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Versions: 00                                                            
Internet Engineering Task Force                          J. Hoyland, Ed.
Internet-Draft                      Royal Holloway, University of London
Intended status: Standards Track                           June 25, 2018
Expires: December 27, 2018

                 Layered Exported Authenticators in TLS


   This document describes an extension that allows for Exported
   Authenticators (EAs) to authenticate each other.  The extension
   includes a reference to a previous EA.  An EA containing this
   extension constitues an attestation of the authenticity of the
   referenced EA.

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 27, 2018.

Copyright Notice

   Copyright (c) 2018 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 Language . . . . . . . . . . . . . . . . . .   3
   2.  Extension Format  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   Exported Authenticators (EAs)[EA] provide a method for authenticating
   one party of a Transport Layer Security (TLS) communication to the
   other after the session has been established.  EAs are defined for
   TLS 1.3[TLS13] and TLS 1.2 with extended master secret, RFC 7627
   [RFC7627].  Multiple EAs sent on the same channel do not prove joint
   authentication.  They prove that the sender is individually
   authoritative over each certificate, but not jointly authoritative
   over all certificates.  By including this extension a sender can
   prove joint authentication.  This extension can be included in
   CertificateRequest messages and Certificate messages.

   Joint authentication could be used, for example, to securely update
   pinned certificates.  When a client connects to a server for which it
   has a pinned certificate, the server could send the new certificate
   to be pinned, and then bind the previously pinned certificate to it.
   This proves to the client that the server is jointly authoritative
   over both certificates.  To defeat this mechanism an attacker is
   required to both compromise the key of the old certificate and
   improperly obtain a certificate from the PKI.

   Another potential use is to provide proof that a certificate has been
   accepted.  Because EAs do not have a response mechanism, the sender
   of an EA does not know the receiver's view of its authentication
   status.  By using this extension to reference EAs sent by its peer, a
   party can prove to its peer that it has accepted a particular

   By constructing a chain of referenced EAs complex joint
   authentication properties can be achieved.

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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Extension Format

   The "extension_data" field of this extension SHALL contain:

   struct {
        opaque prev_certificate_request_context<0..2^8-1>;
        opaque binding[Hash.length];
   } LayeredEA;

   where "prev_certificate_request_context" is the certificate request
   context of the EA you are referencing, and "binding" is the Finished
   message of that same EA.  The hash used is that used in the exported
   authenticator, which is the hash function used by the TLS connection.

   A party who wishes its peer to prove it is jointly authoritative over
   multiple certificates can request a sequence of certificates, each
   bound to its predecessor.  Receipt of a series of EAs binding these
   certificates into a chain proves the sender is jointly authoritative
   over all those certificates.

   A party who receives a CertificateRequest with this extension MUST
   verify that it previously received or sent an EA with the appropriate
   certificate request context and Finished message.  If so then the
   party MAY respond with a Certificate fulfilling the request, or it
   MAY choose to not fulfil the request.

   A party who receives a request from its peer for which it does not
   recognise the referenced certificate or does not want to link to the
   referenced certificate for some other reason, but still wishes to
   respond with an EA MAY send an EA omitting the extension, or it MAY
   choose to not fulfil the request.  If the peer receives an EA with
   the extension omitted it proves the sender is authoritative over the
   certificate in the EA, but makes no claims about the previous EA
   referenced in the request.

   For spontaneous certificates The server MUST include a unique (within
   the context of the connection) certificate_request_context for any EA
   it may wish to bind to.  To be able to verify bindings both parties
   must keep a list of accepted EAs they are willing to bind to,
   including certificate_request_contexts and Finished messages.  A
   client that receives a spontaneous EA with a

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   certificate_request_context that it has already seen and for which it
   is willing to receive a binding MUST ignore it.

3.  Acknowledgements

4.  IANA Considerations

   This document requests IANA to update the TLS ExtensionsType
   registry, defined in [TLS13], to include the
   layered_exported_authenticator extension.

5.  Security Considerations

   For the authentication guarantees to apply, requests, and thus
   responses, must unambiguously identify previous EAs.  Because EAs do
   not place a restriction on both parties to a connection using the
   same certificate_request_context, the certificate_request_context is
   not sufficient to unambiguously identify previous EAs.  Because EAs
   are unidirectional, and the Finished message is dependent on the
   labels used to enforce this, the Finished message is sufficient to
   identify previous EAS unambiguously.  In the case of spontaneous EAs
   a malicious server or an attacker who had compromised the TLS channel
   could send two identical spontaneous EAs.  To militate against this a
   client receiving such an EA MUST check that it has not already
   accepted an EA with the same certificate_request_context that it is
   willing to bind to.  If it previously accepted such a certificate but
   did not add it to the list of certificates which it was willing to
   bind to, adding it to the list is still secure.  The
   certificate_request_context is included in the request to ease
   identification of the previous EA, but is not sufficient alone.

   Both parties can be sure the Finished messages that are used to
   reference previous EAs are unique.  For requested EAs the inclusion
   of the certificate_request_context, which is generated by the
   requestor, guarantees this is the case.  For spontaneous certificates
   the client may only accept EAs after checking it does not have any
   EAs it is willing to bind to with the same

   The Finished messages amount to channel bindings as defined in
   RFC5056 [RFC5056], and thus publication of them should not weaken the
   security of either the referenced EA or the TLS channel.

   This extension only authenticates prior EAs.  Thus, an attacker who
   is able to compromise a TLS connection could append authentications
   to the connection.  Any attempt to bind to these certificates by an
   honest agent would not be accepted by the peer.

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6.  References

6.1.  Normative References

   [EA]       Sullivan, N., "Exported Authenticators in TLS", draft-
              ietf-tls-exported-authenticator-07 (work in progress),
              June 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

6.2.  Informative References

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,

Author's Address

   Jonathan Hoyland (editor)
   Royal Holloway, University of London

   Email: jonathan.hoyland@gmail.com

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