HTTPS Token Binding with TLS Terminating Reverse Proxies
draft-ietf-tokbind-ttrp-01

Internet Engineering Task Force                              B. Campbell
Internet-Draft                                             Ping Identity
Intended status: Standards Track                          August 2, 2017
Expires: February 3, 2018

        HTTPS Token Binding with TLS Terminating Reverse Proxies
                       draft-ietf-tokbind-ttrp-01

Abstract

   This document defines common HTTP header fields that enable a TLS
   terminating reverse proxy to convey information about the validated
   Token Binding Message sent by the client to a backend server, which
   enables that backend server to bind, or verify the binding of,
   cookies and other security tokens to the client's Token Binding key.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on February 3, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   3
   2.  HTTP Header Fields and Processing Rules . . . . . . . . . . .   3
     2.1.  Token Binding ID HTTP Header Fields . . . . . . . . . . .   3
     2.2.  Processing Rules  . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   5
       2.3.1.  Provided Token Binding ID . . . . . . . . . . . . . .   5
       2.3.2.  Provided and Referred Token Binding IDs . . . . . . .   6
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  HTTP Message Header Field Names Registration  . . . . . .   7
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Token Binding over HTTP [I-D.ietf-tokbind-https] provides a mechanism
   that enables HTTP servers to cryptographically bind cookies and other
   security tokens to a key held by the browser or other HTTP client,
   possession of which is proven on the TLS [RFC5246] connections over
   which the tokens are used.  When Token Binding is negotiated in the
   TLS handshake [I-D.ietf-tokbind-negotiation] the client sends an
   encoded Token Binding Message [I-D.ietf-tokbind-protocol] as a header
   in each HTTP request, which proves possession of one or more private
   keys held by the client.  The public portion of the keys are
   represented in the Token Binding IDs of the Token Binding Message and
   for each one there is a signature over some data, which includes the
   exported keying material [RFC5705] of the TLS connection.  An HTTP
   server issuing cookies or other security tokens can associate them
   with the Token Binding ID, which ensures those tokens cannot be used
   successfully over a different TLS connection or by a different client
   than the one to which they were issued.

   A fairly common deployment architecture for HTTPS applications is to
   have the backend HTTP application servers sit behind a reverse proxy
   that terminates TLS.  The proxy is accessible to the internet and
   dispatches client requests to the appropriate backend server within a
   private or protected network.  The backend servers are not directly
   accessible outside the private network and are only reachable through
   the reverse proxy.  The details of such deployments are typically
   opaque to clients who make requests to the proxy server and see
   responses as though they originated from the proxy server itself.

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   TLS connections for HTTPS are established between each client and the
   reverse proxy server.

   Token Binding facilitates a binding of security tokens to a key held
   by the client by way of the TLS connection between that client and
   the server.  In a deployment where TLS is terminated by a reverse
   proxy, however, the TLS connection is between the client and the
   proxy while the backend server is likely the system that will issue
   cookies or other security tokens.  Additional steps are therefore
   needed to enable the use of Token Binding in such deployment
   architectures.  In the absence of a standardized approach, different
   implementations will address it differently, which will make
   interoperability between such implementations difficult or impossible
   without complex configurations or custom integrations.

   This document standardizes HTTP header field names that a TLS
   terminating reverse proxy (TTRP) adds to requests that it sends to
   the backend servers.  The headers contain the information from the
   validated Token Binding Message sent by the client to the proxy with
   the "Sec-Token-Binding" header, thus enabling the backend server to
   bind, or verify the binding of, cookies and other security tokens to
   the client's Token Binding key.  The usage of the headers, both the
   reverse proxy adding it and the application server using them to bind
   cookies or other tokens, are to be configuration options of the
   respective systems as they will not always be applicable.

1.1.  Requirements Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

2.  HTTP Header Fields and Processing Rules

2.1.  Token Binding ID HTTP Header Fields

   The Token Binding Protocol [I-D.ietf-tokbind-protocol] recommends
   that implementations make Token Binding IDs available to the
   application as opaque byte sequences, enabling those applications to
   use the Token Binding IDs when generating and verifying bound tokens.
   In the context of a TLS terminating reverse proxy (TTRP) deployment,
   the provided and referred Token Binding IDs are made available to the
   backend application as the "Sec-Provided-Token-Binding-ID" and "Sec-
   Referred-Token-Binding-ID" HTTP headers respectively.  The value of
   both headers is an "EncodedTokenBindingID", for which the ABNF
   [RFC5234] syntax is shown in Figure 1 below.  "EncodedTokenBindingID"
   is a single HTTP header field-value as defined in Section 3.2 of

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   [RFC7230], which MUST NOT have a list of values or occur multiple
   times in a request.  An "EncodedTokenBindingID" is only for use in
   HTTP requests and MUST NOT to be used in HTTP responses.

       EncodedTokenBindingID = *( DIGIT / ALPHA /  "-" / "_" )

       DIGIT = <Defined in Section B.1 of [RFC5234]>
       ALPHA = <Defined in Section B.1 of [RFC5234]>

              Figure 1: Encoded Token Binding ID Header ABNF

   The value of an "EncodedTokenBindingID" is a base64url encoding of
   the TokenBindingID byte sequence (see section 3 of
   [I-D.ietf-tokbind-protocol]) using the URL and filename safe alphabet
   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.

2.2.  Processing Rules

   This section defines the applicable processing rules for a TLS
   terminating reverse proxy (TTRP) and backend server(s) to provide
   server side support of Token Binding over HTTP
   [I-D.ietf-tokbind-https] using the HTTP headers described in
   Section 2.1.  Use of the technique is to be a configuration or
   deployments option and the processing rules described herein are for
   servers operating with that option enabled.

   A TTRP negotiates the use of Token Binding with the client per
   [I-D.ietf-tokbind-negotiation] and validates the Token Binding
   Message as defined in The Token Binding Protocol
   [I-D.ietf-tokbind-protocol] and Token Binding over HTTP
   [I-D.ietf-tokbind-https] for each HTTP request on the underlying TLS
   connection.  Requests with a valid Token Binding Message (and meeting
   any other authorization or policy requirements of the TTRP) are
   dispatched to the backend server with the following modifications.

   1.  The "Sec-Token-Binding" header in the original incoming request
       MUST be removed from the request that is dispatched to the
       backend server.

   2.  The Token Binding ID of the provided Token Binding of the Token
       Binding Message MUST be placed in the "Sec-Provided-Token-
       Binding-ID" header field of the dispatched request using the
       format defined in Section 2.1.

   3.  If the Token Binding Message contains a referred Token Binding,
       the referred Token Binding ID MUST be placed in the "Sec-

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       Referred-Token-Binding-ID" header field of the dispatched request
       using the format defined in Section 2.1.  Otherwise, the "Sec-
       Referred-Token-Binding-ID" header field MUST NOT be present in
       the dispatched request.

   4.  Any occurrence of the "Sec-Provided-Token-Binding-ID" or "Sec-
       Referred-Token-Binding-ID" header in the original incoming
       request MUST be removed or overwritten before forwarding the
       request.

   Requests made over a TLS connection where the use of Token Binding
   was not negotiated MUST be sanitized by removing any occurrences of
   the "Sec-Provided-Token-Binding-ID" and "Sec-Referred-Token-Binding-
   ID" header fields prior to dispatching the request to the backend
   server.

   Forward proxies and other intermediaries MUST NOT add the "Sec-
   Provided-Token-Binding-ID" or "Sec-Referred-Token-Binding-ID" header
   to requests.

2.3.  Examples

   Extra line breaks and whitespace have been added to the following
   examples for display and formatting purposes only.

2.3.1.  Provided Token Binding ID

   The following "Sec-Token-Binding" header is from an HTTP request made
   over a TLS connection between the client and the TTRP where the use
   of Token Binding has been negotiated (The base64url-encoded
   representation of the exported keying material, which can be used to
   validate the Token Binding Message, for that connection is
   "AYVUayPTP9RmELNpGjFl6Ykm2CUx7pUMxe35yb11dgU").  The encoded Token
   Binding Message has the provided Token Binding the client uses with
   the server.

    Sec-Token-Binding: AIkAAgBBQKzyIrmcY_YCtHVoSHBut69vrGfFdy1_YKTZfFJv
     6BjrZsKD9b9FRzSBxDs1twTqnAS71M1RBumuihhI9xqxXKkAQEtxe4jeUJU0WezxlQ
     XWVSBFeHxFMdXRBIH_LKOSAuSMOJ0XEw1Q8DE248qkOiRKzw3KdSNYukYEPmO21bQi
     3YYAAA

                 Figure 2: Header in HTTP Request to TTRP

   After validating the Token Binding Message, the TTRP removes the
   "Sec-Token-Binding" header and adds the following "Sec-Provided-
   Token-Binding-ID" header with the provided Token Binding ID to the
   request that is dispatched to the backend server.

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    Sec-Provided-Token-Binding-ID: AgBBQKzyIrmcY_YCtHVoSHBut69vrGfFdy1_
      YKTZfFJv6BjrZsKD9b9FRzSBxDs1twTqnAS71M1RBumuihhI9xqxXKk

            Figure 3: Header in HTTP Request to Backend Server

2.3.2.  Provided and Referred Token Binding IDs

   The following "Sec-Token-Binding" header is from an HTTP request made
   over a TLS connection between the client and the TTRP where the use
   of Token Binding has been negotiated (The base64url-encoded
   representation of the exported keying material, which can be used to
   validate the Token Binding Message, for that connection is
   "wEWWCP1KPxfq-QL4NxYII_P4ti_9YYqrTpGs28BZEqE").  The encoded Token
   Binding Message has the provided Token Binding the client uses with
   the server as well as the referred Token Binding that it uses with a
   different server.

    Sec-Token-Binding: ARIAAgBBQCfsI1D1sTq5mvT_2H_dihNIvuHJCHGjHPJchPav
     NbGrOo26-2JgT_IsbvZd4daDFbirYBIwJ-TK1rh8FzrC-psAQMyYIqXj7djGPev1dk
     jV9XxLYGCyqOrBVEtBHrMUCeo22ymLg3OiFcl_fmOPxJbjxI6lKcF0lyfy-dSQmPIe
     zQ0AAAECAEFArPIiuZxj9gK0dWhIcG63r2-sZ8V3LX9gpNl8Um_oGOtmwoP1v0VHNI
     HEOzW3BOqcBLvUzVEG6a6KGEj3GrFcqQBAHQm0pzgUTXKLRamuKE1pmmP9I3UBVpoe
     1DBCe9H2l1VPpsImakUa6crAqZ-0CGBmji7bYzQogpKcyxTTFk5zdwAA

                 Figure 4: Header in HTTP Request to TTRP

   After validating the Token Binding Message, the TTRP removes the
   "Sec-Token-Binding" header and adds the following "Sec-Provided-
   Token-Binding-ID" and "Sec-Referred-Token-Binding-ID" headers, with
   the provided and referred Token Binding IDs respectively, to the
   request that is dispatched to the backend server.

    Sec-Provided-Token-Binding-ID: AgBBQCfsI1D1sTq5mvT_2H_dihNIvuHJCHGj
      HPJchPavNbGrOo26-2JgT_IsbvZd4daDFbirYBIwJ-TK1rh8FzrC-ps
    Sec-Referred-Token-Binding-ID: AgBBQKzyIrmcY_YCtHVoSHBut69vrGfFdy1_
      YKTZfFJv6BjrZsKD9b9FRzSBxDs1twTqnAS71M1RBumuihhI9xqxXKk

            Figure 5: Headers in HTTP Request to Backend Server

3.  Security Considerations

   The headers described herein enable a reverse proxy and backend
   server to function together as though they are single logical server
   side deployment of HTTPS Token Binding.  Use of the headers outside
   that intended use case, however, may undermine the protections
   afforded by Token Binding.  Therefore steps MUST be taken to prevent
   unintended use, both in sending the headers and in relying on their
   value.

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   Producing and consuming the headers SHOULD be a configurable option,
   respectively, in a reverse proxy and backend server (or individual
   application in that server).  The default configuration for both
   should be to not use the headers thus requiring an "opt-in" to the
   functionality.

   Backend servers MUST only accept the headers from trusted reverse
   proxies.  And reverse proxies MUST sanitize the incoming request
   before forwarding it on by removing or overwriting any existing
   instances of the headers.  Otherwise arbitrary clients can control
   the header values as seen and used by the backend server.

   The communication between a reverse proxy and backend server needs to
   be secured against eavesdropping and modification by unintended
   parties.

   The configuration options and request sanitization are necessarily
   functionally of the respective servers.  The other requirements can
   be met in a number of ways, which will vary based on specific
   deployments.  The communication between a reverse proxy and backend
   server, for example, might be over a mutually authenticated TLS with
   the insertion and consumption headers occurring only on that
   connection.  Alternatively the network topology might dictate a
   private network such that the backend application is only able to
   accept requests from the reverse proxy and the proxy can only make
   requests to that server.  Other deployments that meet the
   requirements set forth herein are also possible.

   Employing the "Sec-" header field prefix for the headers defined
   herein denotes them as forbidden header names (see [fetch-spec]),
   which means they cannot be set or modified programmatically by script
   running in-browser.

4.  IANA Considerations

4.1.  HTTP Message Header Field Names Registration

   This document specifies the following new HTTP header fields,
   registration of which is requested in the "Permanent Message Header
   Field Names" registry defined in [RFC3864].

   o  Header Field Name: "Sec-Provided-Token-Binding-ID"
   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/change Controller: IETF
   o  Specification Document(s): [[ this specification ]]

   o  Header Field Name: "Sec-Referred-Token-Binding-ID"

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   o  Applicable protocol: HTTP
   o  Status: standard
   o  Author/change Controller: IETF
   o  Specification Document(s): [[ this specification ]]

5.  References

5.1.  Normative References

   [I-D.ietf-tokbind-https]
              Popov, A., Nystrom, M., Balfanz, D., Langley, A., and J.
              Hodges, "Token Binding over HTTP", draft-ietf-tokbind-
              https-09 (work in progress), April 2017.

   [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-08 (work in progress), April 2017.

   [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-14 (work in progress), April 2017.

   [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>.

   [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>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [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>.

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   [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>.

5.2.  Informative References

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

   [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>.

Appendix A.  Acknowledgements

   The author would like to thank the following people for their various
   contributions to the specification: Vinod Anupam, Dirk Balfanz, John
   Bradley, Nick Harper, Jeff Hodges, Subodh Iyengar, Leif Johansson,
   Yoav Nir, Andrei Popov, Eric Rescorla, Piotr Sikora, Martin Thomson,
   Hans Zandbelt and others (please let me know, if you've contributed
   and I've forgotten you).

Appendix B.  Document History

   [[ to be removed by the RFC Editor before publication as an RFC ]]

   draft-ietf-tokbind-ttrp-01

   o  Prefix the header names with "Sec-" so that they are denoted as
      forbidden header names by Fetch https://fetch.spec.whatwg.org/

   o  Removed potentially confusing sentence from Security
      Considerations per
      https://mailarchive.ietf.org/arch/msg/unbearable/
      O0IpppyyEqMrQjEkyEi8p8CeBGA

   o  Editorial fixes.

   draft-ietf-tokbind-ttrp-00

   o  Initial WG draft from draft-campbell-tokbind-ttrp.

   draft-campbell-tokbind-ttrp-01

   o  Minor editorial fixes.

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   o  Add to the Acknowledgements.

   draft-campbell-tokbind-ttrp-00

   o  Initial draft based on 'consensus to work on the problem' from the
      Seoul meeting [1][2] and reflecting the consensus approach from
      discussions at the Chicago meeting [3].

      [1] https://www.ietf.org/proceedings/97/minutes/minutes-97-
      tokbind-01.txt (minutes from Seoul)
      [2] https://www.ietf.org/proceedings/97/slides/slides-97-tokbind-
      reverse-proxies-00.pdf (slides from Seoul)
      [3] https://mailarchive.ietf.org/arch/msg/
      unbearable/_ZHI8y2Vs5WMP8VMRr7zroo_sNU (summary of discussion)

Author's Address

   Brian Campbell
   Ping Identity

   Email: brian.d.campbell@gmail.com

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