TLS                                                          N. Sullivan
Internet-Draft                                           Cloudflare Inc.
Intended status: Standards Track                           June 20, 2017
Expires: December 22, 2017

                     Exported Authenticators in TLS


   This document describes a mechanism in Transport Layer Security (TLS)
   to provide an exportable proof of ownership of a certificate that can
   be transmitted out of band and verified by the other party.

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   This Internet-Draft will expire on December 22, 2017.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Authenticator . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  API considerations  . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   This document provides a way to authenticate one party of a Transport
   Layer Security (TLS) communication to another using a certificate
   after the session has been established.  This allows both the client
   and server to prove ownership of additional identities at any time
   after the handshake has completed.  This proof of authentication can
   be exported and transmitted out of band from one party to be
   validated by the other party.

   This mechanism provides two advantages over the authentication that
   TLS natively provides:

   multiple identities -  Endpoints that are authoritative for multiple
      identities - but do not have a single certificate that includes
      all of the identities - can authenticate with those identities
      over a single connection.

   spontaneous authentication -  Endpoints can authenticate after a
      connection is established, in response to events in a higher-layer
      protocol, as well as integrating more context.

   This document intends to replace much of the functionality of
   renegotiation in previous versions of TLS.  It has the advantages
   over renegotiation of not requiring additional on-the-wire changes
   during a connection.  For simplicity, only TLS 1.2 and later are

   Post-handshake authentication is defined in TLS 1.3, but it has the
   disadvantage of requiring additional state to be stored in the TLS
   state machine and it composes poorly with multiplexed connection
   protocols like HTTP/2.  It is also only available for client
   authentication.  This mechanism is intended to be used as part of a
   replacement for post-handshake authentication in applications.

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2.  Authenticator

   The authenticator is a structured message that can be exported from
   either party of a TLS connection.  It can be transmitted to the other
   party of the TLS connection at the application layer.  The
   application layer protocol used to send the authenticator SHOULD use
   TLS as its underlying transport.

   An authenticator message can be constructed by either the client or
   the server given an established TLS connection, a certificate, and a
   corresponding private key.  This authenticator uses the message
   structures from section 4.4. of [I-D.ietf-tls-tls13], but different
   parameters.  Also, unlike the Certificate and CertificateRequest
   messages in TLS 1.3, the messages described in this draft are not
   encrypted with a handshake key.

   Each authenticator is computed using a Handshake Context and Finished
   MAC Key derived from the TLS session.  The Handshake Context is and
   Finished MAC Key are dependent on whether the authenticator is
   created by the client or the server.

   o  The Handshake Context is an [RFC5705] (for TLS 1.2) or
      [I-D.ietf-tls-tls13] (for TLS 1.3) exporter value derived using
      the label "EXPORTER-client authenticator handshake context" or
      "EXPORTER-server authenticator handshake context", depending on
      the sender, and length 64 bytes.  The context_value is absent
      (length zero).

   o  The Finished MAC Key is an exporter value derived using the label
      "EXPORTER-server authenticator finished key" or "EXPORTER-client
      authenticator finished key", depending on the sender.  The length
      of this key is equal to the length of the output of the hash
      function selected in TLS for the pseudorandom function (PRF);
      cipher suites that do not use the TLS PRF MUST define a hash
      function that can be used for this purpose or they cannot be used.

   If the connection is TLS 1.2, the master secret MUST have been
   computed with the extended master secret [RFC7627] to avoid key
   synchronization attacks.

   Certificate  The certificate to be used for authentication and any
      supporting certificates in the chain.  This structure is defined
      in [I-D.ietf-tls-tls13] section 4.4.2.

   The certificate message contains an opaque string called
   certificate_request_context which SHOULD be unique for a given
   connection.  Its format is be defined by the application layer
   protocol and SHOULD be non-zero length.  For example, it may be a

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   sequence number used by the higher-level protocol during the
   transport of the authenticator to the other party.  Using a unique
   and unpredictable value ties the authenticator to a given context,
   allowing the application to prevent authenticators from being
   replayed or precomputed by an attacker with temporary access to a
   private key.

   CertificateVerify  A signature over the value Hash(Handshake
      Context || Certificate)

   This is described in section 4.2.3. of [I-D.ietf-tls-tls13].  The
   signature scheme MUST be a valid signature scheme for TLS 1.3.  This
   excludes all RSASSA-PKCS1-v1_5 algorithms and ECDSA algorithms that
   are not supported in TLS 1.3.  For servers, this signature scheme
   must match one of the signature and hash algorithms advertised in the
   signature_algorithms extension of the ClientHello.

   Finished  A HMAC over the value Hash(Handshake Context ||
      Certificate || CertificateVerify) using the hash function from the
      handshake and the Finished MAC Key as a key.

   The certificates used in the Certificate message MUST conform to the
   requirements of a Certificate message in the version of TLS
   negotiated.  This is described in section 4.2.3. of
   [I-D.ietf-tls-tls13] and sections 7.4.2. and 7.4.6. of [RFC5246].
   Alternative certificate formats such as [RFC7250] Raw Public Keys are
   not supported.

   The exported authenticator message is the concatenation of messages:
   Certificate || CertificateVerify || Finished

   A given exported authenticator can be validated by checking the
   validity of the CertificateVerify message and recomputing the
   Finished message to see it it matches.  If the underlying connection
   is TLS 1.3, CertificateVerify messages with an RSASSA-PKCS1-v1_5
   algorithm as its SignatureScheme MUST be rejected.

3.  API considerations

   The creation and validation of exported authenticators SHOULD be
   implemented inside TLS library even if it is possible to implement it
   at the application layer.  TLS implementations supporting the use of
   exported authenticators MUST provide application programming
   interfaces by which clients and servers may request and verify
   exported authenticator messages.

   Given an established connection, the application SHOULD be able to
   call an "authenticate" API which takes as input:

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   o  certificate_request_context (from 1 to 255 bytes)

   o  valid certificate chain for the connection and associated
      extensions (OCSP, SCT, etc.)

   o  signer (either the private key associated with the certificate, or
      interface to perform private key operation)

   o  signature scheme

   The API returns the exported authentiator as output.

   Given an established connection and an exported authenticator
   message, the application SHOULD be able to call a "validate" API that
   takes an exported authenticator as an input.  If the Finished and
   CertificateVerify messages verify correctly, the API returns the
   following as output:

   o  certificate chain and extensions

   o  certificate_request_context

   In order for the application layer to be able to choose the
   certificates and signature schemes to use when constructing an
   authenticator, a TLS server SHOULD expose an API that returns the
   content of the signature_algorithms extension of client's ClientHello

4.  Security Considerations

   The Certificate/Verify/Finished pattern intentionally looks like the
   TLS 1.3 pattern which now has been analyzed several times.  In the
   case where the client presents an authenticator to a server, [SIGMAC]
   presents a relevant framework for analysis.

   Authenticators are independent and unidirectional.  There is no
   explicit state change inside TLS when an authenticator is either
   created or validated.

   o  This property makes it difficult to formally prove that a server
      is jointly authoritative over multiple certificates, rather than
      individually authoritative over each.

   o  There is no indication in the TLS layer about which point in time
      an authenticator was computed.  Any feedback about the time of
      creation or validation of the authenticator should be tracked as
      part of the application layer semantics if required.

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5.  Acknowledgements

   Comments on this proposal were provided by Martin Thomson.
   Suggestions for the security considerations section were provided by
   Karthikeyan Bhargavan.

6.  Normative References

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-20 (work in progress),
              April 2017.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <>.

   [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,

   [SIGMAC]   Krawczyk, H., "A Unilateral-to-Mutual Authentication
              Compiler for Key Exchange (with Applications to Client
              Authentication in TLS 1.3)", 2016,

Author's Address

   Nick Sullivan
   Cloudflare Inc.


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