TLS                                                          N. Sullivan
Internet-Draft                                           Cloudflare Inc.
Intended status: Standards Track                        October 30, 2017
Expires: May 3, 2018

                     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.

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 May 3, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  Authenticator . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  API considerations  . . . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

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.  Conventions and Terminology

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

3.  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 [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.  These values are derived using
   an exporter as described in [RFC5705] (for TLS 1.2) or [TLS13] (for
   TLS 1.3).  These values use different labels depending on the role of
   the sender:

   o  The Handshake Context is an exporter value that is derived using
      the label "EXPORTER-client authenticator handshake context" or
      "EXPORTER-server authenticator handshake context" for
      authenticators sent by the client and server respectively.

   o  The Finished MAC Key is an exporter value derived using the label
      "EXPORTER-client authenticator finished key" or "EXPORTER-server
      authenticator finished key" for authenticators sent by the client
      and server respectively.

   The context_value used for the exporter is absent (length zero) for
   all four values.  The length of the exported value 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.

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   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 [TLS13], Section 4.4.2.

   The certificate message contains an opaque string called
   certificate_request_context.  The format of
   certificate_request_context is defined by the application layer
   protocol and its value can be used to differentiate exported
   authenticators.  For example, the application may use a 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  This message is used to provide explicit proof
      that an endpoint possesses the private key corresponding to its

    struct {
       SignatureScheme algorithm;
       opaque signature<0..2^16-1>;
    } CertificateVerify;

   The algorithm field specifies the signature algorithm used (see
   Section 4.2.3 of [TLS13] for the definition of this field).  The
   signature is a digital signature using that algorithm.  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.  The signature is
   computed using the over the concatenation of:

   o  A string that consists of octet 32 (0x20) repeated 64 times

   o  The context string "Exported Authenticator" (which is not NULL-

   o  A single 0 byte which serves as the separator

   o  The value Hash(Handshake Context || Certificate) where Hash is the
      hash function for the handshake.

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   Finished  A HMAC over the value Hash(Handshake Context ||
      Certificate || CertificateVerify) where Hash is the hash function
      for the handshake, and the HMAC is computed 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 [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 if it matches.

4.  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:

   o  certificate_request_context (from 0 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 authenticator 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:

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   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

5.  IANA Considerations

   This document has no IANA actions.

6.  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.

   The signatures generated with this API cover the context string
   "Exported Authenticator" and therefore cannot be transplanted into
   other protocols.

7.  Acknowledgements

   Comments on this proposal were provided by Martin Thomson.
   Suggestions for Section 6 were provided by Karthikeyan Bhargavan.

8.  References

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8.1.  Normative References

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

   [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,

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
              July 2017.

8.2.  Informative References

   [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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