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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: nick@cloudflare.com