

Abstract

   This document specifies Version 1.0 of the Token Binding protocol.
   The Token Binding protocol allows client/server applications to
   create long-lived, uniquely identifiable TLS [RFC5246] bindings
   spanning multiple TLS sessions and connections.  Applications are
   then enabled to cryptographically bind security tokens to the TLS
   layer, preventing token export and replay attacks.  To protect
   privacy, the TLS Token Binding identifiers are only transmitted
   encrypted and can be reset by the user at any time.

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 http://datatracker.ietf.org/drafts/current/.

   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 April 16, 2015.

Copyright Notice

   Copyright (c) 2014 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
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Token Binding Protocol Overview . . . . . . . . . . . . . . .   3
   3.  Negotiating the Token Binding Protocol and Key Parameters . .   4
   4.  Token Binding Protocol Message  . . . . . . . . . . . . . . .   5
   5.  Establishing a TLS Token Binding  . . . . . . . . . . . . . .   7
   6.  TLS Token Binding ID Format . . . . . . . . . . . . . . . . .   7
   7.  Security Token Validation . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     9.1.  Security Token Replay . . . . . . . . . . . . . . . . . .  11
     9.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .  11
     9.3.  Privacy Considerations  . . . . . . . . . . . . . . . . .  11
     9.4.  Triple Handshake Vulnerability in TLS . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Web services generate various security tokens (e.g.  HTTP cookies,
   OAuth tokens) for web applications to access protected resources.
   Any party in possession of such token gains access to the protected
   resource.  Attackers export bearer tokens from the user's machine,
   present them to web services, and impersonate authenticated users.
   The idea of Token Binding is to prevent such attacks by
   cryptographically binding security tokens to the TLS layer.

   A TLS Token Binding is established by the user agent generating a
   private-public key pair (possibly within a secure hardware module,
   such as TPM) per target server, and proving possession of the private
   key on every TLS connection to the target server.  The proof of
   possession involves signing the tls_unique value [RFC5929] for the
   TLS connection with the private key.  Such TLS Token Binding is
   identified by the corresponding public key.  TLS Token Bindings are
   long-lived, i.e. they encompass multiple TLS connections and TLS
   sessions between a given client and server.  To protect privacy, TLS




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   Token Binding identifiers are never transmitted in clear text and can
   be reset by the user at any time, e.g. when clearing browser cookies.

   When issuing a security token to a client that supports TLS Token
   Binding, a server includes the client's TLS Token Binding ID in the
   token.  Later on, when a client presents a security token containing
   a TLS Token Binding ID, the server makes sure the ID in the token
   matches the ID of the TLS Token Binding established with the client.
   In the case of a mismatch, the server discards the token.

   In order to successfully export and replay a bound security token,
   the attacker needs to also be able to export the client's private
   key, which is hard to do in the case of the key generated in a secure
   hardware module.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  Token Binding Protocol Overview

   The client and server use ALPN protocol IDs [RFC7301] to negotiate
   the use of the Token Binding protocol, in addition to the actual
   application protocol such as HTTP/1.1 [RFC2616] or HTTP/2
   [I-D.ietf-httpbis-http2].  ALPN IDs are also used to negotiate the
   parameters (signature algorithm, length) of the Token Binding key.
   This negotiation does not require TLS protocol changes or additional
   round-trips.

   The "IANA Considerations" section of this document defines an initial
   set of ALPN protocol IDs that allow the use of the Token Binding
   protocol with HTTP/1.1 and HTTP/2.  The initial set of supported key
   parameters includes ECDSA with NIST P256 curve and 2048-bit RSA.  New
   ALPN protocol IDs can be defined in the future to support Token
   Binding usage with other application protocols and key parameters.

   The Token Binding protocol consists of one message sent by the client
   to the server, proving possession of one or more client-generated
   asymmetric keys.  This message is only sent if the client and server
   agree on the use of the Token Binding protocol and the key
   parameters.  The Token Binding message is sent with the application
   protocol data in TLS application_data records.

   A server receiving the Token Binding message verifies that the key
   parameters in the message match the Token Binding parameters
   negotiated via ALPN, and then validates the signatures contained in



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   the Token Binding message.  If either of these checks fails, the
   server terminates the connection, otherwise the TLS Token Binding is
   successfully established with the ID contained in the Token Binding
   message.

   When a server supporting the Token Binding protocol receives a bound
   token, the server compares the TLS Token Binding ID in the security
   token with the TLS Token Binding ID established with the client.  If
   the bound token came from a TLS connection without a Token Binding,
   or if the IDs don't match, the token is discarded.

   This document describes the negotiation of the Token Binding protocol
   and key parameters, the format of the Token Binding protocol message,
   the process of establishing a TLS Token Binding, the format of the
   Token Binding ID, and the process of validating a security token.
   Token Binding over HTTP [HTTPSTB] explains how the Token Binding
   message is encapsulated within application protocol messages.
   [HTTPSTB] also describes Token Binding between multiple communicating
   parties: User Agent, Identity Provider and Relying Party.

3.  Negotiating the Token Binding Protocol and Key Parameters

   The Token Binding protocol is used within TLS connections, in
   combination with an application protocol such as HTTP/1.1 or HTTP/2.
   The "IANA Considerations" section of this document defines a set of
   ALPN protocol IDs that combine application protocol and token binding
   key parameters:

   o  "h2_tb_p256" indicates support for HTTP/2 with Token Binding using
      ECDSA key and NIST P256 curve;

   o  "h2_tb_rsa2048" indicates support for HTTP/2 with Token Binding
      using 2048-bit RSA key;

   o  "http/1.1_tb_p256" indicates support for HTTP/1.1 with Token
      Binding using ECDSA key and NIST P256 curve;

   o  "http/1.1_tb_rsa2048" indicates support for HTTP/1.1 with Token
      Binding using 2048-bit RSA key.

   The client advertises support of the Token Binding protocol by
   sending some of these IDs in the ALPN extension in the ClientHello.
   Application protocol IDs without Token Binding, such as "http/1.1"
   and "h2", can also be included for compatibility with the servers
   that do not support the Token Binding protocol.

   The server indicates support of the Token Binding protocol by sending
   one of the above IDs in the ALPN extension in the ServerHello.  The



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   server implements the protocol selection logic as described in
   section 3.2 "Protocol Selection" of [RFC7301], taking into account
   the application protocols and key parameters supported by the server.

4.  Token Binding Protocol Message

   The Token Binding message is sent by the client and proves possession
   of one or more private keys held by the client.  This message MUST be
   sent if the client and server successfully negotiated the use of the
   Token Binding protocol via ALPN, and MUST NOT be sent otherwise.
   This message MUST be sent in the client's first application protocol
   message.  This message MAY also be sent in subsequent application
   protocol messages, proving possession of other keys by the same
   client, to facilitate token binding between more than two
   communicating parties.  Token Binding over HTTP [HTTPSTB] specifies
   the encapsulation of the Token Binding message in the application
   protocol messages, and the scenarios involving more than two
   communicating parties.  The Token Binding message format is defined
   using TLS specification language, and reuses existing TLS structures
   and IANA registrations where possible:


enum {
    sha256(4), (255)
} HashAlgorithm;

enum {
    rsa(1), ecdsap256(3), (255)
} SignatureAlgorithm;

struct {
    HashAlgorithm hash;
    SignatureAlgorithm signature;
} SignatureAndHashAlgorithm;

struct {
    opaque modulus<1..2^16-1>;
    opaque publicexponent<1..2^8-1>;
} RSAPublicKey;

enum {
    secp256r1 (23), (0xFFFF)
} NamedCurve;

struct {
    opaque point <1..2^8-1>;
} ECPoint;




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struct {
    NamedCurve namedcurve;
    ECPoint point;      // Uncompressed format
} ECDSAParams;

enum {
    provided_token_binding(0), referred_token_binding(1), (255)
} TokenBindingType;

struct {
    TokenBindingType tokenbinding_type;
    SignatureAndHashAlgorithm algorithm;
    select (algorithm.signature) {
        case rsa: RSAPublicKey rsapubkey;
        case ecdsa: ECDSAParams ecdsaparams;
    }
} TokenBindingID;

enum {
    (255)           // No initial ExtensionType registrations
} ExtensionType;

struct {
    ExtensionType extension_type;
    opaque extension_data<0..2^16-1>;
} Extension;

struct {
    TokenBindingID tokenbindingid;
    opaque signature<0..2^16-1>;// Signature over hashed ("token binding", tls_unique)
    Extension extensions<0..2^16-1>;
} TokenBinding;

struct {
    TokenBinding tokenbindings<0..2^16-1>;
} TokenBindingMessage;


   The Token Binding message consists of a series of TokenBinding
   structures containing the TokenBindingID, a signature over the hash
   of the NUL-terminated, ASCII label ("token binding") and the
   tls_unique, optionally followed by Extension structures.  An
   implementation MUST ignore any unknown extensions.  Initially, no
   extension types are defined.  At least one TokenBinding MUST be
   included in the Token Binding message.  The signature algorithm and
   key length used in the TokenBinding MUST match the parameters
   negotiated via ALPN.  The client SHOULD generate and store Token
   Binding keys in a secure manner that prevents key export.  In order



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   to prevent cooperating servers from linking user identities,
   different keys SHOULD be used by the client for connections to
   different servers, according to the token scoping rules of the
   application protocol.

5.  Establishing a TLS Token Binding

   Triple handshake vulnerability in the TLS protocol affects the
   security of the Token Binding protocol, as described in the "Security
   Considerations" section below.  Therefore, the server MUST NOT
   negotiate the use of the Token Binding protocol unless the server
   also negotiates Extended Master Secret TLS extension
   [I-D.ietf-tls-session-hash].

   The server MUST terminate the connection if the use of the Token
   Binding protocol has been successfully negotiated via ALPN within the
   TLS handshake, but the client's first application message does not
   contain the Token Binding message.  The server MUST terminate the
   connection if the use of the Token Binding protocol was not
   negotiated, but the client sends the Token Binding message.

   If the Token Binding type is "provided_token_binding", the server
   MUST verify that the signature algorithm (including elliptic curve in
   the case of ECDSA) and key length in the Token Binding message match
   those negotiated via ALPN.  In the case of a mismatch, the server
   MUST terminate the connection.  As described in [HTTPSTB], Token
   Bindings of type "referred_token_binding" may have different key
   parameters than those negotiated via ALPN.

   If the Token Binding message does not contain at least one
   TokenBinding structure, or the signature contained in a TokenBinding
   structure is invalid, the server MUST terminate the connection.
   Otherwise, the TLS Token Binding is successfully established and its
   ID can be provided to the application for security token validation.

6.  TLS Token Binding ID Format















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   The ID of the TLS Token Binding established as a result of Token
   Binding message processing is a binary representation of the
   following structure:


   struct {
       TokenBindingType tokenbinding_type;
       SignatureAndHashAlgorithm algorithm;
       select (algorithm.signature) {
           case rsa: RSAPublicKey rsapubkey;
           case ecdsa: ECDSAParams ecdsaparams;
       }
   } TokenBindingID;


   TokenBindingID includes the type of the token binding and the key
   parameters negotiated via ALPN.  This document defines two token
   binding types: provided_token_binding used to establish a Token
   Binding when connecting to a server, and referred_token_binding used
   when requesting tokens to be presented to a different server.  Token
   Binding over HTTP [HTTPSTB] describes Token Binding between multiple
   communicating parties: User Agent, Identity Provider and Relying
   Party.  TLS Token Binding ID can be obtained from the TokenBinding
   structure described in the "Token Binding Protocol Message" section
   of this document by discarding the signature and extensions.  TLS
   Token Binding ID will be available at the application layer and used
   by the server to generate and verify bound tokens.

7.  Security Token Validation

   Security tokens can be bound to the TLS layer either by embedding the
   Token Binding ID in the token, or by maintaining a database mapping
   tokens to Token Binding IDs.  The specific method of generating bound
   security tokens is application-defined and beyond the scope of this
   document.

   Upon receipt of a security token, the server attempts to retrieve TLS
   Token Binding ID information from the token and from the TLS
   connection with the client.  Application-provided policy determines
   whether to honor non-bound (bearer) tokens.  If the token is bound
   and a TLS Token Binding has not been established for the client
   connection, the server MUST discard the token.  If the TLS Token
   Binding ID for the token does not match the TLS Token Binding ID
   established for the client connection, the server MUST discard the
   token.






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8.  IANA Considerations

   This document establishes a registry for Token Binding type
   identifiers entitled "Token Binding Types" under the "Token Binding
   Protocol" heading.

   Entries in this registry require the following fields:

   o  Value: The octet value that identifies the Token Binding type
      (0-255).

   o  Description: The description of the Token Binding type.

   o  Specification: A reference to a specification that defines the
      Token Binding type.

   This registry operates under the "Expert Review" policy as defined in
   [RFC5226].  The designated expert is advised to encourage the
   inclusion of a reference to a permanent and readily available
   specification that enables the creation of interoperable
   implementations using the identified Token Binding type.

   An initial set of registrations for this registry follows:

      Value: 0

      Description: provided_token_binding

      Specification: this document

      Value: 1

      Description: referred_token_binding

      Specification: this document

   This document establishes a registry for Token Binding extensions
   entitled "Token Binding Extensions" under the "Token Binding
   Protocol" heading.

   Entries in this registry require the following fields:

   o  Value: The octet value that identifies the Token Binding extension
      (0-255).

   o  Description: The description of the Token Binding extension.





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   o  Specification: A reference to a specification that defines the
      Token Binding extension.

   This registry operates under the "Expert Review" policy as defined in
   [RFC5226].  The designated expert is advised to encourage the
   inclusion of a reference to a permanent and readily available
   specification that enables the creation of interoperable
   implementations using the identified Token Binding extension.  This
   document creates no initial registrations in the "Token Binding
   Extensions" registry.

   This document creates the following registrations for the
   identification of the Token Binding protocol in the "Application
   Layer Protocol Negotiation (ALPN) Protocol IDs" registry originally
   created in [RFC7301]:

      Protocol: HTTP/2 with Token Binding using ECDSA key and NIST P256
      curve

      Identification Sequence: 0x68 0x32 0x5f 0x74 0x62 0x5f 0x70 0x32
      0x35 0x36 ("h2_tb_p256")

      Specification: this document

      Protocol: HTTP/2 with Token Binding using 2048-bit RSA key

      Identification Sequence: 0x68 0x32 0x5f 0x74 0x62 0x5f 0x72 0x73
      0x61 0x32 0x30 0x34 0x38 ("h2_tb_rsa2048")

      Specification: this document

      Protocol: HTTP/1.1 with Token Binding using ECDSA key and NIST
      P256 curve

      Identification Sequence: 0x68 0x74 0x74 0x70 0x2f 0x31 0x2e 0x31
      0x5f 0x74 0x62 0x5f 0x70 0x32 0x35 0x36 ("http/1.1_tb_p256")

      Specification: this document

      Protocol: HTTP/1.1 with Token Binding using 2048-bit RSA key

      Identification Sequence: 0x68 0x74 0x74 0x70 0x2f 0x31 0x2e 0x31
      0x5f 0x74 0x62 0x5f 0x72 0x73 0x61 0x32 0x30 0x34 0x38
      ("http/1.1_tb_rsa2048")

      Specification: this document





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   This document uses "TLS SignatureAlgorithm" and "TLS HashAlgorithm"
   registries originally created in [RFC5246], and "TLS NamedCurve"
   registry originally created in [RFC4492].  This document creates no
   new registrations in these registries.

9.  Security Considerations

9.1.  Security Token Replay

   The goal of the Token Binding protocol is to prevent attackers from
   exporting and replaying security tokens, thereby impersonating
   legitimate users and gaining access to protected resources.  Bound
   tokens can still be replayed by the malware present in the User
   Agent.  In order to export the token to another machine and
   successfully replay it, the attacker also needs to export the
   corresponding private key.  Token Binding private keys are therefore
   high-value assets and SHOULD be strongly protected, ideally by
   generating them in a hardware security module that prevents key
   export.

9.2.  Downgrade Attacks

   The Token Binding protocol is only used when negotiated via ALPN
   within the TLS handshake.  TLS prevents active attackers from
   modifying the messages of the TLS handshake, therefore it is not
   possible for the attacker to remove or modify the ALPN IDs used to
   negotiate the Token Binding protocol and key parameters.  The
   signature algorithm and key length used in the TokenBinding of type
   "provided_token_binding" MUST match the parameters negotiated via
   ALPN.

9.3.  Privacy Considerations

   The Token Binding protocol uses persistent, long-lived TLS Token
   Binding IDs.  To protect privacy, TLS Token Binding IDs are never
   transmitted in clear text and can be reset by the user at any time,
   e.g. when clearing browser cookies.  In order to prevent cooperating
   servers from linking user identities, different keys SHOULD be used
   by the client for connections to different servers, according to the
   token scoping rules of the application protocol.

9.4.  Triple Handshake Vulnerability in TLS

   The Token Binding protocol relies on the tls_unique value to
   associate a TLS connection with a TLS Token Binding.  The triple
   handshake attack [TRIPLE-HS] is a known TLS protocol vulnerability
   allowing the attacker to synchronize tls_unique values between TLS
   connections.  The attacker can then successfully replay bound tokens.



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   For this reason, the Token Binding protocol MUST NOT be negotiated
   unless the Extended Master Secret TLS extension
   [I-D.ietf-tls-session-hash] has also been negotiated.

10.  References

10.1.  Normative References

   [HTTPSTB]  Balfanz, D., Langley, A., Popov, A., and M. Nystroem,
              "Token Binding over HTTP", 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492, May 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, July 2010.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, July 2014.

10.2.  Informative References

   [I-D.ietf-httpbis-http2]
              Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
              Protocol version 2", draft-ietf-httpbis-http2-14 (work in
              progress), July 2014.

   [I-D.ietf-tls-session-hash]
              Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley,
              A., and M. Ray, "Transport Layer Security (TLS) Session
              Hash and Extended Master Secret Extension", draft-ietf-
              tls-session-hash-02 (work in progress), October 2014.



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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", 2014.

Authors' Addresses

   Andrei Popov (editor)
   Microsoft Corp.
   USA

   Email: andreipo@microsoft.com


   Magnus Nystroem
   Microsoft Corp.
   USA

   Email: mnystrom@microsoft.com


   Dirk Balfanz
   Google Inc.
   USA

   Email: balfanz@google.com


   Adam Langley
   Google Inc.
   USA

   Email: agl@google.com

















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