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Versions: (draft-balfanz-https-token-binding) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 8473

Internet Engineering Task Force                                 A. Popov
Internet-Draft                                               M. Nystroem
Intended status: Standards Track                         Microsoft Corp.
Expires: September 22, 2016                              D. Balfanz, Ed.
                                                              A. Langley
                                                             Google Inc.
                                                               J. Hodges
                                                                  Paypal
                                                          March 21, 2016


                        Token Binding over HTTP
                      draft-ietf-tokbind-https-03

Abstract

   This document describes a collection of mechanisms that allow HTTP
   servers to cryptographically bind authentication tokens (such as
   cookies and OAuth tokens) to a TLS [RFC5246] connection.

   We describe both _first-party_ as well as _federated_ scenarios.  In
   a first-party scenario, an HTTP server issues a security token (such
   as a cookie) to a client, and expects the client to send the security
   token back to the server at a later time in order to authenticate.
   Binding the token to the TLS connection between client and server
   protects the security token from theft, and ensures that the security
   token can only be used by the client that it was issued to.

   Federated token bindings, on the other hand, allow servers to
   cryptographically bind security tokens to a TLS [RFC5246] 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 [TBPROTO]

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 September 22, 2016.

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  . . . . . . . . . . . . . . . .   4
   3.  Federation Use Cases  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  HTTP Redirects  . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Negotiated Key Parameters . . . . . . . . . . . . . . . .   7
     3.5.  Federation Example  . . . . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     4.1.  Security Token Replay . . . . . . . . . . . . . . . . . .  10
     4.2.  Triple Handshake Vulnerability in TLS . . . . . . . . . .  10
     4.3.  Sensitivity of the Sec-Token-Binding Header . . . . . . .  10
     4.4.  Securing Federated Sign-On Protocols  . . . . . . . . . .  11
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  13
     5.1.  Scoping of Token Binding Keys . . . . . . . . . . . . . .  13
     5.2.  Life Time of Token Binding Keys . . . . . . . . . . . . .  14
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15








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1.  Introduction

   The Token Binding Protocol [TBPROTO] 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 theft by attackers.

   While the Token Binding Protocol [TBPROTO] defines a message format
   for establishing a Token Binding ID, it doesn't 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 [RFC2616] and 2 [I-D.ietf-httpbis-http2]).
   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 in HTTP requests.  The HTTP header value is a
   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

   Once a client and server have negotiated the Token Binding Protocol
   with HTTP/1.1 or HTTP/2 (see The Token Binding Protocol [TBPROTO]),
   clients MUST include the Sec-Token-Binding header in their HTTP
   requests.  The ABNF of the Sec-Token-Binding header is:


 Sec-Token-Binding = "Sec-Token-Binding" ":" [CFWS] EncodedTokenBindingMessage

   The EncodedTokenBindingMessage is a web-safe Base64-encoding of the
   TokenBindingMessage as defined in the TokenBindingProtocol [TBPROTO].

   The TokenBindingMessage MUST contain a TokenBinding with
   TokenBindingType provided_token_binding, which MUST be signed with
   the Token Binding 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).  The Token
   Binding ID established by this TokenBinding is called a _Provided
   Token Binding ID_

   In HTTP/2, the client SHOULD use Header Compression
   [I-D.ietf-httpbis-header-compression] to avoid the overhead of
   repeating the same header in subsequent HTTP requests.

3.  Federation Use Cases

3.1.  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) gives the client permission to 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 [TBPROTO], and includes a TokenBinding structure in the Sec-
   Token-Binding HTTP header defined above.  What differs between the



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   various mechanisms is _how_ the Token Consumer grants the permission
   to 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.

3.2.  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 (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 an 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 [TBPROTO], 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.





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   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 [TBPROTO].

   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
   Token Provider) only if the Token Consumer permits the client to do
   so.

   Below, we specify how Token Consumers can grant this permission.
   during redirect-based federation protocols.

3.3.  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-
   Referer-Token-Binding-ID HTTP response header in its HTTP response.
   The ABNF of the Include-Referer-Token-Binding-ID header is:


 Include-Referer-Token-Binding-ID = "Include-Referer-Token-Binding-ID" ":"
                                     [CFWS] %x74.72.75.65 ; "true", case-sensitive

   Including this response header 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, the client will not disclose any information about the Token
   Binding used between the client and the Token Consumer to the Token
   Provider.

   When a client receives this header, 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 includes one provided binding
   and one referred binding, the latter constructed from the binding
   between the client and the Token Consumer.

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

   This header 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



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   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 following
   header to the Token Provider with each HTTP request (see above):


    Sec-Token-Binding: EncodedTokenBindingMessage

   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-Referer-Token-
   Binding-ID response header).  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 Privider (more specifically, the web origin that
   the token request sent to).

3.4.  Negotiated Key Parameters

   The Token Binding Protocol [TBPROTO] allows the server and client to
   negotiate a signature algorithm used in the TokenBindingMessage.  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 signature algorithms.  The client MUST
   use the signature algorithm negotiated with the Token Consumer in the
   referred_token_binding TokenBinding of the TokenBindingMessage, even
   if that signature algorithm is different from the one negotiated with
   the origin that the header is sent to.

   Token Providers SHOULD support all the SignatureAndHashAlgorithms
   specified in the Token Binding Protocol [TBPROTO].  If a token
   provider does not support the SignatureAndHashAlgorithm specified in
   the referred_token_binding TokenBinding in the TokenBindingMessage,
   it MUST issue an unbound token.

3.5.  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-Referer-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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4.  Security Considerations

4.1.  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.

4.2.  Triple Handshake Vulnerability in TLS

   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 TLS
   protocol vulnerability allowing the attacker to synchronize keying
   manterial between TLS connections.  The attacker can then
   successfully replay bound tokens.  For this reason, the Token Binding
   protocol MUST NOT be negotiated unless the Extended Master Secret TLS
   extension [I-D.ietf-tls-session-hash] has also been negotiated.

4.3.  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, 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,
   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 cookie theft.  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 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



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   protocol, in this case, didn't successfully bind the cookie to the
   client.

   Likewise, we need integrity protection of the Sec-Token-Binding
   header: A client shouldn't be tricked into sending a Sec-Token-
   Binding header to a server that contains Token Binding messages about
   key pairs that the client doesn't 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's 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 with A's key pair over C's EKM.  If the
   attacker could now trick C to send such a header to S, it would
   appear to S as if C controls a certain key pair when in fact it
   doesn't (the attacker A controls the key pair).

   If A has a pre-existing relationship with S (perhaps has an account
   on S), it now appears to the server S as if A is connecting to it,
   even though it is really C.  (If the server S doesn't 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.  One origin should not be able to set the Sec-Token-Binding
   header (through a DOM API or otherwise) that the User Agent uses with
   another origin.

4.4.  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.



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   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 hasn't
      done so yet (in other words, we're about to run the federated
      sign-on protocol).

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

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

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





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   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 (as opposed to, say, including the referred
   Token Binding key in an application-level message as part of the
   redirect URL) is one way to assure that the man-in-the-middle between
   client and Token Consumer cannot affect the communication of the
   referred Token Binding key to the Token Provider.

   Therefore, the Sec-Token-Binding header 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.

5.  Privacy Considerations

5.1.  Scoping of Token Binding Keys

   Clients must use different Token Binding keys for different servers,
   so as to not allow Token Binding to become a tracking tool across
   different servers.  When Token Binding is used over HTTPS, this key
   scoping should in particular happen at the granularity of "effective
   top-level domain (public suffix) + 1", i.e., at the same granularity
   at which cookies can be set.

   The reason for this is that servers may use Token Binding to secure
   their cookies.  These cookies can be attached to any sub-domain of
   public suffixes, 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.







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5.2.  Life Time of Token Binding Keys

   Token Binding keys don't 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).

   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.

6.  References

6.1.  Normative References

   [I-D.ietf-httpbis-header-compression]
              Peon, R. and H. Ruellan, "HPACK - Header Compression for
              HTTP/2", draft-ietf-httpbis-header-compression-12 (work in
              progress), February 2015.

   [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>.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616,
              DOI 10.17487/RFC2616, June 1999,
              <http://www.rfc-editor.org/info/rfc2616>.

   [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>.

   [TBPROTO]  Popov, A., "The Token Binding Protocol Version 1.0", 2014.




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6.2.  Informative References

   [I-D.ietf-httpbis-http2]
              Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
              Protocol version 2", draft-ietf-httpbis-http2-17 (work in
              progress), February 2015.

   [I-D.ietf-tls-session-hash]
              Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley,
              A., and M. Ray, "Transport Layer Security (TLS) Session
              Hash and Extended Master Secret Extension", draft-ietf-
              tls-session-hash-06 (work in progress), July 2015.

   [TRIPLE-HS]
              Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
              A., and P. Strub, "Triple Handshakes and Cookie Cutters:
              Breaking and Fixing Authentication over TLS. IEEE
              Symposium on Security and Privacy", 2014.

Authors' Addresses

   Andrei Popov
   Microsoft Corp.
   USA

   Email: andreipo@microsoft.com


   Magnus Nystroem
   Microsoft Corp.
   USA

   Email: mnystrom@microsoft.com


   Dirk Balfanz (editor)
   Google Inc.
   USA

   Email: balfanz@google.com


   Adam Langley
   Google Inc.
   USA

   Email: agl@google.com




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   Jeff Hodges
   Paypal
   USA

   Email: Jeff.Hodges@paypal.com














































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