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Versions: (draft-balfanz-https-token-binding) 00 01 02 03 04 05 06 07 08 09 10

Internet Engineering Task Force                                 A. Popov
Internet-Draft                                               M. Nystroem
Intended status: Standards Track                         Microsoft Corp.
Expires: May 27, 2017                                    D. Balfanz, Ed.
                                                              A. Langley
                                                             Google Inc.
                                                               J. Hodges
                                                                  Paypal
                                                       November 23, 2016


                        Token Binding over HTTP
                      draft-ietf-tokbind-https-07

Abstract

   This document describes a collection of mechanisms that allow HTTP
   servers to cryptographically bind authentication tokens (such as
   cookies and OAuth tokens) to TLS [RFC5246] 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 Internet-Draft is a companion document to The Token Binding
   Protocol [I-D.ietf-tokbind-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 http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any




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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 May 27, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   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 Header Field  . . . . . . . . . . . . .   4
     2.1.  HTTPS Token Binding Key Pair Scoping  . . . . . . . . . .   5
   3.  TLS Renegotiation . . . . . . . . . . . . . . . . . . . . . .   5
   4.  First-party Use Cases . . . . . . . . . . . . . . . . . . . .   6
   5.  Federation Use Cases  . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  HTTP Redirects  . . . . . . . . . . . . . . . . . . . . .   8
     5.4.  Negotiated Key Parameters . . . . . . . . . . . . . . . .  10
     5.5.  Federation Example  . . . . . . . . . . . . . . . . . . .  10
   6.  Implementation Considerations . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     7.1.  Security Token Replay . . . . . . . . . . . . . . . . . .  13
     7.2.  Triple Handshake Vulnerability in TLS 1.2 and Older TLS
           Versions  . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.3.  Sensitivity of the Sec-Token-Binding Header . . . . . . .  14
     7.4.  Securing Federated Sign-On Protocols  . . . . . . . . . .  15
   8.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  17
     8.1.  Scoping of Token Binding Keys . . . . . . . . . . . . . .  17
     8.2.  Life Time of Token Binding Keys . . . . . . . . . . . . .  17
     8.3.  Correlation . . . . . . . . . . . . . . . . . . . . . . .  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19



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     11.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     11.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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 related to a private key,
   that the client proves possession of to the server, and is long-lived
   (i.e., subsequent TLS connections between the same client and server
   have the same Token Binding ID).  When issuing a security token (e.g.
   an HTTP cookie or an OAuth token) 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).

   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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].



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2.  The Sec-Token-Binding 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 the Sec-Token-
   Binding header field in their HTTP requests.  The ABNF of the Sec-
   Token-Binding header field is (in [RFC7230] style, see also [RFC7231]
   Section 8.3):

     Sec-Token-Binding = EncodedTokenBindingMessage

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

   For example:

     Sec-Token-Binding: <base64url-encoded TokenBindingMessage>

   The TokenBindingMessage MUST contain one 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 server that the HTTP request is sent to
   (clients use different Token Binding keys 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 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 validaion 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.





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   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 the
   latter 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 Token Binding key pair scoping for those key pairs generated by
   Web browsers in the context of the first-party and federation use
   cases defined in this specification (below), and to be used for
   binding HTTP cookies MUST be at the granularity of "effective top-
   level domain (public suffix) + 1" (eTLD+1), i.e., at the same
   granularity at which cookies can be set (see [RFC6265]).  Key pairs
   used to bind other application tokens, such as OAuth tokens or Open
   ID Connect "ID Tokens", SHOULD generally adhere to the above eTLD+1
   scoping requirement for those tokens being employed in first-party or
   federation scenarios as described below.  Applications other than Web
   browsers MAY use different key 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(s).



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4.  First-party Use Cases

   In a first-party 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.

   See [I-D.ietf-tokbind-protocol] Section 6 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 private keys 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 identity token) 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.

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




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   OpenID Connect (where the identity token is called "ID Token") and
   SAML (where 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
   said client can use the identity token: The Relying Party will
   compare the Token Binding ID in the identity token with the Token
   Binding ID of the TLS connection between it 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 public key that the client
      uses when communicating with the Token Consumer, thus _binding_
      the token to client's Token-Binding keypair.  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 reveals the corresponding public
      key of this keypair as part of the Token Binding ID.  Comparing
      the public key from the token with the public key from the Token
      Binding ID 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 public
      key 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 (proving possession of a
   private key that corresponds to a Token Binding ID between the client
   and the Token Consumer while delivering the token request to the



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   Token Provider) 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 include a redirect from Token Consumer to 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 a 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 [RFC7231] Section 8.3):

     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.  Note that the
   referred token binding is sent only on the request resulting from the
   redirect and not on any subsequent requests to the Token Provider.




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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 301,
   302, 303, 307 or 308, 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
   (see above).

   The TokenBindingMessage SHOULD contain a TokenBinding with
   TokenBindingType referred_token_binding.  If included, this
   TokenBinding MUST be signed with the Token Binding key used by the
   client for connections between itself and the Token Consumer (more
   specifically, the web origin 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 provided_token_binding, which MUST be signed with
   the Token Binding key used by the client for connections between
   itself and the Token Provider (more specifically, the web origin 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
   accomodated 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
      pointing to TP1.  TP1 in turn constructs a security token for the



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      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.  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 origin that the header field is sent to.

   Token Providers SHOULD support all the Token Binding key parameters
   specified in the [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 OIDC, 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:      | Open ID 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 Reponse 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, generic Token Binding implementations intended to support any
   HTTPS-based client-side application (e.g., so-called "native
   applications"), should provide means for applications to have Token
   Binding messages, containing Token Binding IDs of various
   application-specified Token Binding types and for application-
   specified TLS connections, conveyed over an application-specified
   HTTPS connection, i.e., within the TokenBindingMessage conveyed by
   the Sec-Token-Binding header field.

   However, such 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 a Web browser, thus signaling to the Token Binding
   implementation in the Web browser that the Web application associated
   with the server's origin intents to convey the Web browser's Token
   Binding ID to another server.  Other signaling mechanisms are
   possible, 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.  Bound
   tokens can still be replayed by malware present in the client.  In
   order to export the token to another machine and successfully replay
   it, the attacker also needs to export the corresponding private key.
   The Token Binding private key is therefore a high-value asset and
   MUST be strongly protected, ideally by generating it in a hardware
   security module that prevents key export.

7.2.  Triple Handshake Vulnerability in TLS 1.2 and Older TLS Versions

   The Token Binding protocol relies on the exported key material (EKM)
   value [RFC5705] to associate a TLS connection with a TLS Token
   Binding.  The triple handshake attack [TRIPLE-HS] is a known
   vulnerability in TLS 1.2 and older TLS versions, allowing the



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   attacker to synchronize keying material between TLS connections.  The
   attacker can then successfully replay bound tokens.  For this reason,
   the Token Binding protocol MUST NOT be negotiated with these TLS
   versions, unless the Extended Master Secret [RFC7627] and
   Renegotiation Indication [RFC5746] TLS extensions have also been
   negotiated.

7.3.  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's private Token Binding 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 to send 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).



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   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 keys 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
   the account, and can thus indirectly observe A'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 origin 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 origin.  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].

7.4.  Securing Federated Sign-On Protocols

   As explained above, in a federated sign-in scenario a client will
   prove possession of two different key pairs to a Token Provider: One
   key pair is the "provided" Token Binding key pair (which the client
   normally uses with the Token Provider), and the other is the
   "referred" Token Binding key pair (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 key.

   Both proofs (that of the provided Token Binding key and that of the
   referred Token Binding 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 key.

   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).





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   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 key.  Therefore, it can either choose to
   replace the Token Binding key 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 the EKM between man-in-the-middle
   and the Token Provider.  If it chooses the former (and creates its
   own signature, with its own Token Binding key, over the EKM for the
   connection between 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 key in the message
   will not match the Token Binding key 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 key possession of the
   provided Token Binding key is required in the protocol (as we do
   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 Issuer to issue a token
   bound to _its_ Token Binding key, not to the Token Binding key of the
   legitimate client.  To thwart this goal of the man-in-the-middle, the
   client's referred Token Binding key must be communicated to the Token
   Producer in a manner that can not 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 message in the
   Sec-Token-Binding header field (as opposed to, say, including the
   referred Token Binding key in an application-level message as part of



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   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 key 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.

8.  Privacy Considerations

8.1.  Scoping of Token Binding Keys

   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 ([I-D.ietf-tokbind-protocol] section 4.1).

   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, and clients therefore should use the
   same Token Binding key 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 scoping rules, such as using the same
   Token Binding key 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 keys with different servers.

8.2.  Life Time of Token Binding Keys

   Token Binding keys do not have an expiration time.  This means that
   they can potentially be used by a server to track a user across 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 keys (similar to the
   affordances provided to delete 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 should also
   discard Token Binding keys from such modes after the session is over.
   Generally speaking, users should be given the same level of control
   over life time of Token Binding keys 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".

   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].




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     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

   [[TODO: possibly add further considerations wrt the behavior of the
   above header fields, per <https://tools.ietf.org/html/
   rfc7231#section-8.3>]]

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, Nick Harper, Brian Campbell,
   and others.

11.  References

11.1.  Normative References

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

   [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-05 (work in progress), September 2016.

   [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-10 (work in progress), September
              2016.

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




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

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://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,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <http://www.rfc-editor.org/info/rfc5705>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <http://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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7541>.

11.2.  Informative References

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <http://www.rfc-editor.org/info/rfc5746>.

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





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   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <http://www.rfc-editor.org/info/rfc6750>.

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

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <http://www.rfc-editor.org/info/rfc7627>.

   [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








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   Adam Langley
   Google Inc.
   USA

   Email: agl@google.com


   Jeff Hodges
   Paypal
   USA

   Email: Jeff.Hodges@paypal.com







































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