The Token Binding Protocol Version 1.0
RFC 8471

Internet Engineering Task Force (IETF)                     A. Popov, Ed.
Request for Comments: 8471                                   M. Nystroem
Category: Standards Track                                Microsoft Corp.
ISSN: 2070-1721                                               D. Balfanz
                                                             Google Inc.
                                                               J. Hodges
                                                  Kings Mountain Systems
                                                            October 2018

                 The Token Binding Protocol Version 1.0

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 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 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/rfc8471.

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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Token Binding Protocol Overview . . . . . . . . . . . . . . .   4
   3.  Token Binding Protocol Message  . . . . . . . . . . . . . . .   5
     3.1.  TokenBinding.tokenbinding_type  . . . . . . . . . . . . .   6
     3.2.  TokenBinding.tokenbindingid . . . . . . . . . . . . . . .   7
     3.3.  TokenBinding.signature  . . . . . . . . . . . . . . . . .   7
     3.4.  TokenBinding.extensions . . . . . . . . . . . . . . . . .   9
   4.  Establishing a Token Binding  . . . . . . . . . . . . . . . .   9
     4.1.  Client Processing Rules . . . . . . . . . . . . . . . . .   9
     4.2.  Server Processing Rules . . . . . . . . . . . . . . . . .  10
   5.  Bound Security Token Creation and Validation  . . . . . . . .  11
   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 . . . . . . . . . . . . . . . . . . .  14
     7.1.  Security Token Replay . . . . . . . . . . . . . . . . . .  14
     7.2.  Downgrade Attacks . . . . . . . . . . . . . . . . . . . .  14
     7.3.  Token Binding Key-Sharing between Applications  . . . . .  14
     7.4.  Triple Handshake Vulnerability in TLS 1.2 and Older TLS
           Versions  . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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1.  Introduction

   Servers often generate various security tokens (e.g., HTTP cookies,
   OAuth tokens [RFC6749]) for applications to present when accessing
   protected resources.  In general, any party in possession of bearer
   security tokens gains access to certain protected resource(s).
   Attackers take advantage of this by exporting bearer tokens from a
   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
   [RFC5246].  (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.)

   A Token Binding is established by a User Agent generating a
   private-public key pair (possibly within a secure hardware module,
   such as a Trusted Platform Module) 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 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 (or its
   cryptographic hash) in the token.  Later on, when a client presents a
   security token containing a Token Binding ID, the server verifies
   that 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 use the client's private key; this
   is hard to do if the key is specially protected, e.g., generated in a
   secure hardware module.

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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 Protocol Overview

   In the course of a TLS handshake, a client and server can use the
   Token Binding negotiation TLS extension [RFC8472] 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.

   Version 1.0 of the Token Binding protocol is represented by
   TB_ProtocolVersion.major = 1 and TB_ProtocolVersion.minor = 0 in the
   Token Binding negotiation TLS extension; see [RFC8472] ("Transport
   Layer Security (TLS) Extension for Token Binding Protocol
   Negotiation").

   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 (e.g., via [RFC8472]) 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 associated
   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 is
   received on 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.
   [RFC8472] describes the negotiation of the Token Binding protocol and
   key parameters.  [RFC8473] ("Token Binding over HTTP") explains how
   the Token Binding message is encapsulated within HTTP/1.1 messages

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   [RFC7230] or HTTP/2 messages [RFC7540].  [RFC8473] 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 (e.g., via [RFC8472] or a different mechanism)
   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; this
   information can be used to facilitate Token Binding between more than
   two communicating parties.  For example, [RFC8473] specifies an
   encapsulation of the Token Binding message in HTTP application
   protocol messages, as well as scenarios involving more than two
   communicating parties.

   The Token Binding message format is defined using the 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>;
   } TB_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:
               TB_ECPoint point;
       } TokenBindingPublicKey;
   } TokenBindingID;

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

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

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

   struct {
       TokenBindingType tokenbinding_type;
       TokenBindingID tokenbindingid;
       opaque signature<64..2^16-1>; /* Signature over the concatenation
                                        of tokenbinding_type,
                                        key_parameters, and EKM */
       TB_Extension extensions<0..2^16-1>;
   } TokenBinding;

   struct {
       TokenBinding tokenbindings<132..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, and a signature using the Token Binding key,
   optionally followed by TB_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 that are
      intended to be presented to a different server.

   [RFC8473] describes a use case for referred_token_binding where Token
   Bindings are established between multiple communicating parties:
   User Agent, Identity Provider, and Relying Party.  The 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

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   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 referred_token_binding.

   An implementation MUST ignore any unknown Token Binding types.

3.2.  TokenBinding.tokenbindingid

   The ID of the Token Binding established as a result of Token Binding
   message processing contains the identifier of the negotiated key
   parameters, the length (in bytes) of the Token Binding public key,
   and the Token Binding public key itself.  The 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.

   When ecdsap256 is used, TB_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, so that
   applications do not rely on a particular Token Binding ID structure.
   For example, 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 [RFC8017] with SHA256 [FIPS.180-4.2015] as the hash
   function.

   When rsa2048_pss is used, TokenBinding.signature contains the
   signature generated using the RSA Probabilistic Signature Scheme
   (RSASSA-PSS) defined in [RFC8017] with SHA256 as the hash function.
   MGF1 with SHA256 MUST be used as the mask generation function (MGF),
   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 the Elliptic Curve

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   Digital Signature Algorithm (ECDSA) using Curve P-256 and SHA256 as
   defined in [FIPS.186-4.2013] and [ANSI.X9-62.2005].  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  The TokenBindingType value contained in the
      TokenBinding.tokenbinding_type field,

   o  The TokenBindingKeyParameters value contained in the
      TokenBindingID.key_parameters field, and

   o  The 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
   the 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 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 [RFC8473], MAY prohibit
   the use of TLS renegotiation in combination with Token Binding,
   obviating the need for such synchronization.  Alternatively, such
   specifications need to define (1) a way to determine which EKM value
   corresponds to a given TokenBindingMessage and (2) a mechanism that
   prevents a TokenBindingMessage from being split across TLS
   renegotiation boundaries 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 as
   defined in [RFC5705], by supplying the following input values:

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

   o  Context value: No application context supplied.

   o  Length: 32 bytes.

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3.4.  TokenBinding.extensions

   A Token Binding message may optionally contain a series of
   TB_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.

4.  Establishing a Token Binding

4.1.  Client Processing Rules

   The client MUST include at least one TokenBinding structure in the
   Token Binding message.  When a provided_token_binding is included,
   the key parameters used in a provided_token_binding MUST match those
   negotiated with the server (e.g., via [RFC8472] or a different
   mechanism).

   The client MUST generate and store Token Binding keys in a secure
   manner that prevents key export.  In order to prevent cooperating
   servers from linking user identities, the scope of the Token Binding
   keys MUST NOT be broader than the scope of the tokens, as defined by
   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.

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   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 TLS extension [RFC7627] and the renegotiation indication TLS
   extension [RFC5746].

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

   If the Token Binding type is "provided_token_binding", the server
   MUST verify that the signature algorithm (including an elliptic curve
   in the case of ECDSA) and key length in the Token Binding message
   match those negotiated with this client (e.g., via [RFC8472] or a
   different mechanism).  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 presented
   on the current TLS connection 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.

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5.  Bound Security Token Creation and Validation

   Security tokens can be bound to the TLS layer in a variety of ways,
   e.g., by embedding the Token Binding ID or its cryptographic hash in
   the token or by maintaining a database mapping tokens to Token
   Binding IDs.  The specific method of generating bound security tokens
   is defined by the application and is 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 these 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
   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 reject 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 reject the token.

6.  IANA Considerations

   This section establishes a new IANA registry titled "Token Binding
   Protocol" with subregistries "Token Binding Key Parameters", "Token
   Binding Types", and "Token Binding Extensions".  It also registers a
   new TLS exporter label in the "TLS Exporter Labels" registry.

6.1.  Token Binding Key Parameters Registry

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

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

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   This registry operates under the "Specification Required" policy as
   defined in [RFC8126].  The designated expert will require 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
      Specification: This document

      Value: 2
      Description: ecdsap256
      Specification: This document

6.2.  Token Binding Types Registry

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

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

   This registry operates under the "Specification Required" policy as
   defined in [RFC8126].  The designated expert will require 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.

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   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

6.3.  Token Binding Extensions Registry

   This document establishes a subregistry for Token Binding extensions
   titled "Token Binding Extensions" under the "Token Binding Protocol"
   registry.

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

   This registry operates under the "Specification Required" policy as
   defined in [RFC8126].  The designated expert will require 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 the following registration in the "TLS Exporter
   Labels" registry:

      Value: EXPORTER-Token-Binding
      DTLS-OK: Y
      Recommended: Y
      Reference: This document

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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 and from thereby
   impersonating legitimate users and gaining access to protected
   resources.  Bound tokens can be replayed by malware present in
   User Agents; this may be undetectable to 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 defined by
   the application and is beyond the scope of this document.  However,
   the resulting bound token needs to be integrity-protected, so that an
   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 MUST be negotiated using a mechanism that
   prevents downgrade attacks.  For example, [RFC8472] specifies a TLS
   extension for Token Binding negotiation.  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 TokenBinding of type
   "provided_token_binding" MUST match the negotiated parameters.

7.3.  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 shares the same tokens.

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RFC 8471         The Token Binding Protocol Version 1.0     October 2018

7.4.  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 TLS extension [RFC7627] and the renegotiation
   indication TLS extension [RFC5746] have also been negotiated.

8.  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; otherwise, they SHOULD NOT
   negotiate Token Binding at all.

   In order to prevent cooperating servers from linking user identities,
   the scope of the Token Binding keys MUST NOT be broader than the
   scope of the tokens, as defined by 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.

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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, November 2005.

   [FIPS.180-4.2015]
              National Institute of Standards and Technology, "Secure
              Hash Standard (SHS)", FIPS 180-4,
              DOI 10.6028/NIST.FIPS.180-4, August 2015,
              <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

   [FIPS.186-4.2013]
              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS 186-4,
              DOI 10.6028/NIST.FIPS.186-4, July 2013,
              <https://nvlpubs.nist.gov/nistpubs/fips/
              nist.fips.186-4.pdf>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc7230>.

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RFC 8471         The Token Binding Protocol Version 1.0     October 2018

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://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,
              <https://www.rfc-editor.org/info/rfc7627>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC8472]  Popov, A., Ed., Nystroem, M., and D. Balfanz, "Transport
              Layer Security (TLS) Extension for Token Binding Protocol
              Negotiation", RFC 8472, DOI 10.17487/RFC8472, October
              2018, <https://www.rfc-editor.org/info/rfc8472>.

   [RFC8473]  Popov, A., Nystroem, M., Balfanz, D., Ed., Harper, N., and
              J. Hodges, "Token Binding over HTTP", RFC 8473,
              DOI 10.17487/RFC8473, October 2018,
              <https://www.rfc-editor.org/info/rfc8473>.

9.2.  Informative References

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [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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RFC 8471         The Token Binding Protocol Version 1.0     October 2018

   [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

   Jeff Hodges
   Kings Mountain Systems
   United States of America

   Email: Jeff.Hodges@KingsMountain.com

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