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Versions: (draft-balfanz-https-token-binding) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18

Internet Engineering Task Force                                 A. Popov
Internet-Draft                                               M. Nystroem
Intended status: Standards Track                         Microsoft Corp.
Expires: December 28, 2018                               D. Balfanz, Ed.
                                                              A. Langley
                                                               N. Harper
                                                             Google Inc.
                                                               J. Hodges
                                                                  PayPal
                                                           June 26, 2018


                        Token Binding over HTTP
                      draft-ietf-tokbind-https-18

Abstract

   This document describes a collection of mechanisms that allow HTTP
   servers to cryptographically bind security tokens (such as cookies
   and OAuth tokens) to TLS connections.

   We describe both first-party and federated scenarios.  In a first-
   party scenario, an HTTP server is able to cryptographically bind the
   security tokens it issues to a client, and which the client
   subsequently returns to the server, to the TLS connection between the
   client and server.  Such bound security tokens are protected from
   misuse since the server can generally detect if they are replayed
   inappropriately, e.g., over other TLS connections.

   Federated token bindings, on the other hand, allow servers to
   cryptographically bind security tokens to a TLS connection that the
   client has with a different server than the one issuing the token.

   This document is a companion document to The Token Binding Protocol.

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 https://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




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   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 December 28, 2018.

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 . . . . . . . . . . . . . . . . . .   3
   2.  The Sec-Token-Binding HTTP Request Header Field . . . . . . .   4
     2.1.  HTTPS Token Binding Key Pair Scoping  . . . . . . . . . .   5
   3.  TLS Renegotiation . . . . . . . . . . . . . . . . . . . . . .   6
   4.  First-Party Use Cases . . . . . . . . . . . . . . . . . . . .   6
   5.  Federation Use Cases  . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  HTTP Redirects  . . . . . . . . . . . . . . . . . . . . .  10
     5.4.  Negotiated Key Parameters . . . . . . . . . . . . . . . .  12
     5.5.  Federation Example  . . . . . . . . . . . . . . . . . . .  12
   6.  Implementation Considerations . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     7.1.  Security Token Replay . . . . . . . . . . . . . . . . . .  15
     7.2.  Sensitivity of the Sec-Token-Binding Header . . . . . . .  15
     7.3.  Securing Federated Sign-On Protocols  . . . . . . . . . .  17
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  19
     8.1.  Scoping of Token Binding Key Pairs  . . . . . . . . . . .  19
     8.2.  Lifetime of Token Binding Key Pairs . . . . . . . . . . .  20
     8.3.  Correlation . . . . . . . . . . . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     11.2.  Informative References . . . . . . . . . . . . . . . . .  23



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   The Token Binding Protocol [I-D.ietf-tokbind-protocol] defines a
   Token Binding ID for a TLS connection between a client and a server.
   The Token Binding ID of a TLS connection is constructed using the
   public key of a private-public key pair.  The client proves
   possession of the corresponding private key.  This Token Binding key
   pair is long-lived.  I.e., subsequent TLS connections between the
   same client and server have the same Token Binding ID, unless
   specifically reset, e.g., by the user.  When issuing a security token
   (e.g., an HTTP cookie or an OAuth token [RFC6749]) to a client, the
   server can include the Token Binding ID in the token, thus
   cryptographically binding the token to TLS connections between that
   particular client and server, and inoculating the token against abuse
   (re-use, attempted impersonation, etc.) by attackers.

   While the Token Binding Protocol [I-D.ietf-tokbind-protocol] defines
   a message format for establishing a Token Binding ID, it does not
   specify how this message is embedded in higher-level protocols.  The
   purpose of this specification is to define how TokenBindingMessages
   are embedded in HTTP (both versions 1.1 [RFC7230] and 2 [RFC7540]).
   Note that TokenBindingMessages are only defined if the underlying
   transport uses TLS.  This means that Token Binding over HTTP is only
   defined when the HTTP protocol is layered on top of TLS (commonly
   referred to as HTTPS [RFC2818]).

   HTTP clients establish a Token Binding ID with a server by including
   a special HTTP header field in HTTP requests.  The HTTP header field
   value is a base64url-encoded TokenBindingMessage.

   TokenBindingMessages allow clients to establish multiple Token
   Binding IDs with the server, by including multiple TokenBinding
   structures in the TokenBindingMessage.  By default, a client will
   establish a provided Token Binding ID with the server, indicating a
   Token Binding ID that the client will persistently use with the
   server.  Under certain conditions, the client can also include a
   referred Token Binding ID in the TokenBindingMessage, indicating a
   Token Binding ID that the client is using with a different server
   than the one that the TokenBindingMessage is sent to.  This is useful
   in federation scenarios.

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



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   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  The Sec-Token-Binding HTTP Request Header Field

   Once a client and server have negotiated the Token Binding Protocol
   with HTTP/1.1 or HTTP/2 (see [I-D.ietf-tokbind-protocol] and
   [I-D.ietf-tokbind-negotiation]), clients MUST include a Sec-Token-
   Binding header field in their HTTP requests, and MUST include only
   one such header field per HTTP request.  Also, The Sec-Token-Binding
   header field MUST NOT be included in HTTP responses.  The ABNF of the
   Sec-Token-Binding header field is (in [RFC7230] style, see also
   Section 8.3 of [RFC7231]):

     Sec-Token-Binding = EncodedTokenBindingMessage

   The header field name is Sec-Token-Binding and its single value,
   EncodedTokenBindingMessage, is a base64url encoding of a single
   TokenBindingMessage, as defined in [I-D.ietf-tokbind-protocol].  The
   base64url encoding uses the URL- and filename-safe character set
   described in Section 5 of [RFC4648], with all trailing padding
   characters '=' omitted and without the inclusion of any line breaks,
   whitespace, or other additional characters.

   For example:

  Sec-Token-Binding: AIkAAgBBQFzK4_bhAqLDwRQxqJWte33d7hZ0hZWHwk-miKPg4E\
                     9fcgs7gBPoz-9RfuDfN9WCw6keHEw1ZPQMGs9CxpuHm-YAQM_j\
                     aOwwej6a-cQBGU7CJpUHOvXG4VvjNq8jDsvta9Y8_bPEPj25Gg\
                     mKiPjhJEtZA6mJ_9SNifLvVBTi7fR9wSAAAA

   (Note that the backslashes and line breaks are provided to ease
   readability, they are not part of the actual encoded message.)

   If the server receives more than one Sec-Token-Binding header field
   in an HTTP request, then the server MUST reject the message with a
   400 (Bad Request) HTTP status code.  Additionally, the Sec-Token-
   Binding header field:

      SHOULD NOT be stored by origin servers on PUT requests,

      MAY be listed by a server in a Vary response header field, and,

      MUST NOT be used in HTTP trailers.

   The TokenBindingMessage MUST contain exactly one TokenBinding
   structure with TokenBindingType of provided_token_binding, which MUST
   be signed with the Token Binding private key used by the client for



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   connections between itself and the server that the HTTP request is
   sent to (clients use different Token Binding key pairs for different
   servers, see Section 2.1 below).  The Token Binding ID established by
   this TokenBinding is called a Provided Token Binding ID.

   The TokenBindingMessage MAY also contain exactly one TokenBinding
   structure with TokenBindingType of referred_token_binding, as
   specified in Section 5.3.  In addition to the latter, or rather than
   the latter, the TokenBindingMessage MAY contain other TokenBinding
   structures.  This is use case-specific, and such use cases are
   outside the scope of this specification.

   A TokenBindingMessage is validated by the server as described in
   Section 4.2 ("Server Processing Rules") of
   [I-D.ietf-tokbind-protocol].  If validation fails and a Token Binding
   is rejected, any associated bound tokens MUST also be rejected by the
   server.  HTTP requests containing invalid tokens MUST be rejected.
   In this case, the server application MAY return HTTP status code 400
   (Bad Request) or proceed with an application-specific invalid token
   response (e.g., directing the client to re-authenticate and present a
   different token), or terminate the connection.

   In HTTP/2, the client SHOULD use Header Compression [RFC7541] to
   avoid the overhead of repeating the same header field in subsequent
   HTTP requests.

2.1.  HTTPS Token Binding Key Pair Scoping

   HTTPS is used in conjunction with various application protocols and
   application contexts, in various ways.  For example, general-purpose
   Web browsing is one such HTTP-based application context.  Within that
   context, HTTP cookies [RFC6265] are typically utilized for state
   management, including client authentication.  A related, though
   distinct, example of other HTTP-based application contexts is where
   OAuth tokens [RFC6749] are utilized to manage authorization for
   third-party application access to resources.  The token scoping rules
   of these two examples can differ: the scoping rules for cookies are
   concisely specified in [RFC6265], whereas OAuth is a framework and
   defines various token types with various scopings, some of which are
   determined by the encompassing application.

   The scoping of Token Binding key pairs generated by Web browsers for
   the purpose of binding HTTP cookies MUST be no wider than the
   granularity of a "registered domain" (also known as "effective top-
   level domain + 1", or "eTLD+1").  An origin's "registered domain" is
   the origin's host's public suffix plus the label to its left, with
   the term "public suffix" being defined in a note in Section 5.3 of
   [RFC6265] as "a domain that is controlled by a public registry".  For



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   example, for "https://www.example.com", the public suffix (eTLD) is
   "com", and the registered domain (eTLD+1) is "example.com".  User
   agents SHOULD use an up-to-date public suffix list, such as the one
   maintained by Mozilla [PSL].

   This means that in practice the scope of a Token Binding key pair is
   no larger than the scope of a cookie allowed by a Web browser.  If a
   Web browser restricts cookies to a narrower scope than registered
   domains, the scope of Token Binding key pairs MAY also be more
   narrow.  This applies to the use of Token Binding key pairs in first-
   party use cases, as well as in federation use cases defined in this
   specification (Section 5).

   Key pairs used to bind other application tokens, such as OAuth tokens
   or OpenID Connect ID Tokens, SHOULD adhere to the above eTLD+1
   scoping requirement for those tokens being employed in first-party or
   federation scenarios.  Applications other than Web browsers MAY use
   different key pair scoping rules.  See also Section 8.1, below.

   Scoping rules for other HTTP-based application contexts are outside
   the scope of this specification.

3.  TLS Renegotiation

   Token Binding over HTTP/1.1 [RFC7230] can be performed in combination
   with TLS renegotiation.  In this case, renegotiation MUST only occur
   between a client's HTTP request and the server's response, the client
   MUST NOT send any pipelined requests, and the client MUST NOT
   initiate renegotiation.  (I.e., the client may only send a
   renegotiation ClientHello in response to the server's HelloRequest.)
   These conditions ensure that both the client and the server can
   clearly identify which TLS Exported Keying Material value [RFC5705]
   to use when generating or verifying the TokenBindingMessage.  This
   also prevents a TokenBindingMessage from being split across TLS
   renegotiation boundaries.  (I.e., due to TLS message fragmentation -
   see Section 6.2.1 of [RFC5246].)

4.  First-Party Use Cases

   In a first-party use case (also known as a "same-site" use case), an
   HTTP server issues a security token such as a cookie (or similar) to
   a client, and expects the client to return the security token at a
   later time, e.g., in order to authenticate.  Binding the security
   token to the TLS connection between client and server protects the
   security token from misuse, since the server can detect if the
   security token is replayed inappropriately, e.g., over other TLS
   connections.




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   See Section 5 of [I-D.ietf-tokbind-protocol] for general guidance
   regarding binding of security tokens and their subsequent validation.

5.  Federation Use Cases

5.1.  Introduction

   For privacy reasons, clients use different Token Binding key pairs to
   establish Provided Token Binding IDs with different servers.  As a
   result, a server cannot bind a security token (such as an OAuth token
   or an OpenID Connect ID Token [OpenID.Core]) to a TLS connection that
   the client has with a different server.  This is, however, a common
   requirement in federation scenarios: For example, an Identity
   Provider may wish to issue an identity token to a client and
   cryptographically bind that token to the TLS connection between the
   client and a Relying Party.

   In this section, we describe mechanisms to achieve this.  The common
   idea among these mechanisms is that a server (called the Token
   Consumer in this document) signals to the client that it should
   reveal the Provided Token Binding ID that is used between the client
   and itself to another server (called the Token Provider in this
   document).  Also common across the mechanisms is how the Token
   Binding ID is revealed to the Token Provider: The client uses the
   Token Binding Protocol [I-D.ietf-tokbind-protocol], and includes a
   TokenBinding structure in the Sec-Token-Binding HTTP header field
   defined above.  What differs between the various mechanisms is how
   the Token Consumer signals to the client that it should reveal the
   Token Binding ID to the Token Provider.  Below, we specify one such
   mechanism, which is suitable for redirect-based interactions between
   Token Consumers and Token Providers.




















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     Client                        Token Consumer         Token Provider
     +--------+                        +----+                +-----+
     | Client |                        | TC |                | TP  |
     +--------+                        +----+                +-----+
         |                               |                      |
         |                               |                      |
         |                               |                      |
         | Client interacts w/TC         |                      |
         | using TokenBindingID TBID1:   |                      |
         | TBMSG[[provided_token_binding,|                      |
         |        TBID1, signature]]     |                      |
         |------------------------------>|                      |
         |                               |                      |
         | Client interacts w/TP                                |
         | using TokenBindingID TBID2:                          |
         | TBMSG[[provided_token_binding,                       |
         |        TBID2, signature]]                            |
         |----------------------------------------------------->|
         |                                                      |
         |                               |                      |
         | TC signals permission to      |                      |
         | reveal TBID1 to TP            |                      |
         |<------------------------------|                      |
         |                               |                      |
         |                                                      |
         | Client interacts w/TP                                |
         | using TokenBindingID TBID1 and TBID2:                |
         | TBMSG[[provided_token_binding,                       |
         |        TBID2, signature],                            |
         |       [referred_token_binding,                       |
         |        TBID1, signature]]                            |
         |----------------------------------------------------->|
         |                                                      |
         |                               |                      |
         |                               |                      |

5.2.  Overview

   In a Federated Sign-On protocol, an Identity Provider issues an
   identity token to a client, which sends the identity token to a
   Relying Party to authenticate itself.  Examples of this include
   OpenID Connect (in which the identity token is called an "ID Token")
   and SAML [OASIS.saml-core-2.0-os] (in which the identity token is a
   SAML assertion).

   To better protect the security of the identity token, the Identity
   Provider may wish to bind the identity token to the TLS connection
   between the client and the Relying Party, thus ensuring that only



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   said client can use the identity token.  The Relying Party will
   compare the Token Binding ID (or a cryptographic hash of it) in the
   identity token with the Token Binding ID (or a hash thereof) of the
   TLS connection between this Relying Party and the client.

   This is an example of a federation scenario, which more generally can
   be described as follows:

   o  A Token Consumer causes the client to issue a token request to the
      Token Provider.  The goal is for the client to obtain a token and
      then use it with the Token Consumer.

   o  The client delivers the token request to the Token Provider.

   o  The Token Provider issues the token.  The token is issued for the
      specific Token Consumer who requested it (thus preventing
      malicious Token Consumers from using tokens with other Token
      Consumers).  The token is, however, typically a bearer token,
      meaning that any client can use it with the Token Consumer, not
      just the client to which it was issued.

   o  Therefore, in the previous step, the Token Provider may want to
      include in the token the Token Binding ID (or a cryptographic hash
      of it) that the client uses when communicating with the Token
      Consumer, thus binding the token to the client's Token Binding key
      pair.  The client proves possession of the private key when
      communicating with the Token Consumer through the Token Binding
      Protocol [I-D.ietf-tokbind-protocol], and uses the corresponding
      public key of this key pair as a component of the Token Binding
      ID.  Comparing the Token Binding ID from the token to the Token
      Binding ID established with the client allows the Token Consumer
      to verify that the token was sent to it by the legitimate client.

   o  To allow the Token Provider to include the Token Binding ID in the
      token, the Token Binding ID between client and Token Consumer must
      therefore be communicated to the Token Provider along with the
      token request.  Communicating a Token Binding ID involves proving
      possession of a private key and is described in the Token Binding
      Protocol [I-D.ietf-tokbind-protocol].

   The client will perform this last operation only if the Token
   Consumer requests the client to do so.

   Below, we specify how Token Consumers can signal this request in
   redirect-based federation protocols.  Note that this assumes that the
   federated sign-on flow starts at the Token Consumer, or at the very
   least, includes a redirect from the Token Consumer to the Token




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   Provider.  It is outside the scope of this document to specify
   similar mechanisms for flows that do not include such redirects.

5.3.  HTTP Redirects

   When a Token Consumer redirects the client to a Token Provider as a
   means to deliver the token request, it SHOULD include an Include-
   Referred-Token-Binding-ID HTTP response header field in its HTTP
   response.  The ABNF of the Include-Referred-Token-Binding-ID header
   is (in [RFC7230] style, see also Section 8.3 of [RFC7231]):

     Include-Referred-Token-Binding-ID = "true"

   Where the header field name is "Include-Referred-Token-Binding-ID",
   and the field-value of "true" is case-insensitive.  For example:

     Include-Referred-Token-Binding-ID: true

   Including this response header field signals to the client that it
   should reveal, to the Token Provider, the Token Binding ID used
   between itself and the Token Consumer.  In the absence of this
   response header field, the client will not disclose any information
   about the Token Binding used between the client and the Token
   Consumer to the Token Provider.

   As illustrated in Section 5.5, when a client receives this header
   field, it should take the TokenBindingID of the provided TokenBinding
   from the referrer and create a referred TokenBinding with it to
   include in the TokenBindingMessage on the redirect request.  In other
   words, the Token Binding message in the redirect request to the Token
   Provider now includes one provided binding and one referred binding,
   the latter constructed from the binding between the client and the
   Token Consumer.

   When a client receives the Include-Referred-Token-Binding-ID header,
   it includes the referred token binding even if both the Token
   Provider and the Token Consumer fall under the same eTLD+1 and the
   provided and referred token binding IDs are the same.

   The referred token binding is sent only on the initial request
   resulting from the HTTP response that included the Include-Referred-
   Token-Binding-ID header.  Should the response to that initial request
   be a further redirect, the original referred token binding is no
   longer included in subsequent requests.  (A new referred token
   binding may be included if the redirecting endpoint itself responded
   with a Include-Referred-Token-Binding-ID response header.)





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   If the Include-Referred-Token-Binding-ID header field is received in
   response to a request that did not include the Token-Binding header
   field, the client MUST ignore the Include-Referred-Token-Binding-ID
   header field.

   This header field has only meaning if the HTTP status code is a
   redirection code (300-399), and MUST be ignored by the client for any
   other status codes.  If the client supports the Token Binding
   Protocol, and has negotiated the Token Binding Protocol with both the
   Token Consumer and the Token Provider, it already sends the Sec-
   Token-Binding header field to the Token Provider with each HTTP
   request (as described in Section 2 above).

   The TokenBindingMessage included in the redirect request to the Token
   Provider SHOULD contain a TokenBinding with TokenBindingType
   referred_token_binding.  If included, this TokenBinding MUST be
   signed with the Token Binding private key used by the client for
   connections between itself and the Token Consumer (more specifically,
   the server that issued the Include-Referred-Token-Binding-ID response
   header field).  The Token Binding ID established by this TokenBinding
   is called a Referred Token Binding ID.

   As described above, the TokenBindingMessage MUST additionally contain
   a Provided Token Binding ID, i.e., a TokenBinding structure with
   TokenBindingType of provided_token_binding, which MUST be signed with
   the Token Binding private key used by the client for connections
   between itself and the Token Provider (more specifically, the server
   that the token request is being sent to).

   If, for some deployment-specific reason, the initial Token Provider
   ("TP1") needs to redirect the client to another Token Provider
   ("TP2"), rather than directly back to the Token Consumer, it can be
   accommodated using the header fields defined in this specification in
   the following fashion ("the redirect-chain approach"):

      Initially, the client is redirected to TP1 by the Token Consumer
      ("TC"), as described above.  Upon receiving the client's request,
      containing a TokenBindingMessage which contains both provided and
      referred TokenBindings (for TP1 and TC, respectively), TP1
      responds to the client with a redirect response containing the
      Include-Referred-Token-Binding-ID header field and directing the
      client to send a request to TP2.  This causes the client to follow
      the same pattern and send a request containing a
      TokenBindingMessage which contains both provided and referred
      TokenBindings (for TP2 and TP1, respectively) to TP2.  Note that
      this pattern can continue to further Token Providers.  In this
      case, TP2 issues a security token, bound to the client's
      TokenBinding with TP1, and sends a redirect response to the client



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      pointing to TP1.  TP1 in turn constructs a security token for the
      Token Consumer, bound to the TC's referred TokenBinding which had
      been conveyed earlier, and sends a redirect response pointing to
      the TC, containing the bound security token, to the client.

   The above is intended as only a non-normative example.  Details are
   specific to deployment contexts.  Other approaches are possible, but
   are outside the scope of this specification.

5.4.  Negotiated Key Parameters

   The TLS Extension for Token Binding Protocol Negotiation
   [I-D.ietf-tokbind-negotiation] allows the server and client to
   negotiate the parameters (signature algorithm, length) of the Token
   Binding key pair.  It is possible that the Token Binding ID used
   between the client and the Token Consumer, and the Token Binding ID
   used between the client and Token Provider, use different key
   parameters.  The client MUST use the key parameters negotiated with
   the Token Consumer in the referred_token_binding TokenBinding of the
   TokenBindingMessage, even if those key parameters are different from
   the ones negotiated with the server that the header field is sent to.

   Token Providers SHOULD support all the Token Binding key parameters
   specified in [I-D.ietf-tokbind-protocol].  If a token provider does
   not support the key parameters specified in the
   referred_token_binding TokenBinding in the TokenBindingMessage, it
   MUST NOT issue a bound token.

5.5.  Federation Example

   The diagram below shows a typical HTTP Redirect-based Web Browser SSO
   Profile (no artifact, no callbacks), featuring binding of, e.g., a
   TLS Token Binding ID into an OpenID Connect ID Token.


















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                                  Legend:

   +------------+------------------------------------------------------+
   | EKM:       | TLS Exported Keying Material [RFC5705]               |
   | {EKMn}Ksm: | EKM for server "n", signed by private key of TBID    |
   |            | "m", where "n" must represent server receiving the   |
   |            | ETBMSG.  If a conveyed TB's type is                  |
   |            | provided_token_binding, then m = n, else if TB's     |
   |            | type is referred_token_binding, then m != n. E.g.,   |
   |            | see step 1b in diagram below.                        |
   | ETBMSG:    | "Sec-Token-Binding" HTTP header field conveying an   |
   |            | EncodedTokenBindingMessage, in turn conveying        |
   |            | TokenBinding (TB)struct(s), e.g.: ETBMSG[[TB]] or    |
   |            | ETBMSG[[TB1],[TB2]]                                  |
   | ID Token:  | the ID Token in OpenID Connect, it is the semantic   |
   |            | equivalent of a SAML "authentication assertion". "ID |
   |            | Token w/TBIDn" denotes a "token bound" ID Token      |
   |            | containing TBIDn.                                    |
   | Ks & Kp:   | private (aka secret) key, and public key,            |
   |            | respectively, of client-side Token Binding key pair  |
   | OIDC:      | OpenID Connect                                       |
   | TB:        | TokenBinding struct containing signed EKM, TBID, and |
   |            | TB type, e.g.:                                       |
   |            | [{EKM1}Ks1,TBID1,provided_token_binding]             |
   | TBIDn:     | Token Binding ID for client and server n's token-    |
   |            | bound TLS association. TBIDn contains Kpn.           |
   +------------+------------------------------------------------------+

 Client,                      Token Consumer,       Token Provider,
 aka:                         aka:                  aka:
 User Agent                   OpenID Client,        OpenID Provider,
                              OIDC Relying Party,   OIDC Provider,
                              SAML Relying Party    SAML Identity Provider
                              [ server "1" ]        [ server "2" ]
 +--------+                        +----+                +-----+
 | Client |                        | TC |                | TP  |
 +--------+                        +----+                +-----+
     |                               |                      |
     |                               |                      |
     |                               |                      |
     | 0. Client interacts w/TC      |                      |
     | over HTTPS, establishes Ks1 & Kp1, TBID1             |
     | ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]]     |
     |------------------------------>|                      |
     |                               |                      |
     |                               |                      |
     |                               |                      |
     | 1a. OIDC ID Token request, aka|                      |



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     | "Authentication Request", conveyed with              |
     | HTTP response header field of:                       |
     | Include-Referred-Token-Binding-ID:true               |
     | any security-relevant cookies |                      |
     | should contain TBID1          |                      |
   +<- - - - - - - - - - - - - - - - |                      |
   . | (redirect to TP via 301, 302, |                      |
   . |  303, 307, or 308)            |                      |
   . |                               |                      |
   +------------------------------------------------------->|
     | 1b. opens HTTPS w/TP,                                |
     | establishes Ks2, Kp2, TBID2;                         |
     | sends GET or POST with                               |
     | ETBMSG[[{EKM2}Ks2,TBID2,provided_token_binding],     |
     |        [{EKM2}Ks1,TBID1,referred_token_binding]]     |
     | as well as the ID Token request                      |
     |                               |                      |
     |                               |                      |
     |                               |                      |
     | 2. user authentication (if applicable,               |
     |    methods vary, particulars are out of scope)       |
     |<====================================================>|
     | (TP generates ID Token for TC containing TBID1, may  |
     |  also set cookie(s) containing TBID2 and/or TBID1,   |
     |  details vary, particulars are out of scope)         |
     |                               |                      |
     |                               |                      |
     |                               |                      |
     | 3a. ID Token containing Kp1, issued for TC,          |
     |    conveyed via OIDC "Authentication Response"       |
   +<- - - - - - - - - - - - - - - - - - - - - - - - - - - -|
   . |   (redirect to TC)            |                      |
   . |                               |                      |
   . |                               |                      |
   +-------------------------------->|                      |
     | 3b. HTTPS GET or POST with                           |
     | ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]]     |
     | conveying Authn Response containing                   |
     | ID Token w/TBID1, issued for TC                      |
     |                               |                      |
     |                               |                      |
     |                               |                      |
     | 4. user is signed-on, any security-relevant cookie(s)|
     | that are set SHOULD contain TBID1                    |
     |<------------------------------|                      |
     |                               |                      |
     |                               |                      |




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6.  Implementation Considerations

   HTTPS-based applications may have multi-party use cases other than,
   or in addition to, the HTTP redirect-based signaling-and-conveyance
   of referred token bindings, as presented above in Section 5.3.

   Thus, Token Binding implementations should provide APIs for such
   applications to generate Token Binding messages containing Token
   Binding IDs of various application-specified Token Binding types, to
   be conveyed by the Sec-Token-Binding header field.

   However, Token Binding implementations MUST only convey Token Binding
   IDs to servers if signaled to do so by an application.  For example,
   a server can return an Include-Referred-Token-Binding-ID HTTP
   response header field to an application, which then signals to the
   Token Binding implementation that it intends to convey the Token
   Binding ID used with this server to another server.  Other signaling
   mechanisms are possible, and are specific to the application layer
   protocol, but are outside the scope of this specification.

   NOTE:  See Section 8 ("Privacy Considerations"), for privacy guidance
          regarding the use of this functionality.

7.  Security Considerations

7.1.  Security Token Replay

   The goal of the Federated Token Binding mechanisms is to prevent
   attackers from exporting and replaying tokens used in protocols
   between the client and Token Consumer, thereby impersonating
   legitimate users and gaining access to protected resources.  Although
   bound tokens can still be replayed by any malware present in clients
   (which may be undetectable by a server), in order to export bound
   tokens to other machines and successfully replay them, attackers also
   need to export the 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.

   This consideration is a special case of the Security Token Replay
   security consideration laid out in the The Token Binding Protocol
   [I-D.ietf-tokbind-protocol] specification.

7.2.  Sensitivity of the Sec-Token-Binding Header

   The purpose of the Token Binding protocol is to convince the server
   that the client that initiated the TLS connection controls a certain
   key pair.  For the server to correctly draw this conclusion after



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   processing the Sec-Token-Binding header field, certain secrecy and
   integrity requirements must be met.

   For example, the client's Token Binding private key must be kept
   secret by the client.  If the private key is not secret, then another
   actor in the system could create a valid Token Binding header field,
   impersonating the client.  This can render the main purpose of the
   protocol - to bind bearer tokens to certain clients - moot.
   Consider, for example, an attacker who obtained (perhaps through a
   network intrusion) an authentication cookie that a client uses with a
   certain server.  Consider further that the server bound that cookie
   to the client's Token Binding ID precisely to thwart misuse of the
   cookie.  If the attacker were to come into possession of the client's
   private key, he could then establish a TLS connection with the server
   and craft a Sec-Token-Binding header field that matches the binding
   present in the cookie, thus successfully authenticating as the
   client, and gaining access to the client's data at the server.  The
   Token Binding protocol, in this case, did not successfully bind the
   cookie to the client.

   Likewise, we need integrity protection of the Sec-Token-Binding
   header field.  A client should not be tricked into sending a Sec-
   Token-Binding header field to a server that contains Token Binding
   messages about key pairs that the client does not control.  Consider
   an attacker A that somehow has knowledge of the exported keying
   material (EKM) for a TLS connection between a client C and a server
   S.  (While that is somewhat unlikely, it is also not entirely out of
   the question, since the client might not treat the EKM as a secret -
   after all, a pre-image-resistant hash function has been applied to
   the TLS master secret, making it impossible for someone knowing the
   EKM to recover the TLS master secret.  Such considerations might lead
   some clients to not treat the EKM as a secret.)  Such an attacker A
   could craft a Sec-Token-Binding header field with A's key pair over
   C's EKM.  If the attacker could now trick C into sending such a
   header field to S, it would appear to S as if C controls a certain
   key pair, when in fact it does not (the attacker A controls the key
   pair).

   If A has a pre-existing relationship with S (perhaps has an account
   on S), it now appears to the server S as if A is connecting to it,
   even though it is really C.  (If the server S does not simply use
   Token Binding IDs to identify clients, but also uses bound
   authentication cookies, then A would also have to trick C into
   sending one of A's cookies to S, which it can do through a variety of
   means - inserting cookies through Javascript APIs, setting cookies
   through related-domain attacks, etc.)  In other words, A tricked C
   into logging into A's account on S.  This could lead to a loss of
   privacy for C, since A presumably has some other way to also access



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   the account, and can thus indirectly observe C's behavior (for
   example, if S has a feature that lets account holders see their
   activity history on S).

   Therefore, we need to protect the integrity of the Sec-Token-Binding
   header field.  One eTLD+1 should not be able to set the Sec-Token-
   Binding header field (through a DOM API or otherwise) that the User
   Agent uses with another eTLD+1.  Employing the "Sec-" header field
   prefix helps to meet this requirement by denoting the header field
   name to be a "forbidden header name", see [fetch-spec].

7.3.  Securing Federated Sign-On Protocols

   As explained above, in a federated sign-in scenario, a client will
   prove possession of two different Token Binding private keys to a
   Token Provider: One private key corresponds to the "provided" Token
   Binding ID (which the client normally uses with the Token Provider),
   and the other is the Token Binding private key corresponding to the
   "referred" Token Binding ID (which the client normally uses with the
   Token Consumer).  The Token Provider is expected to issue a token
   that is bound to the referred Token Binding ID.

   Both proofs (that of the provided Token Binding private key and that
   of the referred Token Binding private key) are necessary.  To show
   this, consider the following scenario:

   o  The client has an authentication token with the Token Provider
      that is bound to the client's Token Binding ID used with that
      Token Provider.

   o  The client wants to establish a secure (i.e., free of men-in-the-
      middle) authenticated session with the Token Consumer, but has not
      done so yet (in other words, we are about to run the federated
      sign-on protocol).

   o  A man-in-the-middle is allowed to intercept the connection between
      client and Token Consumer or between Client and Token Provider (or
      both).

   The goal is to detect the presence of the man-in-the-middle in these
   scenarios.

   First, consider a man-in-the-middle between the client and the Token
   Provider.  Recall that we assume that the client possesses a bound
   authentication token (e.g., cookie) for the Token Provider.  The man-
   in-the-middle can intercept and modify any message sent by the client
   to the Token Provider, and any message sent by the Token Provider to
   the client.  (This means, among other things, that the man-in-the-



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   middle controls the Javascript running at the client in the origin of
   the Token Provider.)  It is not, however, in possession of the
   client's Token Binding private key.  Therefore, it can either choose
   to replace the Token Binding ID in requests from the client to the
   Token Provider, and create a Sec-Token-Binding header field that
   matches the TLS connection between the man-in-the-middle and the
   Token Provider, or it can choose to leave the Sec-Token-Binding
   header field unchanged.  If it chooses the latter, the signature in
   the Token Binding message (created by the original client on the
   exported keying material (EKM) for the connection between client and
   man-in-the-middle) will not match a signature on the EKM between man-
   in-the-middle and the Token Provider.  If it chooses the former (and
   creates its own signature, using its own Token Binding private key,
   over the EKM for the connection between itself, the man-in-the-
   middle, and Token Provider), then the Token Binding message will
   match the connection between man-in-the-middle and Token Provider,
   but the Token Binding ID in the message will not match the Token
   Binding ID that the client's authentication token is bound to.
   Either way, the man-in-the-middle is detected by the Token Provider,
   but only if the proof of possession of the provided Token Binding
   private key is required in the protocol (as is done above).

   Next, consider the presence of a man-in-the-middle between client and
   Token Consumer.  That man-in-the-middle can intercept and modify any
   message sent by the client to the Token Consumer and any message sent
   by the Token Consumer to the client.  The Token Consumer is the party
   that redirects the client to the Token Provider.  In this case, the
   man-in-the-middle controls the redirect URL and can tamper with any
   redirect URL issued by the Token Consumer (as well as with any
   Javascript running in the origin of the Token Consumer).  The goal of
   the man-in-the-middle is to trick the Token Provider into issuing a
   token bound to its Token Binding ID, not to the Token Binding ID of
   the legitimate client.  To thwart this goal of the man-in-the-middle,
   the client's referred Token Binding ID must be communicated to the
   Token Producer in a manner that cannot be affected by the man-in-the-
   middle (who, as we recall, can modify redirect URLs and Javascript at
   the client).  Including the referred Token Binding structure in the
   Sec-Token-Binding header field (as opposed to, say, including the
   referred Token Binding ID in an application-level message as part of
   the redirect URL) is one way to assure that the man-in-the-middle
   between client and Token Consumer cannot affect the communication of
   the referred Token Binding ID to the Token Provider.

   Therefore, the Sec-Token-Binding header field in the federated sign-
   on use case contains both: a proof of possession of the provided
   Token Binding key, as well as a proof of possession of the referred
   Token Binding key.




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   Note that the presence of Token Binding does not relieve the Token
   Provider and Token Consumer from performing various checks to ensure
   the security of clients during federated sign-on protocols.  These
   include the following:

   o  The Token Provider should not issue tokens to Token Consumers that
      have been shown to act maliciously.  To aid in this, the
      federation protocol should identify the Token Consumer to the
      Token Provider (e.g., through OAuth client IDs or similar
      mechanisms), and the Token Provider should ensure that tokens are
      indeed issued to the Token Consumer identified in the token
      request (e.g., by verifying that the redirect URI is associated
      with the OAuth client ID.)

   o  The Token Consumer should verify that the tokens were issued for
      it, and not some other token consumer.  To aid in this, the
      federation protocol should include an audience parameter in the
      token response, or apply equivalent mechanisms (the implicit OAuth
      flow requires Token Consumers to identify themselves when they
      exchange OAuth authorization codes for OAuth refresh tokens,
      leaving it up to the Token Provider to verify that the OAuth
      authorization was delivered to the correct Token Consumer).

8.  Privacy Considerations

8.1.  Scoping of Token Binding Key Pairs

   Clients use different Token Binding key pairs for different servers,
   so as to not allow Token Binding to become a tracking tool across
   different servers.  However, the scoping of the Token Binding key
   pairs to servers varies according to the scoping rules of the
   application protocol (Section 4.1 of [I-D.ietf-tokbind-protocol]).

   In the case of HTTP cookies, servers may use Token Binding to secure
   their cookies.  These cookies can be attached to any sub-domain of
   effective top-level domains (eTLDs), and clients therefore should use
   the same Token Binding key pair across such subdomains.  This will
   ensure that any server capable of receiving the cookie will see the
   same Token Binding ID from the client, and thus be able to verify the
   token binding of the cookie.  See Section 2.1, above.

   If the client application is not a Web browser, it may have
   additional knowledge about the relationship between different
   servers.  For example, the client application might be aware of the
   fact that two servers play the role of Relying Party and Identity
   Provider in a federated sign-on protocol, and that they therefore
   share the identity of the user.  In such cases, it is permissible to
   use different Token Binding key pair scoping rules, such as using the



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   same Token Binding key pair for both the Relying Party and the
   Identity Provider.  Absent such special knowledge, conservative key-
   scoping rules should be used, assuring that clients use different
   Token Binding key pairs with different servers.

8.2.  Lifetime of Token Binding Key Pairs

   Token Binding key pairs do not have an expiration time.  This means
   that they can potentially be used by a server to track a user for an
   extended period of time (similar to a long-lived cookie).  HTTPS
   clients such as Web user agents SHOULD therefore provide a user
   interface for discarding Token Binding key pairs (similar to the
   affordances provided to delete cookies).

   If a user agent provides modes such as private browsing mode in which
   the user is promised that browsing state such as cookies are
   discarded after the session is over, the user agent MUST also discard
   Token Binding key pairs from such modes after the session is over.
   Generally speaking, users should be given the same level of control
   over lifetime of Token Binding key pairs as they have over cookies or
   other potential tracking mechanisms.

8.3.  Correlation

   An application's various communicating endpoints that receive Token
   Binding IDs for TLS connections other than their own obtain
   information about the application's other TLS connections.  (In this
   context, "an application" is a combination of client-side and server-
   side components, communicating over HTTPS, where the client side may
   be either or both Web browser-based or native application-based.)
   These other Token Binding IDs can serve as correlation handles for
   the endpoints of the other connections.  If the receiving endpoints
   are otherwise aware of these other connections, then no additional
   information is being exposed.  For instance, if in a redirect-based
   federation protocol, the Identity Provider and Relying Party already
   possess URLs for one another, also having Token Binding IDs for these
   connections does not provide additional correlation information.  If
   not, then, by providing the other Token Binding IDs, additional
   information is exposed that can be used to correlate the other
   endpoints.  In such cases, a privacy analysis of enabled correlations
   and their potential privacy impacts should be performed as part of
   the application design decisions of how, and whether, to utilize
   Token Binding.

   Also, Token Binding implementations must take care to only reveal
   Token Binding IDs to other endpoints if the application associated
   with a Token Binding ID signals to do so, see Section 6
   ("Implementation Considerations").



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   Finally, care should be taken to ensure that unrelated applications
   do not obtain information about each other's Token Bindings.  For
   instance, a Token Binding implementation shared between multiple
   applications on a given system should prevent unrelated applications
   from obtaining each other's Token Binding information.  This may be
   accomplished by using techniques such as application isolation and
   key segregation, depending upon system capabilities.

9.  IANA Considerations

   Below are the Internet Assigned Numbers Authority (IANA) Permanent
   Message Header Field registration information per [RFC3864].

     Header field name:           Sec-Token-Binding
     Applicable protocol:         HTTP
     Status:                      standard
     Author/Change controller:    IETF
     Specification document(s):   this one

     Header field name:           Include-Referred-Token-Binding-ID
     Applicable protocol:         HTTP
     Status:                      standard
     Author/Change controller:    IETF
     Specification document(s):   this one

10.  Acknowledgements

   This document incorporates comments and suggestions offered by Eric
   Rescorla, Gabriel Montenegro, Martin Thomson, Vinod Anupam, Anthony
   Nadalin, Michael B.  Jones, Bill Cox, Brian Campbell, 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.

11.  References

11.1.  Normative References

   [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-14 (work in progress), May 2018.







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   [I-D.ietf-tokbind-protocol]
              Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
              Hodges, "The Token Binding Protocol Version 1.0", draft-
              ietf-tokbind-protocol-19 (work in progress), May 2018.

   [PSL]      Mozilla, "Public Suffix List, https://publicsuffix.org/",
              <https://publicsuffix.org/>.

   [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>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [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>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

   [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>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.



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   [RFC7541]  Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
              <https://www.rfc-editor.org/info/rfc7541>.

   [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>.

11.2.  Informative References

   [fetch-spec]
              WhatWG, "Fetch", Living Standard ,
              <https://fetch.spec.whatwg.org/>.

   [I-D.ietf-tokbind-tls13]
              Harper, N., "Token Binding for Transport Layer Security
              (TLS) Version 1.3 Connections", draft-ietf-tokbind-
              tls13-01 (work in progress), May 2018.

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocol for the OASIS Security Assertion
              Markup Language (SAML) V2.0", OASIS Standard saml-core-
              2.0-os, March 2005, <http://docs.oasis-
              open.org/security/saml/v2.0/saml-core-2.0-os.pdf>.

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0", August 2015,
              <http://openid.net/specs/openid-connect-core-1_0.html>.

   [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>.

   [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>.

   [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>.







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   [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>.

   [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
   Microsoft Corp.
   USA

   Email: andreipo@microsoft.com


   Magnus Nystroem
   Microsoft Corp.
   USA

   Email: mnystrom@microsoft.com


   Dirk Balfanz (editor)
   Google Inc.
   USA

   Email: balfanz@google.com


   Adam Langley
   Google Inc.
   USA

   Email: agl@google.com


   Nick Harper
   Google Inc.
   USA

   Email: nharper@google.com




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Internet-Draft           Token Binding over HTTP               June 2018


   Jeff Hodges
   PayPal
   USA

   Email: Jeff.Hodges@paypal.com














































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