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PROPOSED STANDARD

Internet Engineering Task Force (IETF)                          A. Popov
Request for Comments: 8473                                   M. Nystroem
Category: Standards Track                                Microsoft Corp.
ISSN: 2070-1721                                          D. Balfanz, Ed.
                                                               N. Harper
                                                             Google Inc.
                                                               J. Hodges
                                                  Kings Mountain Systems
                                                            October 2018


                        Token Binding over HTTP

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 that it issues to a client -- and that the client
   subsequently returns to the server -- to the TLS connection between
   the client and the 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
   Version 1.0" (RFC 8471).

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/rfc8473.




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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 . . . . . . . . . . . . . . . . . .   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 . . . . . . . . . . . . . . . . . . . .   7
   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  . . . . . . . . . . . . . . . . . . .  13
   6.  Implementation Considerations . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
     7.1.  Security Token Replay . . . . . . . . . . . . . . . . . .  16
     7.2.  Sensitivity of the Sec-Token-Binding Header . . . . . . .  16
     7.3.  Securing Federated Sign-On Protocols  . . . . . . . . . .  17
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  20
     8.1.  Scoping of Token Binding Key Pairs  . . . . . . . . . . .  20
     8.2.  Lifetime of Token Binding Key Pairs . . . . . . . . . . .  20
     8.3.  Correlation . . . . . . . . . . . . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     10.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25







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1.  Introduction

   The Token Binding protocol [RFC8471] 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.  That is, 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 (reuse, attempted
   impersonation, etc.) by attackers.

   While the Token Binding protocol [RFC8471] 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 HTTP 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.

   A TokenBindingMessage allows a client to establish multiple Token
   Binding IDs with the server by including multiple TokenBinding
   structures.  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 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.



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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 [RFC8471] and [RFC8472]), 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 (per
   the style of [RFC7230]; 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 [RFC8471].  The base64url encoding
   uses the URL and filename safe character set described in Section 5
   of [RFC4648], with all trailing padding characters (i.e., "=")
   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:

   o  SHOULD NOT be stored by origin servers on PUT requests,

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

   o  MUST NOT be used in HTTP trailers.

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




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   established by this TokenBinding is called a "Provided Token
   Binding ID".

   The TokenBindingMessage MAY also contain exactly one TokenBinding
   structure with a TokenBindingType value 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 specific to the use case in question; 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 [RFC8471].  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
   (where the term "public suffix" is defined in the "NOTE:" paragraph
   in Section 5.3 of [RFC6265] as "a domain that is controlled by a
   public registry").  For example, for "https://www.example.com", the
   public suffix (eTLD) is "com", and the registered domain (eTLD+1) is



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   "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 narrower.
   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 [OpenID.Core], 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.  (That is, 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 due to TLS message fragmentation; see
   Section 6.2.1 of [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.)












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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 the client and the 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.

   See Section 5 of [RFC8471] for general guidance regarding the 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 [RFC8471] 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 the Security Assertion Markup Language (SAML)
   [OASIS.saml-core-2.0-os] (in which the identity token is a SAML
   assertion).






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   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
   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 [RFC8471] 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 the client and the 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 [RFC8471].

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






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   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
   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 (per the style of [RFC7230]; 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 [RFC8471] of the provided
   TokenBinding from the referrer and create a referred TokenBinding
   with it to include in the TokenBindingMessage in 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 in 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



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   Token Binding is no longer included in subsequent requests.  (A new
   referred Token Binding may be included if the redirecting endpoint
   itself responded with an Include-Referred-Token-Binding-ID response
   header.)

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

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

   The TokenBindingMessage included in the redirect request to the Token
   Provider SHOULD contain a TokenBinding with a TokenBindingType value
   of 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 a
   TokenBindingType value 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 a client's request
      that contains a TokenBindingMessage that in turn contains both
      provided and referred TokenBindings (for TP1 and TC,
      respectively), TP1 responds to the client with a redirect response
      that (1) contains the Include-Referred-Token-Binding-ID header
      field and (2) directs the client to send a request to TP2.  This
      causes the client to follow the same pattern and send a request



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      containing a TokenBindingMessage that contains both provided and
      referred TokenBindings (for TP2 and TP1, respectively) to TP2.
      Note that this pattern can continue to additional 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
      pointing to TP1.  TP1 in turn constructs a security token for the
      Token Consumer, bound to the TC's referred TokenBinding that 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 [RFC8472]
   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 the
   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 [RFC8471].  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.




















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5.5.  Federation Example

   The diagram below shows a typical HTTP redirect-based web browser
   single sign-on (SSO) profile (Section 4.1 of
   [OASIS.saml-prof-2.0-os]) (no artifact, no callbacks), featuring the
   binding of, for example, a TLS Token Binding ID into an OpenID
   Connect ID Token.

                                  Legend:

   +------------+------------------------------------------------------+
   | EKM:       | TLS Exported Keying Material [RFC5705]               |
   |            |                                                      |
   | {EKMn}Ksm: | EKM for server "n", signed by the private key of     |
   |            | TBID "m", where "n" must represent the 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.  For    |
   |            | example, see step 1b in the 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 and Kp: | private (aka secret) key and public key,             |
   |            | respectively, of the client-side Token Binding key   |
   |            | pair                                                 |
   |            |                                                      |
   | OIDC:      | OpenID Connect                                       |
   |            |                                                      |
   | TB:        | TokenBinding struct containing a 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.          |
   +------------+------------------------------------------------------+








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Client, aka                Token Consumer, aka    Token Provider, 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 and Kp1, TBID1           |
    | ETBMSG[[{EKM1}Ks1,TBID1,provided_token_binding]]     |
    |------------------------------>|                      |
    |                               |                      |
    |                               |                      |
    |                               |                      |
    | 1a. OIDC ID Token request, aka|                      |
    | "Authentication Request", conveyed with              |
    | an 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 a 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)         |
    |                               |                      |
    |                               |                      |





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    |                               |                      |
    | 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 an Authentication Response containing      |
    | an ID Token w/TBID1, issued for TC                   |
    |                               |                      |
    |                               |                      |
    |                               |                      |
    | 4. user is signed on; any security-relevant cookie(s)|
    | that is set SHOULD contain TBID1                     |
    |<------------------------------|                      |
    |                               |                      |
    |                               |                      |

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.  Signaling
   mechanisms other than the Include-Referred-Token-Binding-ID HTTP
   response header field are possible, but these mechanisms are outside
   the scope of this specification.

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











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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 the 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 to 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 scenario described in
   Section 7.1 ("Security Token Replay") of [RFC8471].

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

   For example, the client must keep its Token Binding private key
   secret.  If the private key is not secret, then another actor in the
   system could create a valid Token Binding header field and thereby
   impersonate 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, they 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 to a
   server a Sec-Token-Binding header field that contains Token Bindings
   signed with any Token Binding keys that the client does not control.



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   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 (e.g., 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, in this
   scenario, A can trick 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 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 Document Object Model (DOM)
   API [W3C.REC-DOM-Level-3-Core-20040407] 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 as a "forbidden header name"; see [fetch-spec].

7.3.  Securing Federated Sign-On Protocols

   As explained above, in a federated sign-on 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.





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   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
      yet done so (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
      the client and the Token Consumer or between the client and the
      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-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 choose
   to either (1) 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 (2) 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 EKM for
   the connection between the client and the man-in-the-middle) will not
   match a signature on the EKM between the 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 the Token
   Provider), then the Token Binding message will match the connection
   between the man-in-the-middle and the 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).




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   Next, consider the presence of a man-in-the-middle between the client
   and the 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 and 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 Provider in a manner that cannot be
   affected by the man-in-the-middle (who, as mentioned above, can
   modify redirect URLs and JavaScript at the client).  Including the
   referred TokenBinding 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 the client and the 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 and a proof of possession of the referred Token
   Binding key.

   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 the use of 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).











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   o  The Token Consumer should verify that the tokens were issued for
      it and not for 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 [RFC8471]).

   In the case of HTTP cookies, servers may use Token Binding to secure
   their cookies.  These cookies can be attached to any subdomain 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 roles of Relying Party and Identity
   Provider, respectively, 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 same Token Binding key pair for both the Relying
   Party and the Identity Provider.  Absent such special knowledge,
   conservative key-pair 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
   controls provided for deleting cookies).




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   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 the 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 web-browser-based, native-application-based, or both.)
   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, then also having Token Binding IDs for
   these connections does not provide additional correlation
   information.  If not, by providing the other Token Binding IDs,
   additional information is then 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 signaled to do so by the
   application associated with a Token Binding ID; see Section 6
   ("Implementation Considerations").

   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.









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9.  IANA Considerations

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

      Header Field name:           Sec-Token-Binding
      Protocol:                    HTTP
      Status:                      standard
      Reference:                   This document

      Header Field name:           Include-Referred-Token-Binding-ID
      Protocol:                    HTTP
      Status:                      standard
      Reference:                   This document

10.  References

10.1.  Normative References

   [PSL]      Mozilla, "Public Suffix List",
              <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>.



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   [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>.

   [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>.

   [RFC8471]  Popov, A., Ed., Nystroem, M., Balfanz, D., and J. Hodges,
              "The Token Binding Protocol Version 1.0", RFC 8471,
              DOI 10.17487/RFC8471, October 2018,
              <https://www.rfc-editor.org/info/rfc8471>.

   [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>.

10.2.  Informative References

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

   [OASIS.saml-core-2.0-os]
              Cantor, S., Kemp, J., Philpott, R., and E. Maler,
              "Assertions and Protocols 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>.







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   [OASIS.saml-prof-2.0-os]
              Hughes, J., Ed., Cantor, S., Ed., Hodges, J., Ed., Hirsch,
              F., Ed., Mishra, P., Ed., Philpott, R., Ed., and E. Maler,
              Ed., "Profiles for the OASIS Security Assertion Markup
              Language (SAML) V2.0", OASIS Standard
              OASIS.saml-profiles-2.0-os, March 2005,
              <http://docs.oasis-open.org/security/
              saml/v2.0/saml-profiles-2.0-os.pdf>.

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", November 2014,
              <http://openid.net/specs/openid-connect-core-1_0.html>.

   [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>.

   [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.

   [W3C.REC-DOM-Level-3-Core-20040407]
              Le Hors, A., Ed., Le Hegaret, P., Ed., Wood, L., Ed.,
              Nicol, G., Ed., Robie, J., Ed., Champion, M., Ed., and S.
              Byrne, Ed., "Document Object Model (DOM) Level 3 Core
              Specification", World Wide Web Consortium Recommendation
              REC-DOM-Level-3-Core-20040407, April 2004,
              <https://www.w3.org/TR/2004/
              REC-DOM-Level-3-Core-20040407>.














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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.

Authors' Addresses

   Andrei Popov
   Microsoft Corp.
   United States of America

   Email: andreipo@microsoft.com


   Magnus Nystroem
   Microsoft Corp.
   United States of America

   Email: mnystrom@microsoft.com


   Dirk Balfanz (editor)
   Google Inc.
   United States of America

   Email: balfanz@google.com


   Nick Harper
   Google Inc.
   United States of America

   Email: nharper@google.com


   Jeff Hodges
   Kings Mountain Systems
   United States of America

   Email: Jeff.Hodges@KingsMountain.com






Popov, et al.                Standards Track                   [Page 25]


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