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Versions: (draft-popov-token-binding) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15

Internet Engineering Task Force                            A. Popov, Ed.
Internet-Draft                                               M. Nystroem
Intended status: Standards Track                         Microsoft Corp.
Expires: March 17, 2016                                       D. Balfanz
                                                              A. Langley
                                                             Google Inc.
                                                      September 14, 2015


                 The Token Binding Protocol Version 1.0
                     draft-ietf-tokbind-protocol-02

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 March 17, 2016.

Copyright Notice

   Copyright (c) 2015 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.  Token Binding Protocol Message  . . . . . . . . . . . . . . .   4
   4.  Establishing a TLS Token Binding  . . . . . . . . . . . . . .   6
   5.  TLS Token Binding ID Format . . . . . . . . . . . . . . . . .   7
   6.  Security Token Validation . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
     8.1.  Security Token Replay . . . . . . . . . . . . . . . . . .   9
     8.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .   9
     8.3.  Privacy Considerations  . . . . . . . . . . . . . . . . .  10
     8.4.  Token Binding Key Sharing Between Applications  . . . . .  10
     8.5.  Triple Handshake Vulnerability in TLS . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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 exported keying material [RFC5705]
   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 the Token Binding Negotiation TLS Extension
   [I-D.ietf-tokbind-negotiation] to negotiate the Token Binding
   protocol version and the parameters (signature algorithm, length) of
   the Token Binding key.  This negotiation does not require additional
   round-trips.

   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 [I-D.ietf-tokbind-negotiation], and then validates the
   signatures contained in 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




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   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 defines 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 Negotiation TLS Extension
   [I-D.ietf-tokbind-negotiation] describes the negotiation of the Token
   Binding protocol and key parameters.  Token Binding over HTTP
   [I-D.ietf-tokbind-https] explains how the Token Binding message is
   encapsulated within HTTP/1.1 [RFC7230] or HTTP/2 [RFC7540] messages.
   [I-D.ietf-tokbind-https] also describes Token Binding between
   multiple communicating parties: User Agent, Identity Provider and
   Relying Party.

3.  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 [I-D.ietf-tokbind-negotiation], 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
   [I-D.ietf-tokbind-https] 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 {
    rsa2048_pkcs1.5_sha256(0), rsa2048_pss_sha256(1), ecdsap256_sha256(2), (255)
} TokenBindingKeyParameters;

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

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

struct {



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

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

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

struct {
    TokenBindingType tokenbinding_type;
    TokenBindingKeyParameters key_parameters;
    select (key_parameters) {
        case rsa2048_pkcs1.5_sha256:
        case rsa2048_pss_sha256:
            RSAPublicKey rsapubkey;
        case ecdsap256_sha256:
            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 the exported keying material value
    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
   exported keying material (EKM) value, optionally followed by
   Extension structures.



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   The EKM is obtained using the Keying Material Exporters for TLS
   defined in [RFC5705], by supplying the following input values:

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

   o  Context value: NULL (no application context supplied).

   o  Length: 32 bytes.

   An implementation MUST ignore any unknown extensions.  Initially, no
   extension types are defined.  One of the possible uses of extensions
   envisioned at the time of this writing is attestation: cryptographic
   proof that allows the server to verify that the Token Binding key is
   hardware-bound.  The definitions of such Token Binding protocol
   extensions are outside the scope of this specification.

   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
   [I-D.ietf-tokbind-negotiation].  The client SHOULD generate and store
   Token Binding keys in a secure manner that prevents key export.  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.

4.  Establishing a TLS Token Binding

   The 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 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
   [I-D.ietf-tokbind-negotiation].  In the case of a mismatch, the
   server MUST terminate the connection.  As described in
   [I-D.ietf-tokbind-https], Token Bindings of type
   "referred_token_binding" may have different key parameters than those
   negotiated via [I-D.ietf-tokbind-negotiation].




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   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.

5.  TLS Token Binding ID Format

   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;
       TokenBindingKeyParameters key_parameters;
       select (key_parameters) {
           case rsa2048_pkcs1.5_sha256:
           case rsa2048_pss_sha256:
               RSAPublicKey rsapubkey;
           case ecdsap256_sha256:
               ECDSAParams ecdsaparams;
       }
   } TokenBindingID;


   TokenBindingID includes the type of the token binding and the key
   parameters negotiated via [I-D.ietf-tokbind-negotiation].  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 [I-D.ietf-tokbind-https]
   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.

6.  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.




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   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.

7.  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






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   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.

   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 uses "Token Binding Key Parameters" registry originally
   created in [I-D.ietf-tokbind-negotiation] and "TLS NamedCurve"
   registry originally created in [RFC4492].  This document creates no
   new registrations in these registries.

8.  Security Considerations

8.1.  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.

8.2.  Downgrade Attacks

   The Token Binding protocol is only used when negotiated via
   [I-D.ietf-tokbind-negotiation] 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



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   modify the Token Binding Negotiation TLS Extension 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
   [I-D.ietf-tokbind-negotiation].

8.3.  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.  Some applications offer special
   privacy modes where they don't store or use tokens supplied by the
   server, e.g.  "in private" browsing.  Connections made in these
   special privacy modes SHOULD NOT negotiate Token Binding.  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.

8.4.  Token Binding Key Sharing Between Applications

   Existing systems provide a variety of platform-specific mechanisms
   for certain applications to share tokens, e.g. to enable single sign-
   on scenarios.  For these scenarios to keep working with bound tokens,
   the applications that are allowed to share tokens will need to also
   share Token Binding keys.  Care must be taken to restrict the sharing
   of Token Binding keys to the same group(s) of applications that share
   the same tokens.

8.5.  Triple Handshake Vulnerability in TLS

   The Token Binding protocol relies on the exported keying material
   (EKM) to associate a TLS connection with a Token Binding.  The triple
   handshake attack [TRIPLE-HS] is a known TLS protocol vulnerability
   allowing the 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
   unless the Extended Master Secret TLS extension
   [I-D.ietf-tls-session-hash] has also been negotiated.

9.  Acknowledgements

   This document incorporates comments and suggestions offered by Eric
   Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Bill Cox,
   Nick Harper and others.





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10.  References

10.1.  Normative References

   [I-D.ietf-tokbind-https]
              Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
              "Token Binding over HTTP", draft-ietf-tokbind-https-01
              (work in progress), June 2015.

   [I-D.ietf-tokbind-negotiation]
              Popov, A., Nystrom, M., Balfanz, D., and A. Langley,
              "Transport Layer Security (TLS) Extension for Token
              Binding Protocol Negotiation", draft-ietf-tokbind-
              negotiation-00 (work in progress), September 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              DOI 10.17487/RFC4492, May 2006,
              <http://www.rfc-editor.org/info/rfc4492>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://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, <http://www.rfc-editor.org/info/rfc5705>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.







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   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <http://www.rfc-editor.org/info/rfc7540>.

10.2.  Informative References

   [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-06 (work in progress), July 2015.

   [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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