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Exported Authenticators in TLS
draft-ietf-tls-exported-authenticator-15

Document Type Active Internet-Draft (tls WG)
Author Nick Sullivan
Last updated 2022-05-16 (Latest revision 2022-03-04)
Replaces draft-sullivan-tls-exported-authenticator
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Details
draft-ietf-tls-exported-authenticator-15
TLS                                                          N. Sullivan
Internet-Draft                                           Cloudflare Inc.
Intended status: Standards Track                            4 March 2022
Expires: 5 September 2022

                     Exported Authenticators in TLS
                draft-ietf-tls-exported-authenticator-15

Abstract

   This document describes a mechanism that builds on Transport Layer
   Security (TLS) or Datagram Transport Layer Security (DTLS) and
   enables peers to provide a proof of ownership of an identity, such as
   an X.509 certificate.  This proof can be exported by one peer,
   transmitted out-of-band to the other peer, and verified by the
   receiving peer.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 5 September 2022.

Copyright Notice

   Copyright (c) 2022 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  Message Sequences . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Authenticator Request . . . . . . . . . . . . . . . . . . . .   4
   5.  Authenticator . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Authenticator Keys  . . . . . . . . . . . . . . . . . . .   6
     5.2.  Authenticator Construction  . . . . . . . . . . . . . . .   7
       5.2.1.  Certificate . . . . . . . . . . . . . . . . . . . . .   8
       5.2.2.  CertificateVerify . . . . . . . . . . . . . . . . . .   8
       5.2.3.  Finished  . . . . . . . . . . . . . . . . . . . . . .  10
       5.2.4.  Authenticator Creation  . . . . . . . . . . . . . . .  10
   6.  Empty Authenticator . . . . . . . . . . . . . . . . . . . . .  10
   7.  API considerations  . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  The "request" API . . . . . . . . . . . . . . . . . . . .  11
     7.2.  The "get context" API . . . . . . . . . . . . . . . . . .  11
     7.3.  The "authenticate" API  . . . . . . . . . . . . . . . . .  11
     7.4.  The "validate" API  . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Update of the TLS ExtensionType Registry  . . . . . . . .  13
     8.2.  Update of the TLS Exporter Labels Registry  . . . . . . .  13
     8.3.  Update of the TLS HandshakeType Registry  . . . . . . . .  13
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   This document provides a way to authenticate one party of a Transport
   Layer Security (TLS) or Datagram Transport Layer Security (DTLS)
   connection to its peer using authentication messages created 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 its peer.

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

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

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   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 (such as context
      from the application).

   Versions of TLS prior to TLS 1.3 used renegotiation as a way to
   enable post-handshake client authentication given an existing TLS
   connection.  The mechanism described in this document may be used to
   replace the post-handshake authentication functionality provided by
   renegotiation.  Unlike renegotiation, exported Authenticator-based
   post-handshake authentication does not require any changes at the TLS
   layer.

   Post-handshake authentication is defined in section 4.6.3 of TLS 1.3
   [RFC8446], but it has the disadvantage of requiring additional state
   to be stored as part of the TLS state machine.  Furthermore, the
   authentication boundaries of TLS 1.3 post-handshake authentication
   align with TLS record boundaries, which are often not aligned with
   the authentication boundaries of the higher-layer protocol.  For
   example, multiplexed connection protocols like HTTP/2 [RFC7540] do
   not have a notion of which TLS record a given message is a part of.

   Exported Authenticators are meant to be used as a building block for
   application protocols.  Mechanisms such as those required to
   advertise support and handle authentication errors are not handled by
   TLS (or DTLS).

   The minimum version of TLS and DTLS required to implement the
   mechanisms decribed in this document are TLS 1.2 [RFC6347] and DTLS
   1.2 [RFC5246].

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.

   This document uses terminology such as client, server, connection,
   handshake, endpoint, peer that are defined in section 1.1 of
   [RFC8446].  The term "initial connection" refers to the (D)TLS
   connection from which the exported authenticator messages are
   derived.

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3.  Message Sequences

   There are two types of messages defined in this document:
   Authenticator Requests and Authenticators.  These can be combined in
   the following three sequences:

   Client Authentication

   *  Server generates Authenticator Request

   *  Client generates Authenticator from Server's Authenticator Request

   *  Server validates Client's Authenticator

   Server Authentication

   *  Client generates Authenticator Request

   *  Server generates Authenticator from Client's Authenticator Request

   *  Client validates Server's Authenticator

   Spontaneous Server Authentication

   *  Server generates Authenticator

   *  Client validates Server's Authenticator

4.  Authenticator Request

   The authenticator request is a structured message that can be created
   by either party of a (D)TLS connection using data exported from that
   connection.  It can be transmitted to the other party of the (D)TLS
   connection at the application layer.  The application layer protocol
   used to send the authenticator request SHOULD use a secure transport
   channel with equivalent security to TLS, such as QUIC [RFC9001], as
   its underlying transport to keep the request confidential.  The
   application MAY use the existing (D)TLS connection to transport the
   authenticator.

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   An authenticator request message can be constructed by either the
   client or the server.  Server-generated authenticator requests use
   the CertificateRequest message from Section 4.3.2 of [RFC8446].
   Client-generated authenticator requests use a new message, called the
   ClientCertificateRequest, which uses the same structure as
   CertificateRequest.  (Note that the latter is not a request for a
   client certificate, but rather a certificate request generated by the
   client.)  These message structures are used even if the connection
   protocol is TLS 1.2 or DTLS 1.2.

   The CertificateRequest and ClientCertificateRequest messages are used
   to define the parameters in a request for an authenticator.  These
   are encoded as TLS handshake messages, including length and type
   fields.  They do not include any TLS record layer framing and are not
   encrypted with a handshake or application-data key.

   The structures are defined to be:

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

      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
      authenticator request and which will be echoed in the
      authenticator message.  A certificate_request_context value MUST
      be unique for each authenticator request within the scope of a
      connection (preventing replay and context confusion).  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.  For example, the
      application may choose this value to correspond to a value used in
      an existing datastructure in the software to simplify
      implementation.

   extensions:  The set of extensions allowed in the CertificateRequest
      structure and the ClientCertificateRequest structure are those
      defined in the TLS ExtensionType Values IANA registry [RFC8447]
      containing CR in the TLS 1.3 column.  In addition, the set of
      extensions in the ClientCertificateRequest structure MAY include
      the server_name [RFC6066] extension.

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   The uniqueness requirements of the certificate_request_context apply
   only to CertificateRequest and ClientCertificateRequest messages that
   are used as part of authenticator requests, but do apply across
   CertificateRequest and ClientCertificateRequest messages.  A
   certificate_request_context value used in a ClientCertificateRequest
   cannot be used in an authenticator CertificateRequest on the same
   connection, and vice versa.  There is no impact if the value of a
   certificate_request_context used in an authenticator request matches
   the value of a certificate_request_context in the handshake or in a
   post-handshake message.

5.  Authenticator

   The authenticator is a structured message that can be exported from
   either party of a (D)TLS connection.  It can be transmitted to the
   other party of the (D)TLS connection at the application layer.  The
   application layer protocol used to send the authenticator SHOULD use
   a secure transport channel with equivalent security to TLS, such as
   QUIC [RFC9001], as its underlying transport to keep the authenticator
   confidential.  The application MAY use the existing (D)TLS connection
   to transport the authenticator.

   An authenticator message can be constructed by either the client or
   the server given an established (D)TLS connection, an identity, such
   as an X.509 certificate, and a corresponding private key.  Clients
   MUST NOT send an authenticator without a preceding authenticator
   request; for servers an authenticator request is optional.  For
   authenticators that do not correspond to authenticator requests, the
   certificate_request_context is chosen by the server.

5.1.  Authenticator Keys

   Each authenticator is computed using a Handshake Context and Finished
   MAC Key derived from the (D)TLS connection.  These values are derived
   using an exporter as described in Section 4 of [RFC5705] (for (D)TLS
   1.2) or Section 7.5 of [RFC8446] (for (D)TLS 1.3).  For (D)TLS 1.3,
   the exporter_master_secret MUST be used, not the
   early_exporter_master_secret.  These values use different labels
   depending on the role of the sender:

   *  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 or server respectively.

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   *  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
      or server respectively.

   The context_value used for the exporter is empty (zero length) for
   all four values.  There is no need to include additional context
   information at this stage since the application-supplied context is
   included in the authenticator itself.  The length of the exported
   value is equal to the length of the output of the hash function
   associated with the selected cipher suite (for TLS 1.3) or the hash
   function used for the pseudorandom function (PRF) (for (D)TLS 1.2).
   Exported authenticators cannot be used with (D)TLS 1.2 cipher suites
   that do not use the TLS PRF and with TLS 1.3 cipher suites that do
   not have an associated hash function.  This hash is referred to as
   the authenticator hash.

   To avoid key synchronization attacks, Exported Authenticators MUST
   NOT be generated or accepted on (D)TLS 1.2 connections that did not
   negotiate the extended master secret extension [RFC7627].

5.2.  Authenticator Construction

   An authenticator is formed from the concatenation of TLS 1.3
   [RFC8446] Certificate, CertificateVerify, and Finished messages.
   These messages are encoded as TLS handshake messages, including
   length and type fields.  They do not include any TLS record layer
   framing and are not encrypted with a handshake or application-data
   key.

   If the peer populating the certificate_request_context field in an
   authenticator's Certificate message has already created or correctly
   validated an authenticator with the same value, then no authenticator
   should be constructed.  If there is no authenticator request, the
   extensions are chosen from those presented in the (D)TLS handshake's
   ClientHello.  Only servers can provide an authenticator without a
   corresponding request.

   ClientHello extensions are used to determine permissible extensions
   in the server's unsolicited Certificate message in order to follow
   the general model for extensions in (D)TLS in which extensions can
   only be included as part of a Certificate message if they were
   previously sent as part of a CertificateRequest message or
   ClientHello message.  This ensures that the recipient will be able to
   process such extensions.

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5.2.1.  Certificate

   The Certificate message contains the identity to be used for
   authentication, such as the end-entity certificate and any supporting
   certificates in the chain.  This structure is defined in [RFC8446],
   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
   but MUST be unique within the scope of the connection and be
   unpredictable to the peer.

   Certificates chosen in the Certificate message MUST conform to the
   requirements of a Certificate message in the negotiated version of
   (D)TLS.  In particular, the entries of certificate_list MUST be valid
   for the signature algorithms indicated by the peer in the
   "signature_algorithms" and "signature_algorithms_cert" extension, as
   described in Section 4.2.3 of [RFC8446] for (D)TLS 1.3 or from
   Sections 7.4.2 and 7.4.6 of [RFC5246] for (D)TLS 1.2.

   In addition to "signature_algorithms" and
   "signature_algorithms_cert", the "server_name" [RFC6066],
   "certificate_authorities" (Section 4.2.4. of [RFC8446]), and
   "oid_filters" (Section 4.2.5. of [RFC8446]) extensions are used to
   guide certificate selection.

   Only the X.509 certificate type defined in [RFC8446] is supported.
   Alternative certificate formats such as [RFC7250] Raw Public Keys are
   not supported in this version of the specification and their use in
   this context has not yet been analysed.

   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 (D)TLS ClientHello.  Unrecognized extensions in the
   authenticator request MUST be ignored.

5.2.2.  CertificateVerify

   This message is used to provide explicit proof that an endpoint
   possesses the private key corresponding to its identity.  The format
   of this message is taken from TLS 1.3:

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

   The algorithm field specifies the signature algorithm used (see
   Section 4.2.3 of [RFC8446] 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 present in the
   "signature_algorithms" extensino of the authenticator request.
   Otherwise, with spontaneous server authentication, the signature
   algorithm used MUST be chosen from the "signature_algorithms" sent by
   the peer in the ClientHello of the (D)TLS handshake.  If there are no
   available signature algorithms, then no authenticator should be
   constructed.

   The signature is computed using the chosen signature scheme over the
   concatenation of:

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

   *  The context string "Exported Authenticator" (which is not NUL-
      terminated)

   *  A single 0 octet which serves as the separator

   *  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 authenticator hash defined in section 4.1.  If the
   authenticator request is not present, it is omitted from this
   construction, i.e., 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

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   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 (such as the
   Handshake Context) before taking a user-visible action.

5.2.3.  Finished

   An HMAC [HMAC] over the hashed authenticator transcript, which is the
   concatenation of the Handshake Context, authenticator request (if
   present), Certificate, and CertificateVerify.  The HMAC is computed
   using the authenticator hash, using the Finished MAC Key as a key.

   Finished = HMAC(Finished MAC Key, Hash(Handshake Context ||
        authenticator request || Certificate || CertificateVerify))

5.2.4.  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

   An authenticator is valid if the CertificateVerify message is
   correctly constructed given the authenticator request (if used) and
   the Finished message matches the expected value.  When validating an
   authenticator, constant-time comparisons SHOULD be used for signature
   and MAC validation.

6.  Empty Authenticator

   If, given an authenticator request, the endpoint does not have an
   appropriate identity or does not want to return one, it constructs an
   authenticated refusal called an empty authenticator.  This is a
   Finished message sent without a Certificate or CertificateVerify.
   This message is an HMAC over the hashed authenticator transcript with
   a Certificate message containing no CertificateEntries and the
   CertificateVerify message omitted.  The HMAC is computed using the
   authenticator hash, using the Finished MAC Key as a key.  This
   message is encoded as a TLS handshake message, including length and
   type field.  It does not include TLS record layer framing and is not
   encrypted with a handshake or application-data key.

   Finished = HMAC(Finished MAC Key, Hash(Handshake Context ||
        authenticator request || Certificate))

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7.  API considerations

   The creation and validation of both authenticator requests and
   authenticators SHOULD be implemented inside the (D)TLS library even
   if it is possible to implement it at the application layer.  (D)TLS
   implementations supporting the use of exported authenticators SHOULD
   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:

   *  the connection uses a (D)TLS version of 1.1 or earlier, or

   *  the connection is (D)TLS 1.2 and the extended master secret
      extension [RFC7627] was not negotiated

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

7.1.  The "request" API

   The "request" API takes as input:

   *  certificate_request_context (from 0 to 255 octets)

   *  set of extensions to include (this MUST include
      signature_algorithms) and the contents thereof

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

7.2.  The "get context" API

   The "get context" API takes as input:

   *  authenticator or authenticator request

   It returns the certificate_request_context.

7.3.  The "authenticate" API

   The "authenticate" API takes as input:

   *  a reference to the initial connection

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   *  an identity, such as a set of certificate chains and associated
      extensions (OCSP [RFC6960], SCT [RFC6962], etc.)

   *  a signer (either the private key associated with the identity, or
      interface to perform private key operations) for each chain

   *  an authenticator request or certificate_request_context (from 0 to
      255 octets)

   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 and more easily remember which
   extensions were sent in the ClientHello.

   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.

7.4.  The "validate" API

   The "validate" API takes as input:

   *  a reference to the initial connection

   *  an optional authenticator request

   *  an authenticator

   *  a function for validating a certificate chain

   It returns a status to indicate whether the authenticator is valid or
   not after applying the function for validating the certificate chain
   to the chain contained in the authenticator.  If validation is
   successful, it also returns the identity, such as the certificate
   chain and its extensions.

   The API should return a failure if the certificate_request_context of
   the authenticator was used in a different authenticator that was
   previously validated.  Well-formed empty authenticators are returned
   as invalid.

   When validating an authenticator, constant-time comparison should be
   used.

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8.  IANA Considerations

8.1.  Update of the TLS ExtensionType Registry

   IANA is requested to update the entry for server_name(0) in the
   registry for ExtensionType (defined in [RFC8446]) by replacing the
   value in the "TLS 1.3" column with the value "CH, EE, CR" and adding
   this document in the "Reference" column.

   IANA is also requested to add the following note to the registry:

   The addition of the "CR" to the "TLS 1.3" column for the
   server_name(0) extension only marks the extension as valid in a
   ClientCertificateRequest created as part of client-generated
   authenticator requests.

8.2.  Update of the TLS Exporter Labels Registry

   IANA is requested to add the following entries to the registry for
   Exporter Labels (defined in [RFC5705]): "EXPORTER-client
   authenticator handshake context", "EXPORTER-server authenticator
   handshake context", "EXPORTER-client authenticator handshake
   context", "EXPORTER-client authenticator finished key" and "EXPORTER-
   server authenticator finished key" with "DTLS-OK" and "Recommended"
   set to "Y" and this document added to the "Reference" column.

8.3.  Update of the TLS HandshakeType Registry

   IANA is requested to add the following entry to the registry for
   HandshakeType (defined in [RFC8446]): "client_certificate_request"
   with "DTLS-OK" and "Recommended" set to "Y" and this document added
   to the "Reference" column with the following in the "Note" column:
   "Used in TLS versions prior to 1.3."

9.  Security Considerations

   The Certificate/Verify/Finished pattern intentionally looks like the
   TLS 1.3 pattern which now has been analyzed several times.  For
   example, [SIGMAC] presents a relevant framework for analysis, and
   section 10. of [RFC8446] contains a conprehensive set of references.

   Authenticators are independent and unidirectional.  There is no
   explicit state change inside TLS when an authenticator is either
   created or validated.  The application in possession of a validated
   authenticator can rely on any semantics associated with data in the
   certificate_request_context.

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

   *  There is no indication in (D)TLS 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.

   In TLS 1.3 the client can not explicitly learn from the TLS layer
   whether its Finished message was accepted.  Because the application
   traffic keys are not dependent on the client's final flight,
   receiving messages from the server does not prove that the server
   received the client's Finished.  To avoid disagreement between the
   client and server on the authentication status of EAs, servers MUST
   verify the client Finished before sending an EA or processing a
   received EA.

10.  Acknowledgements

   Comments on this proposal were provided by Martin Thomson.
   Suggestions for Section 9 were provided by Karthikeyan Bhargavan.

11.  References

11.1.  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>.

   [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>.

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   [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>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8447]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
              <https://www.rfc-editor.org/info/rfc8447>.

11.2.  Informative References

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <https://www.rfc-editor.org/info/rfc6960>.

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
              <https://www.rfc-editor.org/info/rfc6962>.

   [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>.

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   [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>.

   [RFC9001]  Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/info/rfc9001>.

   [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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