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Versions: (draft-sullivan-tls-exported-authenticator) 00 01 02 03 04 05

TLS                                                          N. Sullivan
Internet-Draft                                           Cloudflare Inc.
Intended status: Standards Track                       December 13, 2017
Expires: June 16, 2018


                     Exported Authenticators in TLS
                draft-ietf-tls-exported-authenticator-05

Abstract

   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.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 16, 2018.

Copyright Notice

   Copyright (c) 2017 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  Authenticator Request . . . . . . . . . . . . . . . . . . . .   3
   4.  Authenticator . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  API considerations  . . . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

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
   supported.

   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




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   authentication.  This mechanism is intended to be used as part of a
   replacement for post-handshake authentication in applications.

2.  Conventions and Terminology

   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].

3.  Authenticator Request

   The authenticator request 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 to keep the
   request confidential.

   An authenticator request message can be constructed by either the
   client or the server.  This authenticator request uses the
   CertificateRequest message structure from Section 4.3.2 of [TLS13].
   This message does not include the TLS record layer and is therefore
   not encrypted with a handshake key.

   CertificateRequest  This message is used to define the parameters in
      a request for an authenticator.

    struct {
       opaque certificate_request_context<0..2^8-1>;
       Extension extensions<2..2^16-1>;
    } CertificateRequest;

   certificate_request_context  An opaque string which identifies the
      certificate request and which will be echoed in the authenticator
      message.  The certificate_request_context MUST be unique within
      the scope of this connection (thus preventing replay of
      authenticators).  The certificate_request_context SHOULD be chosen
      to be unpredictable to the peer (e.g., by randomly generating it)
      in order to prevent an attacker who has temporary access to the
      peer's private key from pre-computing valid authenticators.

   extensions  The extensions that are allowed in this structure include
      the extensions defined for CertificateRequest messages defined in
      Section 4.2. of [TLS13] and the server_name [RFC6066] extension,
      which is allowed for client-generated authenticator requests.





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4.  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 to keep the certificate confidential.

   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.  For clients, an authenticator request is
   required; for servers an authenticator request is optional.  The
   authenticator uses the message structures from Section 4.4 of
   [TLS13], but different parameters.  These messages do not include the
   TLS record layer and are therefore 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.

   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.




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   The certificate message contains an opaque string called
   certificate_request_context, which is extracted from the
   authenticator request if present.  If no authenticator request is
   provided, it is zero-length.

   CertificateVerify  This message is used to provide explicit proof
      that an endpoint possesses the private key corresponding to its
      certificate.

    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.  If an authenticator request is present, the
   signature algorithm MUST be chosen from one of the signature schemes
   in the authenticator request.  Otherwise, the signature algorithm
   used should be chosen from the "signature_algorithms" extension of
   the ClientHello used in the connection handshake.

   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-
      terminated)

   o  A single 0 byte which serves as the separator

   o  If the authenticator request is present, the value "Hash(Handshake
      Context || authenticator request || Certificate)"

   o  If an authenticator request is not present, the value
      "Hash(Handshake Context || Certificate)"

   where Hash is the hash function for the handshake.

   Finished  A HMAC over the value Hash(Handshake Context ||
      Certificate || CertificateVerify) if an authenticator is present,
      or Hash(Handshake Context || authenticator request ||
      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.



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   The certificates chosen in the Certificate message MUST conform to
   the requirements of a Certificate message in the version of TLS
   negotiated.  If an authenticator request is present, the signature
   algorithms used to choose the algorithm are taken from the
   "signature_algorithms" in the from the authenticator.  If there is no
   authenticator request, the signature algorithms are chosen from the
   "signature_algorithms" extension from the ClientHello used in the
   connection.  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
   "server_name" [RFC6066], "certificate_authorities" (Section 4.2.4. of
   [TLS13]), or "oid_filters" (Section 4.2.5. of [TLS13]) extensions are
   used to guide certificate selection, with the extensions provided in
   the authenticator request taking precedence over the extensions
   provided in the connection handshake.

   If an authenticator request was provided, the Certificate message
   MUST contain only extensions present in the authenticator request.
   Otherwise, the Certificate message MUST contain only extensions
   present in the ClientHello.

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

   A given authenticator can be validated by checking the validity of
   the CertificateVerify message given the authenticator request (if
   used) and recomputing the Finished message to see if it matches.

5.  API considerations

   The creation and validation of both authenticator requests and
   authenticators SHOULD be implemented inside the 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 the following APIs:

   "request", which takes as input:

   o  certificate_request_context (from 0 to 255 bytes)

   o  set of extensions to include (this MUST include
      signature_algorithms)





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   It returns an authenticator request, which is a sequence of octets
   that includes a CertificateRequest message.

   "get context", which takes as input

   o  authenticator

   It returns the certificate_request_context.

   "authenticate", which takes as input:

   o  a set of certificate chains and associated extensions (OCSP, SCT,
      etc.)

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

   o  an optional authenticator request

   It returns the exported authenticator as output.  It is RECOMMENDED
   that the logic for selecting the certificates and extensions to
   include in the exporter is implemented in the TLS library.
   Implementing this in the TLS library lets the implementer take
   advantage of existing extension and certificate selection logic.

   It is also possible to implement this API outside of the TLS library
   using TLS exporters.  This may be preferable in cases where the
   application does not have access to a TLS library with these APIs or
   when TLS is handled independently of the application layer protocol.

   "validate", which takes as input: * an optional authenticator request
   * an authenticator

   It returns the certificate chain and extensions.

6.  IANA Considerations

   This document has no IANA actions.

7.  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.






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   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.

8.  Acknowledgements

   Comments on this proposal were provided by Martin Thomson.
   Suggestions for Section 7 were provided by Karthikeyan Bhargavan.

9.  References

9.1.  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, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008, <https://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, <https://www.rfc-editor.org/info/rfc5705>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011, <https://www.rfc-
              editor.org/info/rfc6066>.







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   [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, <https://www.rfc-editor.org/info/rfc7250>.

   [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,
              <https://www.rfc-editor.org/info/rfc7627>.

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-22 (work in progress),
              November 2017.

9.2.  Informative References

   [SIGMAC]   Krawczyk, H., "A Unilateral-to-Mutual Authentication
              Compiler for Key Exchange (with Applications to Client
              Authentication in TLS 1.3)", 2016,
              <https://eprint.iacr.org/2016/711.pdf>.

Author's Address

   Nick Sullivan
   Cloudflare Inc.

   Email: nick@cloudflare.com






















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