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Versions: 00 draft-ietf-tokbind-https

Internet Engineering Task Force                                 A. Popov
Internet-Draft                                               M. Nystroem
Intended status: Standards Track                         Microsoft Corp.
Expires: April 16, 2015                                  D. Balfanz, Ed.
                                                              A. Langley
                                                             Google Inc.
                                                        October 13, 2014


                        Token Binding over HTTP
                  draft-balfanz-https-token-binding-00

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 [DraftPopov]

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
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on April 16, 2015.

Copyright Notice

   Copyright (c) 2014 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  The Token-Binding Header  . . . . . . . . . . . . . . . . . .   3
   3.  Federation Use Cases  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  HTTP Redirects  . . . . . . . . . . . . . . . . . . . . .   5
     3.4.  Cross-Origin Resource Sharing . . . . . . . . . . . . . .   6
     3.5.  Negotiated Key Parameters . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     4.1.  Security Token Replay . . . . . . . . . . . . . . . . . .   7
     4.2.  Privacy Considerations  . . . . . . . . . . . . . . . . .   7
     4.3.  Triple Handshake Vulnerability in TLS . . . . . . . . . .   8
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Token Binding Protocol [DraftPopov] 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




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   token to TLS connections between that particular client and server,
   and inoculating the token against theft by attackers.

   While the Token Binding Protocol [DraftPopov] 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].

2.  The 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
   [DraftPopov]), clients MUST include the following header in their
   HTTP requests:


    Token-Binding: EncodedTokenBindingMessage

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




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   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 [DraftPopov], and includes a TokenBinding structure in the
   Token-Binding HTTP header defined above.  What differs between the
   various mechanisms is _how_ the Token Consumer grants the permission
   to reveal the Token Binding ID to the Token Provider.

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



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   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 the Token Binding ID of the TLS connection between the
      client and the Token Consumer in the token.

   o  That Token Binding ID 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 [DraftPopov].

   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 will enumerate a number of mechanisms available to Token
   Consumers to grant this permission.

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 the following
   HTTP response header in its HTTP response:


    Include-Referer-Token-Binding-ID: true




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   Including this response header signals to the client that it should
   reveal the Token Binding ID used between the client and the Token
   Consumer to the Token Provider.  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.

   This header has only meaning if the HTTP status code is 302 or 301,
   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 following header to the Token Provider
   with each HTTP request (see above):


    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.  Cross-Origin Resource Sharing

   When issuing an XML HTTP request across origins to a Token Provider,
   a Token Consumer can reveal its Token Binding ID through the
   withRefererTokenBindingID property of the XmlHttpRequest object.
   Example:


    var xhr = new XMLHttpRequest();
    xhr.withCredentials = true; // send cookies
    xhr.withRefererTokenBindingID = true;
    xhr.open(method, url, true);

   The client SHOULD include the Token-Binding: header to the outgoing
   request as described above if:




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   o  the withRefererTokenBindingID property of the XmlHttpRequest
      object is set to true, and

   o  the client has negotiated the Token Binding Protocol both with the
      web origin that issued the XmlHttpRequest, and the web origin to
      which the XmlHttpRequest is addressed.

3.5.  Negotiated Key Parameters

   The Token Binding Protocol [DraftPopov] 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 [DraftPopov].  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.

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.  Privacy Considerations

   The Token Binding protocol uses persistent, long-lived TLS Token
   Binding IDs.  To protect privacy, TLS Token Binding IDs are never
   transmitted in clear text and can be reset by the user at any time,
   e.g. when clearing browser cookies.  Unique Token Binding IDs MUST be
   generated for connections to different origins, so they cannot be
   used by cooperating servers to link user identities.



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4.3.  Triple Handshake Vulnerability in TLS

   The Token Binding protocol relies on the tls_unique value 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 tls_unique values 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.

5.  References

5.1.  Normative References

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

   [I-D.ietf-httpbis-header-compression]
              Peon, R. and H. Ruellan, "HPACK - Header Compression for
              HTTP/2", draft-ietf-httpbis-header-compression-09 (work in
              progress), July 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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, June 1999.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492, May 2006.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, July 2010.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, July 2014.




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5.2.  Informative References

   [I-D.ietf-httpbis-http2]
              Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
              Protocol version 2", draft-ietf-httpbis-http2-14 (work in
              progress), July 2014.

   [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-02 (work in progress), October 2014.

   [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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