Internet Engineering Task Force (IETF)                       N. Sullivan
Request for Comments: 9261                               Cloudflare Inc.
Category: Standards Track                                      July 2022
ISSN: 2070-1721


                    Exported Authenticators in TLS

Abstract

  This document describes a mechanism that builds on Transport Layer
  Security (TLS) or Datagram Transport Layer Security (DTLS) and
  enables peers to provide 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 is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc9261.

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.

Table of Contents

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

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 that do not have a single certificate that
     includes all of the identities, can authenticate additional
     identities over a single connection.

  spontaneous authentication:  After a connection is established,
     endpoints can authenticate in response to events in a higher-layer
     protocol; they can also integrate 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 TLS 1.3 Section 4.6.2 of
  [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 [RFC9113] 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 described in this document are TLS 1.2 [RFC5246] and DTLS
  1.2 [RFC6347].  (These were obsoleted by TLS 1.3 [RFC8446] and DTLS
  1.3 [RFC9147].)

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, and 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.

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.

  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", that 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 that identifies the
     authenticator request and that 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 precomputing valid authenticators.  For example, the
     application may choose this value to correspond to a value used in
     an existing data structure in the software to simplify
     implementation.

  extensions:  The set of extensions allowed in the structures of
     CertificateRequest and ClientCertificateRequest is comprised of
     those defined in the "TLS ExtensionType Values" IANA registry
     containing CR in the "TLS 1.3" column (see [IANA-TLS] and
     [RFC8447]).  In addition, the set of extensions in the
     ClientCertificateRequest structure MAY include the server_name
     extension [RFC6066].

  The uniqueness requirements of the certificate_request_context apply
  across CertificateRequest and ClientCertificateRequest messages that
  are used as part of authenticator requests.  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 (Message Authentication Code) 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.

  *  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 because 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 ciphersuite (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 ciphersuites
  that do not use the TLS PRF and with TLS 1.3 ciphersuites 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
  Certificate, CertificateVerify, and Finished messages [RFC8446].
  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.

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
  Section 4.4.2 of [RFC8446].

  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;
  however, it 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" extensions, as
  described in Section 4.2.3 of [RFC8446] for (D)TLS 1.3 or in 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 Raw Public Keys as described
  in [RFC7250] are not supported in this version of the specification
  and their use in this context has not yet been analyzed.

  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:

     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
  Elliptic Curve Digital Signature Algorithm (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" extension 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 that serves as the separator, and

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

5.2.3.  Finished

  An HMAC [HMAC] over the hashed authenticator transcript 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))

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)

  *  the 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

  *  an identity, such as a set of certificate chains and associated
     extensions (OCSP [RFC6960], SCT [RFC6962] (obsoleted by
     [RFC9162]), etc.)

  *  a signer (either the private key associated with the identity or
     the 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 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 be
  implemented in the TLS library.  Implementing this in the TLS library
  lets the implementer take advantage of existing extension and
  certificate selection logic, and the implementer can 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 or not the authenticator is
  valid 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.

8.  IANA Considerations

8.1.  Update of the TLS ExtensionType Registry

  IANA has updated the entry for server_name(0) in the "TLS
  ExtensionType Values" registry [IANA-TLS] (defined in [RFC8446]) by
  replacing the value in the "TLS 1.3" column with the value "CH, EE,
  CR" and listing this document in the "Reference" column.

  IANA has also added 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 has added the following entries to the "TLS Exporter Labels"
  registry [IANA-EXPORT] (defined in [RFC5705]): "EXPORTER-client
  authenticator handshake context", "EXPORTER-server 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 listed as the reference.

8.3.  Update of the TLS HandshakeType Registry

  IANA has added the following entry to the "TLS HandshakeType"
  registry [IANA-HANDSHAKE] (defined in [RFC8446]):
  "client_certificate_request" (17) with "DTLS-OK" set to "Y" and this
  document listed as the reference.  In addition, the following appears
  in the "Comment" 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 that now has been analyzed several times.  For
  example, [SIGMAC] presents a relevant framework for analysis, and
  Appendix E.1.6 of [RFC8446] contains a comprehensive 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.

  *  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"; therefore, they cannot be transplanted into
  other protocols.

  In TLS 1.3, the client cannot 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 message.  To avoid disagreement
  between the client and server on the authentication status of
  Exported Authenticators, servers MUST verify the client Finished
  message before sending an EA or processing a received Exported
  Authenticator.

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>.

  [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>.

  [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>.

  [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
             Datagram Transport Layer Security (DTLS) Protocol Version
             1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
             <https://www.rfc-editor.org/info/rfc9147>.

10.2.  Informative References

  [IANA-EXPORT]
             IANA, "TLS Exporter Labels",
             <https://www.iana.org/assignments/tls-parameters/>.

  [IANA-HANDSHAKE]
             IANA, "TLS HandshakeType",
             <https://www.iana.org/assignments/tls-parameters/>.

  [IANA-TLS] IANA, "TLS ExtensionType Values",
             <https://www.iana.org/assignments/tls-extensiontype-
             values/>.

  [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>.

  [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>.

  [RFC9113]  Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
             DOI 10.17487/RFC9113, June 2022,
             <https://www.rfc-editor.org/info/rfc9113>.

  [RFC9162]  Laurie, B., Messeri, E., and R. Stradling, "Certificate
             Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
             December 2021, <https://www.rfc-editor.org/info/rfc9162>.

  [SIGMAC]   Krawczyk, H., "A Unilateral-to-Mutual Authentication
             Compiler for Key Exchange (with Applications to Client
             Authentication in TLS 1.3)", Proceedings of the 2016 ACM
             SIGSAC Conference on Computer and Communications Security,
             DOI 10.1145/2976749.2978325, August 2016,
             <https://eprint.iacr.org/2016/711.pdf>.

Acknowledgements

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

Author's Address

  Nick Sullivan
  Cloudflare Inc.
  Email: [email protected]

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