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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 16 17 18 19 RFC 8471

Internet Engineering Task Force                            A. Popov, Ed.
Internet-Draft                                               M. Nystroem
Intended status: Standards Track                         Microsoft Corp.
Expires: August 20, 2017                                      D. Balfanz
                                                              A. Langley
                                                             Google Inc.
                                                               J. Hodges
                                                                  Paypal
                                                       February 16, 2017


                 The Token Binding Protocol Version 1.0
                     draft-ietf-tokbind-protocol-13

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 Token Binding identifiers are only conveyed over TLS 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 August 20, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   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
   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
     3.1.  TokenBinding.tokenbinding_type  . . . . . . . . . . . . .   6
     3.2.  TokenBinding.tokenbindingid . . . . . . . . . . . . . . .   6
     3.3.  TokenBinding.signature  . . . . . . . . . . . . . . . . .   7
     3.4.  TokenBinding.extensions . . . . . . . . . . . . . . . . .   8
   4.  Establishing a Token Binding  . . . . . . . . . . . . . . . .   9
     4.1.  Client Processing Rules . . . . . . . . . . . . . . . . .   9
     4.2.  Server Processing Rules . . . . . . . . . . . . . . . . .   9
   5.  Bound Security Token Creation and Validation  . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Token Binding Key Parameters Registry . . . . . . . . . .  11
     6.2.  Token Binding Types Registry  . . . . . . . . . . . . . .  12
     6.3.  Token Binding Extensions Registry . . . . . . . . . . . .  13
     6.4.  Registration of Token Binding TLS Exporter Label  . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     7.1.  Security Token Replay . . . . . . . . . . . . . . . . . .  13
     7.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .  14
     7.3.  Privacy Considerations  . . . . . . . . . . . . . . . . .  14
     7.4.  Token Binding Key Sharing Between Applications  . . . . .  14
     7.5.  Triple Handshake Vulnerability in TLS 1.2 and Older TLS
           Versions  . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Often, servers generate various security tokens (e.g.  HTTP cookies,
   OAuth tokens) for applications to present when accessing protected
   resources.  In general, any party in possession of bearer security
   tokens gain access to certain protected resource(s).  Attackers take



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   advantage of this by exporting bearer tokens from user's application
   connections or machines, presenting them to application servers, and
   impersonating authenticated users.  The idea of Token Binding is to
   prevent such attacks by cryptographically binding application
   security tokens to the underlying TLS layer.

   A Token Binding is established by a user agent generating a private-
   public key pair (possibly, within a secure hardware module, such as
   TPM) per target server, providing the public key to the server, and
   proving possession of the corresponding private key, on every TLS
   connection to the server.  The proof of possession involves signing
   the exported keying material (EKM) [RFC5705] from the TLS connection
   with the private key.  The corresponding public key is included in
   the Token Binding identifier structure (described in the Section 3.2
   "TokenBinding.tokenbindingid").  Token Bindings are long-lived, i.e.,
   they encompass multiple TLS connections and TLS sessions between a
   given client and server.  To protect privacy, Token Binding IDs are
   never conveyed over insecure connections 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 Token
   Binding, a server includes the client's Token Binding ID in the
   token.  Later on, when a client presents a security token containing
   a Token Binding ID, the server ensures the ID in the token matches
   the ID of the Token Binding established with the client.  In the case
   of a mismatch, the server rejects the token (details are application-
   specific).

   In order to successfully export and replay a bound security token, an
   attacker needs to also be able to export the client's private key,
   which is hard to do if the key is specially protected, e.g.,
   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

   In the course of a TLS handshake, a 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.





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   The Token Binding protocol consists of one message sent by the client
   to the server, proving possession of one or more client-generated
   asymmetric private keys.  This message is not sent if the Token
   Binding Negotiation has been unsuccessful.  The Token Binding message
   is sent with the application protocol data over TLS.

   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 rejects the binding, along with all
   associeted bound tokens.  Otherwise the 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 Token Binding ID in the token with the
   Token Binding ID established with the client.  If the bound token
   came from a TLS connection without a Token Binding, or if the Token
   Binding IDs do not match, the token is rejected.

   This document defines the format of the Token Binding protocol
   message, the process of establishing a Token Binding, the format of
   the Token Binding ID, and the process of validating a bound 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 to prove 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
   additional private keys held by the same client, which can be used to
   facilitate token binding between more than two communicating parties.
   For example, Token Binding over HTTP [I-D.ietf-tokbind-https]
   specifies an encapsulation of the Token Binding message in HTTP
   application protocol messages, as well as scenarios involving more
   than two communicating parties.




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   The Token Binding message format is defined using TLS Presentation
   Language (see Section 4 of [RFC5246]):

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

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

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

    struct {
        TokenBindingKeyParameters key_parameters;
        uint16 key_length;  /* Length (in bytes) of the following
                               TokenBindingID.TokenBindingPublicKey */
        select (key_parameters) {
            case rsa2048_pkcs1.5:
            case rsa2048_pss:
                RSAPublicKey rsapubkey;
            case ecdsap256:
                ECPoint point;
        } TokenBindingPublicKey;
    } TokenBindingID;

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

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

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

    struct {
        TokenBindingType tokenbinding_type;
        TokenBindingID tokenbindingid;
        opaque signature<0..2^16-1>; /* Signature over the concatenation
                                        of tokenbinding_type,
                                        key_parameters and exported



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                                        keying material (EKM) */
        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, each containing the type of the token binding, the
   TokenBindingID, a signature using the Token Binding key, optionally
   followed by Extension structures.

3.1.  TokenBinding.tokenbinding_type

   This document defines two Token Binding types:

   o  provided_token_binding - used to establish a Token Binding when
      connecting to a server.

   o  referred_token_binding - used when requesting tokens to be
      presented to a different server.

   Token Binding over HTTP [I-D.ietf-tokbind-https] describes a use case
   for referred_token_binding where Token Bindings are established
   between multiple communicating parties: User Agent, Identity Provider
   and Relying Party.  User Agent sends referred_token_binding to the
   Identity Provider in order to prove possession of the Token Binding
   key it uses with the Relying Party.  The Identity Provider can then
   bind the token it is supplying (for presentation to the Relying
   Party) to the Token Binding ID contained in the
   referred_token_binding.  Such a bound token enjoys the protections
   discussed below in Section 7 "Security Considerations".

3.2.  TokenBinding.tokenbindingid

   The ID of the Token Binding established as a result of Token Binding
   message processing contains the identifier of the key parameters
   negotiated via [I-D.ietf-tokbind-negotiation], the length (in bytes)
   of the Token Binding public key, and the Token Binding public key
   itself.  Token Binding ID can be obtained from the TokenBinding
   structure by discarding the Token Binding type, signature and
   extensions.

   When rsa2048_pkcs1.5 or rsa2048_pss is used, RSAPublicKey.modulus and
   RSAPublicKey.publicexponent contain the modulus and exponent of a
   2048-bit RSA public key represented in big-endian format, with
   leading zero bytes omitted.



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   When ecdsap256 is used, ECPoint.point contains the X coordinate
   followed by the Y coordinate of a Curve P-256 key.  The X and Y
   coordinates are unsigned 32-byte integers encoded in big-endian
   format, preserving any leading zero bytes.  Future specifications may
   define Token Binding keys using other elliptic curves with their
   corresponding signature and point formats.

   Token Binding protocol implementations SHOULD make Token Binding IDs
   available to the application as opaque byte sequences.  E.g., server
   applications will use Token Binding IDs when generating and verifying
   bound tokens.

3.3.  TokenBinding.signature

   When rsa2048_pkcs1.5 is used, TokenBinding.signature contains the
   signature generated using the RSASSA-PKCS1-v1_5 signature scheme
   defined in [RFC3447] with SHA256 as the hash function.

   When rsa2048_pss is used, TokenBinding.signature contains the
   signature generated using the RSASSA-PSS signature scheme defined in
   [RFC3447] with SHA256 as the hash function.  MGF1 with SHA256 MUST be
   used as the mask generation function, and the salt length MUST equal
   32 bytes.

   When ecdsap256 is used, TokenBinding.signature contains a pair of
   32-byte integers, R followed by S, generated with ECDSA using Curve
   P-256 and SHA256 as defined in [ANSI.X9-62.2005] and
   [FIPS.186-4.2013].  R and S are encoded in big-endian format,
   preserving any leading zero bytes.

   The signature is computed over the byte string representing the
   concatenation of:

   o  TokenBindingType value contained in the
      TokenBinding.tokenbinding_type field;

   o  TokenBindingKeyParameters value contained in the
      TokenBindingID.key_parameters field;

   o  Exported keying material (EKM) value obtained from the current TLS
      connection.

   Please note that TLS 1.2 and earlier versions support renegotiation,
   which produces a new TLS master secret for the same connection, with
   associated session keys and EKM value.  TokenBinding.signature MUST
   be a signature of the EKM value derived from the TLS master secret
   that produced the session keys encrypting the TLS application_data
   record(s) containing this TokenBinding.  Such use of the current EKM



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   for the TLS connection makes replay of bound tokens within
   renegotiated TLS sessions detectable, but requires the application to
   synchronize Token Binding message generation and verification with
   the TLS handshake state.

   Specifications defining the use of Token Binding with application
   protocols, such as Token Binding over HTTP [I-D.ietf-tokbind-https],
   MAY prohibit the use of TLS renegotiation in combination with Token
   Binding, obviating the need for such synchronization.  Alternatively,
   such specifications need to define a way to determine which EKM value
   corresponds to a given TokenBindingMessage, and a mechanism
   preventing a TokenBindingMessage from being split across TLS
   renegotiation boundaries (i.e., due to TLS message fragmentation -
   see Section 6.2.1 of [RFC5246]).  Note that application layer
   messages conveying a TokenBindingMessage may cross renegotiation
   boundaries in ways that make processing difficult.

   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, for the signature schemes defined in this
      document.  Other signature schemes may require a longer exporter
      output.

3.4.  TokenBinding.extensions

   A Token Binding message may optionally contain a series of Extension
   structures, each consisting of an extension_type and extension_data.
   The structure and meaning of extension_data depends on the specific
   extension_type.

   Initially, no extension types are defined (see Section 6.3
   "Token Binding Extensions Registry").  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.

   An implementation MUST ignore any unknown Token Binding types.







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4.  Establishing a Token Binding

4.1.  Client Processing Rules

   The client MUST include at least one TokenBinding structure in the
   Token Binding message.  The key parameters used in the
   provided_token_binding MUST match those negotiated with the server
   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.

   When the client needs to send a referred_token_binding to the
   Identity Provider, the client SHALL construct the referred
   TokenBinding structure in the following manner:

   o  Set TokenBinding.tokenbinding_type to referred_token_binding.

   o  Set TokenBinding.tokenbindingid to the Token Binding ID used with
      the Relying Party.

   o  Generate TokenBinding.signature, using the EKM value of the TLS
      connection to the Identity Provider, the Token Binding key
      established with the Relying Party and the signature algorithm
      indicated by the associated key parameters.  Note that these key
      parameters may differ from the key parameters negotiated with the
      Identity Provider.

   Conveying referred Token Bindings in this fashion allows the Identity
   Provider to verify that the client controls the Token Binding key
   used with the Relying Party.

4.2.  Server Processing Rules

   The triple handshake vulnerability in TLS 1.2 and older TLS versions
   affects the security of the Token Binding protocol, as described in
   Section 7 "Security Considerations".  Therefore, the server MUST NOT
   negotiate the use of the Token Binding protocol with these TLS
   versions, unless the server also negotiates the Extended Master
   Secret [RFC7627] and Renegotiation Indication [RFC5746] TLS
   extensions.

   If the use of the Token Binding protocol was not negotiated, but the
   client sends the Token Binding message, the server MUST reject any
   contained bindings.  If the Token Binding type is



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   "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 reject the binding.  Token Bindings of type
   "referred_token_binding" may use different key parameters than those
   negotiated with this client.

   If the Token Binding message does not contain at least one
   TokenBinding structure, or if a signature contained in any
   TokenBinding structure is invalid, the server MUST reject the
   binding.

   Servers MUST ignore any unknown extensions.  Initially, no extension
   types are defined (see Section 6.3
   "Token Binding Extensions Registry").

   If all checks defined above have passed successfully, the Token
   Binding between this client and server is established.  The Token
   Binding ID(s) conveyed in the Token Binding Message can be provided
   to the server-side application.  The application may then use the
   Token Binding IDs for bound security token creation and validation,
   see Section 5.

   If a Token Binding is rejected, any associated bound tokens MUST also
   be rejected by the server.  The effect of this is application-
   specific, e.g. failing requests, a requirement for the client to re-
   authenticate and present a different token, or connection
   termination.

5.  Bound Security Token Creation and 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.  Note that applicable security considerations are outlined
   in Section 7.

   Either or both clients and servers MAY create bound security tokens.
   For example, HTTPS servers employing Token Binding for securing their
   HTTP cookies will bind the cookies.  In the case of a server-
   initiated challenge-response protocol employing Token Binding and
   TLS, the client can, for example, incorporate the Token Binding ID
   within the signed object it returns, thus binding the object.

   Upon receipt of a security token, the server attempts to retrieve
   Token Binding ID information from the token and from the TLS



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

6.  IANA Considerations

   This section establishes three IANA registries: "Token Binding Key
   Parameters", "Token Binding Types" and "Token Binding Extensions".
   It also registers a new TLS exporter label in the TLS Exporter Label
   Registry.

6.1.  Token Binding Key Parameters Registry

   This document establishes a registry for identifiers of Token Binding
   key parameters entitled "Token Binding Key Parameters" under the
   "Token Binding Protocol" heading.

   Entries in this registry require the following fields:

   o  Value: The octet value that identifies a set of Token Binding key
      parameters (0-255).

   o  Description: The description of the Token Binding key parameters.

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

   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 set of Token Binding key
   parameters.

   An initial set of registrations for this registry follows:

      Value: 0

      Description: rsa2048_pkcs1.5

      Specification: this document

      Value: 1

      Description: rsa2048_pss



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      Specification: this document

      Value: 2

      Description: ecdsap256

      Specification: this document

6.2.  Token Binding Types Registry

   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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6.3.  Token Binding Extensions Registry

   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.

6.4.  Registration of Token Binding TLS Exporter Label

   This document adds a registration for the "EXPORTER-Token-Binding"
   value in the TLS Exporter Label Registry to correspond to this
   specification.

7.  Security Considerations

7.1.  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 be replayed by the malware present in User Agents, which
   may be undetectable by a server.  However, in order to export bound
   tokens to other machines and successfully replay them, attackers also
   need to export corresponding Token Binding private keys.  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.

   The manner in which a token is bound to the TLS layer is application-
   defined and beyond the scope of this document.  However, the
   resulting bound token needs to be integrity-protected, so that an



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   attacker cannot remove the binding or substitute a Token Binding ID
   of their choice without detection.

   The Token Binding protocol does not prevent cooperating clients from
   sharing a bound token.  A client could intentionally export a bound
   token with the corresponding Token Binding private key, or perform
   signatures using this key on behalf of another client.

7.2.  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
   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].

7.3.  Privacy Considerations

   The Token Binding protocol uses persistent, long-lived Token Binding
   IDs.  To protect privacy, 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 a special privacy
   mode where they don't store or use tokens supplied by the server,
   e.g. "in private" browsing.  When operating in this special privacy
   mode, applications SHOULD use newly generated Token Binding keys and
   delete them when exiting this mode, or else SHOULD NOT negotiate
   Token Binding at all.

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

   A server can use tokens and Token Binding IDs to track clients.
   Client applications that automatically limit the lifetime or scope of
   tokens to maintain user privacy SHOULD apply the same validity time
   and scope limits to Token Binding keys.

7.4.  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



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   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.

7.5.  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, 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 with these TLS versions, unless the Extended
   Master Secret [RFC7627] and Renegotiation Indication [RFC5746] TLS
   extensions have also been negotiated.

8.  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,
   and others.

9.  References

9.1.  Normative References

   [ANSI.X9-62.2005]
              American National Standards Institute, "Public Key
              Cryptography for the Financial Services Industry, The
              Elliptic Curve Digital Signature Algorithm (ECDSA)",
              ANSI X9.62, 2005.

   [FIPS.186-4.2013]
              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS 186-4, 2013.

   [I-D.ietf-tokbind-https]
              Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
              Hodges, "Token Binding over HTTP", draft-ietf-tokbind-
              https-08 (work in progress), February 2017.

   [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-07 (work in progress), February 2017.




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   [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>.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
              2003, <http://www.rfc-editor.org/info/rfc3447>.

   [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>.

   [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,
              <http://www.rfc-editor.org/info/rfc5746>.

   [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>.

   [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>.

   [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,
              <http://www.rfc-editor.org/info/rfc7627>.







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9.2.  Informative References

   [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


   Jeff Hodges
   Paypal
   USA

   Email: Jeff.Hodges@paypal.com








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