Internet DRAFT - draft-ietf-tls-exported-authenticator
draft-ietf-tls-exported-authenticator
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
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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
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Please review these documents carefully, as they describe your rights
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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>.
Sullivan Expires 5 September 2022 [Page 15]
Internet-Draft TLS Exported Authenticator March 2022
[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
Sullivan Expires 5 September 2022 [Page 16]
