Exported Authenticators in TLS
draft-ietf-tls-exported-authenticator-07

TLS                                                          N. Sullivan
Internet-Draft                                           Cloudflare Inc.
Intended status: Standards Track                           June 05, 2018
Expires: December 7, 2018

                     Exported Authenticators in TLS
                draft-ietf-tls-exported-authenticator-07

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.

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   This Internet-Draft will expire on December 7, 2018.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  Authenticator Request . . . . . . . . . . . . . . . . . . . .   3
   4.  Authenticator . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Authenticator Keys  . . . . . . . . . . . . . . . . . . .   4
     4.2.  Authenticator Construction  . . . . . . . . . . . . . . .   5
       4.2.1.  Certificate . . . . . . . . . . . . . . . . . . . . .   5
       4.2.2.  CertificateVerify . . . . . . . . . . . . . . . . . .   6
       4.2.3.  Finished  . . . . . . . . . . . . . . . . . . . . . .   7
       4.2.4.  Authenticator Creation  . . . . . . . . . . . . . . .   7
   5.  Empty Authenticator . . . . . . . . . . . . . . . . . . . . .   8
   6.  API considerations  . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  The "request" API . . . . . . . . . . . . . . . . . . . .   8
     6.2.  The "get context" API . . . . . . . . . . . . . . . . . .   9
     6.3.  The "authenticate" API  . . . . . . . . . . . . . . . . .   9
     6.4.  The "validate" API  . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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.

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   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 [RFC7540].  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.

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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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.

   The CertificateRequest is used to define the parameters in a request
   for an authenticator.  The definition for TLS 1.3 is:

      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.  A certificate_request_context value MUST NOT be repeated
      for multiple authenticator requests generated by the same party

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      within the scope of a 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.  This
      value is unrelated to the certificate_request_context used in
      post-handshake authentication and collisions do not need to be
      avoided.

   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.

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.  Clients MUST NOT send an authenticator
   without a preceding authenticator request; 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.

4.1.  Authenticator Keys

   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

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      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.  For TLS 1.3 symmetric cipher suites, the hash
   algorithm used with HKDF is 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.

4.2.  Authenticator Construction

   An authenticator is formed from the concatenation of TLS 1.3 [TLS13]
   Certificate, CertificateVerify, and Finished messages.

   If an authenticator request is present, the extensions used to guide
   the construction of these messages are taken from the authenticator
   request.  If there is no authenticator request, the extensions are
   chosen from the TLS handshake.  Only servers can provide an
   authenticator without a corresponding request.  In such cases,
   ClientHello extensions are used to determine permissible extensions
   in the Certificate message.

4.2.1.  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, which is extracted from the
   authenticator request if present.  If no authenticator request is
   provided, the certificate_request_context can be chosen arbitrarily.

   The certificates chosen in the Certificate message MUST conform to
   the requirements of a Certificate message in the negotiated version
   of TLS.  In particular, the certificate chain MUST be valid for the a
   signature algorithms indicated by the peer in the
   "signature_algorithms" and "signature_algorithms_cert" extension, as
   described in Section 4.2.3 of [TLS13] for TLS 1.3 or the
   "signature_algorithms" extension from Sections 7.4.2 and 7.4.6 of
   [RFC5246] for TLS 1.2.

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   In addition to "signature_algorithms" and
   "signature_algorithms_cert", the "server_name" [RFC6066],
   "certificate_authorities" (Section 4.2.4. of [TLS13]), and
   "oid_filters" (Section 4.2.5. of [TLS13]) extensions are used to
   guide certificate selection.  The extensions, or others that might
   affect certificate selection, are taken from the authenticator
   request if present, or the TLS handshake if not.

   Alternative certificate formats such as [RFC7250] Raw Public Keys are
   not supported in this version of the specification.

   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 TLS handshake.

4.2.2.  CertificateVerify

   This message is used to provide explicit proof that an endpoint
   possesses the private key corresponding to its certificate.  The
   definition for TLS 1.3 is:

      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 combinations of
   ECDSA and hash 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" sent by the peer in the TLS
   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)

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   o  A single 0 byte which serves as the separator

   o  The hashed authenticator transcript

   The authenticator transcript is the hash of the concatenated
   Handshake Context, authenticator request (if present), and
   Certificate message:

   Hash(Handshake Context || authenticator request || Certificate)

   Where Hash is the hash function negotiated by TLS.  If the
   authenticator request is not present, it is omitted from this
   construction (that is, it is zero length).

   If the party that generates the exported authenticator does so with a
   different connection than the party that is validating it, then the
   Handshake Context will not match, resulting in a CertificateVerify
   message that does not validate.  This includes situations in which
   the application data is sent via TLS-terminating proxy.  Given a
   failed CertificateVerify validation, it may be helpful for the
   application to confirm that both peers share the same connection
   using a value derived from the connection secrets before taking a
   user-visible action.

4.2.3.  Finished

   A HMAC [HMAC] over the hashed authenticator transcript, which is the
   concatenated Handshake Context, authenticator request (if present),
   Certificate, and CertificateVerify:

   Hash(Handshake Context || authenticator request ||
        Certificate || CertificateVerify)

   The HMAC is computed using the same hash function using the Finished
   MAC Key as a key.

4.2.4.  Authenticator Creation

   An endpoint constructs an authenticator by serializing the
   Certificate, CertificateVerify, and Finished as TLS handshake
   messages and concatenating the octets:

   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.

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5.  Empty Authenticator

   If, given an authenticator request, the endpoint does have an
   appropriate certificate or does not want to return one, it constructs
   an authenticated refusal called an empty authenticator.  This is an
   HMAC over the hashed authenticator transcript with a Certificate
   message containing no CertificateEntries and the CertificateVerify
   message omitted:

   "Hash(Handshake Context || authenticator request || Certificate) "

   The HMAC is computed using the same hash function using the Finished
   MAC Key as a key.

6.  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.

   Notwithstanding the success conditions described below, all APIs MUST
   fail if:

   o  the connection uses a TLS version of 1.1 or earlier, or

   o  the connection is TLS 1.2 and the extended master secret [RFC7627]
      was not used

   The following sections describes APIs that are considered necessary
   to implement exported authenticators.  These are informative only.

6.1.  The "request" API

   The "request" API takes as input:

   o  certificate_request_context (from 0 to 255 bytes)

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

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

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6.2.  The "get context" API

   The "get context" API takes as input:

   o  authenticator

   It returns the certificate_request_context.

6.3.  The "authenticate" API

   The "authenticate" 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 or certificate_request_context
      (from 0 to 255 bytes)

   It returns either the exported authenticator or an empty
   authenticator as a sequence of octets.  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.

6.4.  The "validate" API

   The "validate" API takes as input:

   o  an optional authenticator request

   o  an authenticator

   It returns the certificate chain and extensions and a status to
   indicate whether the authenticator is valid or not.  If the
   authenticator was empty - that is, it did not contain a certificate -
   the certificate chain will contain no certificates.

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7.  IANA Considerations

   This document has no IANA actions.

8.  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.

9.  Acknowledgements

   Comments on this proposal were provided by Martin Thomson.
   Suggestions for Section 8 were provided by Karthikeyan Bhargavan.

10.  References

10.1.  Normative References

   [HMAC]     Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [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>.

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   [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>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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

10.2.  Informative References

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [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>.

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Author's Address

   Nick Sullivan
   Cloudflare Inc.

   Email: nick@cloudflare.com

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