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<front> <front>
<title abbrev="TLS Exported Authenticator">Exported Authenticators in TLS</t itle> <title abbrev="TLS Exported Authenticator">Exported Authenticators in TLS</t itle>
<seriesInfo name="RFC" value="9261"/>
<author initials="N." surname="Sullivan" fullname="Nick Sullivan"> <author initials="N." surname="Sullivan" fullname="Nick Sullivan">
<organization>Cloudflare Inc.</organization> <organization>Cloudflare Inc.</organization>
<address> <address>
<email>nick@cloudflare.com</email> <email>nick@cloudflare.com</email>
</address> </address>
</author> </author>
<date year="2022" month="July"/>
<date year="2022" month="March" day="04"/> <area>sec</area>
<workgroup>tls</workgroup>
<area>Security</area>
<workgroup>TLS</workgroup>
<keyword>Internet-Draft</keyword>
<abstract> <abstract>
<t>This document describes a mechanism that builds on Transport Layer Secu
<t>This document describes a mechanism that builds on Transport Layer Security ( rity (TLS) or
TLS) or
Datagram Transport Layer Security (DTLS) and enables peers to Datagram Transport Layer Security (DTLS) and enables peers to
provide a proof of ownership of an identity, such as an X.509 certificate. This provide proof of ownership of an identity, such as an X.509 certificate. This p
proof can roof can
be exported by one peer, transmitted out-of-band to the other peer, and verified be exported by one peer, transmitted out of band to the other peer, and verified
by the by the
receiving peer.</t> receiving peer.</t>
</abstract> </abstract>
</front> </front>
<middle> <middle>
<section anchor="introduction" numbered="true" toc="default">
<section anchor="introduction" title="Introduction"> <name>Introduction</name>
<t>This document provides a way to authenticate one party of a Transport L
<t>This document provides a way to authenticate one party of a Transport Layer ayer
Security (TLS) or Datagram Transport Layer Security (DTLS) connection to its pee r Security (TLS) or Datagram Transport Layer Security (DTLS) connection to its pee r
using authentication messages created after the session using authentication messages created after the session
has been established. This allows both the client and server to prove ownership 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 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 proof of authentication can be exported and transmitted out of band from one
party to be validated by its peer.</t> party to be validated by its peer.</t>
<t>This mechanism provides two advantages over the authentication that TLS
<t>This mechanism provides two advantages over the authentication that TLS and D and DTLS natively
TLS natively
provide:</t> provide:</t>
<dl newline="false" spacing="normal">
<dt>multiple identities:</dt>
<dd>
Endpoints that are authoritative for multiple identities, but that do not have
a
single certificate that includes all of the identities, can authenticate additio
nal
identities over a single connection.</dd>
<t><list style="hanging"> <dt>spontaneous authentication:</dt>
<t hangText="multiple identities -"> <dd>
Endpoints that are authoritative for multiple identities - but do not have a After a connection is established, endpoints can authenticate in response to
single certificate that includes all of the identities - can authenticate additi events in a higher-layer protocol; they can also integrate more context (such as
onal context from the application).</dd>
identities over a single connection.</t> </dl>
<t hangText="spontaneous authentication -"> <t>Versions of TLS prior to TLS 1.3 used renegotiation as a way to enable
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).</t>
</list></t>
<t>Versions of TLS prior to TLS 1.3 used renegotiation as a way to enable
post-handshake client authentication given an existing TLS connection. post-handshake client authentication given an existing TLS connection.
The mechanism described in this document may be used to replace the The mechanism described in this document may be used to replace the
post-handshake authentication functionality provided by renegotiation. post-handshake authentication functionality provided by renegotiation.
Unlike renegotiation, exported Authenticator-based post-handshake Unlike renegotiation, Exported Authenticator-based post-handshake
authentication does not require any changes at the TLS layer.</t> authentication does not require any changes at the TLS layer.</t>
<t>Post-handshake authentication is defined in section 4.6.3 of TLS 1.3 <xref ta rget="RFC8446"/>, but it has the <t>Post-handshake authentication is defined in TLS 1.3 <xref target="RFC84 46" sectionFormat="of" section="4.6.2"/>, but it has the
disadvantage of requiring additional state to be stored as part of the TLS disadvantage of requiring additional state to be stored as part of the TLS
state machine. Furthermore, the authentication boundaries of TLS state machine. Furthermore, the authentication boundaries of TLS 1.3 post-hands
1.3 post-handshake authentication align with TLS record boundaries, hake authentication align with TLS record boundaries,
which are often not aligned with the authentication boundaries of the which are often not aligned with the authentication boundaries of the
higher-layer protocol. For example, multiplexed connection protocols higher-layer protocol. For example, multiplexed connection protocols
like HTTP/2 <xref target="RFC7540"/> do not have a notion of which TLS record like HTTP/2 <xref target="RFC9113" format="default"/> do not have a notion of wh ich TLS record
a given message is a part of.</t> a given message is a part of.</t>
<t>Exported Authenticators are meant to be used as a building block for
<t>Exported Authenticators are meant to be used as a building block for
application protocols. Mechanisms such as those required to advertise application protocols. Mechanisms such as those required to advertise
support and handle authentication errors are not handled by TLS (or DTLS).</t> support and handle authentication errors are not handled by TLS (or DTLS).</t>
<t>The minimum version of TLS and DTLS required to implement the mechanism
s
described in this document are TLS 1.2 <xref target="RFC5246" format="default"/>
and DTLS 1.2 <xref target="RFC6347" format="default"/>.</t>
</section>
<section anchor="conventions-and-terminology" numbered="true" toc="default">
<name>Conventions and Terminology</name>
<t>
The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQU
IRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>
RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to
be interpreted as
described in BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
when, and only when, they appear in all capitals, as shown here.
</t>
<t>This document uses terminology such as client, server, connection,
handshake, endpoint, and peer that are defined in
<xref target="RFC8446" sectionFormat="of" section="1.1"/>. The term "initial co
nnection" refers to the (D)TLS
connection from which the Exported Authenticator messages are derived.</t>
</section>
<section anchor="message-sequences" numbered="true" toc="default">
<name>Message Sequences</name>
<t>There are two types of messages defined in this document: authenticator
requests and authenticators. These can be combined in the following three sequ
ences:</t>
<t>Client Authentication</t>
<t>The minimum version of TLS and DTLS required to implement the mechanisms <ul spacing="normal">
decribed in this document are TLS 1.2 <xref target="RFC6347"/> and DTLS 1.2 <xre <li>Server generates authenticator request</li>
f target="RFC5246"/>.</t> <li>Client generates Authenticator from Server's authenticator request</
li>
</section> <li>Server validates Client's authenticator</li>
<section anchor="conventions-and-terminology" title="Conventions and Terminology </ul>
"> <t>Server Authentication</t>
<ul spacing="normal">
<t>The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, <li>Client generates authenticator request</li>
“SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and <li>Server generates authenticator from Client's authenticator request</
“OPTIONAL” in this document are to be interpreted as described in BCP li>
14 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only when, they a <li>Client validates Server's authenticator</li>
ppear in all </ul>
capitals, as shown here.</t> <t>Spontaneous Server Authentication</t>
<ul spacing="normal">
<t>This document uses terminology such as client, server, connection, <li>Server generates authenticator</li>
handshake, endpoint, peer that are defined in section 1.1 of <li>Client validates Server's authenticator</li>
<xref target="RFC8446"/>. The term “initial connection” refers to the (D)TLS </ul>
connection from which the exported authenticator messages are derived.</t> </section>
<section anchor="authenticator-request" numbered="true" toc="default">
</section> <name>Authenticator Request</name>
<section anchor="message-sequences" title="Message Sequences"> <t>The authenticator request is a structured message that can be created b
y either
<t>There are two types of messages defined in this document: Authenticator Reque
sts and Authenticators. These can be combined in the following three sequences:
</t>
<t>Client Authentication</t>
<t><list style="symbols">
<t>Server generates Authenticator Request</t>
<t>Client generates Authenticator from Server’s Authenticator Request</t>
<t>Server validates Client’s Authenticator</t>
</list></t>
<t>Server Authentication</t>
<t><list style="symbols">
<t>Client generates Authenticator Request</t>
<t>Server generates Authenticator from Client’s Authenticator Request</t>
<t>Client validates Server’s Authenticator</t>
</list></t>
<t>Spontaneous Server Authentication</t>
<t><list style="symbols">
<t>Server generates Authenticator</t>
<t>Client validates Server’s Authenticator</t>
</list></t>
</section>
<section anchor="authenticator-request" title="Authenticator Request">
<t>The authenticator request is a structured message that can be created by eith
er
party of a (D)TLS connection using data exported from that connection. It can 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 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 layer. The application-layer protocol used to send the authenticator request
SHOULD use a secure transport channel with equivalent security to TLS, such as Q <bcp14>SHOULD</bcp14> use a secure transport channel with equivalent security to
UIC <xref target="RFC9001"/>, as its TLS, such as QUIC <xref target="RFC9001" format="default"/>, as its
underlying transport to keep the request confidential. The underlying transport to keep the request confidential. The
application MAY use the existing (D)TLS connection to transport the authenticato application <bcp14>MAY</bcp14> use the existing (D)TLS connection to transport t
r.</t> he authenticator.</t>
<t>An authenticator request message can be constructed by either the clien
<t>An authenticator request message can be constructed by either the client or t t or the
he
server. Server-generated authenticator requests use the CertificateRequest server. Server-generated authenticator requests use the CertificateRequest
message from Section 4.3.2 of <xref target="RFC8446"/>. Client-generated message from <xref target="RFC8446" sectionFormat="of" section="4.3.2"/>. Clien
authenticator requests use a new message, called the ClientCertificateRequest, t-generated
which uses the same structure as CertificateRequest. (Note that the latter 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 is not a request for a client certificate, but rather a certificate request
generated by the client.) These message generated by the client.) These message
structures are used even if the connection protocol is TLS 1.2 or DTLS 1.2.</t> structures are used even if the connection protocol is TLS 1.2 or DTLS 1.2.</t>
<t>The CertificateRequest and ClientCertificateRequest messages are used t
<t>The CertificateRequest and ClientCertificateRequest messages are used to defi o define the
ne the
parameters in a request for an authenticator. These are encoded as TLS handshak e messages, including parameters in a request for an authenticator. These are encoded as TLS handshak e messages, including
length and type fields. They do not include any TLS record layer framing and are length and type fields. They do not include any TLS record-layer framing and are
not encrypted with a handshake or application-data key.</t> not encrypted with a handshake or application-data key.</t>
<t>The structures are defined to be:</t>
<t>The structures are defined to be:</t> <artwork name="" type="" align="left" alt=""><![CDATA[
<figure><artwork><![CDATA[
struct { struct {
opaque certificate_request_context<0..2^8-1>; opaque certificate_request_context<0..2^8-1>;
Extension extensions<2..2^16-1>; Extension extensions<2..2^16-1>;
} ClientCertificateRequest; } ClientCertificateRequest;
struct { struct {
opaque certificate_request_context<0..2^8-1>; opaque certificate_request_context<0..2^8-1>;
Extension extensions<2..2^16-1>; Extension extensions<2..2^16-1>;
} CertificateRequest; } CertificateRequest;
]]></artwork></figure> ]]></artwork>
<dl newline="false" spacing="normal">
<t><list style="hanging"> <dt>certificate_request_context:</dt>
<t hangText="certificate_request_context:"> <dd>
An opaque string which identifies the authenticator request and which will An opaque string that identifies the authenticator request and that will
be echoed in the authenticator message. A certificate_request_context value MUS be echoed in the authenticator message. A certificate_request_context value <bc
T be unique for p14>MUST</bcp14> be unique for
each authenticator request within the scope of a connection each authenticator request within the scope of a connection
(preventing replay and context confusion). The (preventing replay and context confusion). The
certificate_request_context SHOULD be chosen to be unpredictable certificate_request_context <bcp14>SHOULD</bcp14> be chosen to be unpredictable
to the peer (e.g., by randomly generating it) in order to prevent 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 an attacker who has temporary access to the peer's private key
from pre-computing valid authenticators. For example, the application from precomputing valid authenticators. For example, the application
may choose this value to correspond to a value used in an existing may choose this value to correspond to a value used in an existing
datastructure in the software to simplify implementation.</t> data structure in the software to simplify implementation.</dd>
<t hangText="extensions:"> <dt>extensions:</dt>
The set of extensions allowed in the CertificateRequest <dd>
structure and the ClientCertificateRequest structure are those The set of extensions allowed in the structures of CertificateRequest
defined in the TLS ExtensionType Values IANA registry <xref target="RFC8447"/> and ClientCertificateRequest is comprised of those
containing CR in the TLS 1.3 column. In addition, the set of defined in the "TLS ExtensionType Values" IANA registry containing CR in the "TL
extensions in the ClientCertificateRequest structure MAY S 1.3" column (see <xref target="IANA-TLS" format='default' /> and <xref target=
include the server_name <xref target="RFC6066"/> extension.</t> "RFC8447" format="default"/>). In addition, the set of
</list></t> extensions in the ClientCertificateRequest structure <bcp14>MAY</bcp14>
include the server_name extension <xref target="RFC6066" format="default"/>.</dd
>
</dl>
<t>The uniqueness requirements of the certificate_request_context apply <t>The uniqueness requirements of the certificate_request_context apply
only to CertificateRequest and ClientCertificateRequest messages that are across CertificateRequest and ClientCertificateRequest messages that
used as part of authenticator requests, but do apply across CertificateRequest are used as part of authenticator requests. A certificate_request_context value
and ClientCertificateRequest messages. A certificate_request_context value used used
in a ClientCertificateRequest cannot be used in an authenticator in a ClientCertificateRequest cannot be used in an authenticator
CertificateRequest on the same connection, and vice versa. There is no impact CertificateRequest on the same connection, and vice versa. There is no impact
if the value if the value
of a certificate_request_context used in an authenticator of a certificate_request_context used in an authenticator
request matches the value of a certificate_request_context in the handshake or request matches the value of a certificate_request_context in the handshake or
in a post-handshake message.</t> in a post-handshake message.</t>
</section>
</section> <section anchor="authenticator" numbered="true" toc="default">
<section anchor="authenticator" title="Authenticator"> <name>Authenticator</name>
<t>The authenticator is a structured message that can be exported from eit
<t>The authenticator is a structured message that can be exported from either her
party of a (D)TLS connection. It can be transmitted to the other party of 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 the (D)TLS connection at the application layer. The application-layer protocol
used to send the authenticator used to send the authenticator
SHOULD use a secure transport channel with equivalent security to TLS, such as Q <bcp14>SHOULD</bcp14> use a secure transport channel with equivalent security to
UIC <xref target="RFC9001"/>, as its TLS, such as QUIC <xref target="RFC9001" format="default"/>, as its
underlying transport to keep the authenticator confidential. underlying transport to keep the authenticator confidential.
The application MAY use the existing (D)TLS connection to transport the authenti The application <bcp14>MAY</bcp14> use the existing (D)TLS connection to transpo
cator.</t> rt the authenticator.</t>
<t>An authenticator message can be constructed by either the client or the
<t>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 cer
server given an established (D)TLS connection, an identity, such as an X.509 cer tificate;
tificate, and a corresponding private key. Clients <bcp14>MUST NOT</bcp14> send an authen
and a corresponding private key. Clients MUST NOT send an authenticator ticator
without a preceding authenticator request; for servers an without a preceding authenticator request; for servers, an
authenticator request is optional. For authenticators that do not correspond authenticator request is optional. For authenticators that do not correspond
to authenticator requests, the certificate_request_context is chosen by to authenticator requests, the certificate_request_context is chosen by
the server.</t> the server.</t>
<section anchor="authenticator-keys" numbered="true" toc="default">
<name>Authenticator Keys</name>
<section anchor="authenticator-keys" title="Authenticator Keys"> <t>Each authenticator is computed using a Handshake Context and Finished
MAC (Message Authentication Code) Key
<t>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 derived from the (D)TLS connection. These values are derived using an exporter as
described in Section 4 of <xref target="RFC5705"/> (for (D)TLS 1.2) or Section 7 described in <xref target="RFC5705" sectionFormat="of" section="4"/> (for (D)TLS
.5 of <xref target="RFC8446"/> (for 1.2) or <xref target="RFC8446" sectionFormat="of" section="7.5"/> (for
(D)TLS 1.3). For (D)TLS 1.3, the exporter_master_secret MUST be used, not the (D)TLS 1.3). For (D)TLS 1.3, the exporter_master_secret <bcp14>MUST</bcp14> be
used, not the
early_exporter_master_secret. These values use different labels depending on th e role of the early_exporter_master_secret. These values use different labels depending on th e role of the
sender:</t> sender:</t>
<ul spacing="normal">
<t><list style="symbols"> <li>The Handshake Context is an exporter value that is derived using t
<t>The Handshake Context is an exporter value that is derived using the label he label
“EXPORTER-client authenticator handshake context” or “EXPORTER-server "EXPORTER-client authenticator handshake context" or "EXPORTER-server
authenticator handshake context” for authenticators sent by the client or authenticator handshake context" for authenticators sent by the client or
server respectively.</t> server, respectively.</li>
<t>The Finished MAC Key is an exporter value derived using the label <li>The Finished MAC Key is an exporter value derived using the label
“EXPORTER-client authenticator finished key” or “EXPORTER-server authenticator "EXPORTER-client authenticator finished key" or "EXPORTER-server authenticator
finished key” for authenticators sent by the client or server respectively.</t> finished key" for authenticators sent by the client or server, respectively.</li
</list></t> >
</ul>
<t>The context_value used for the exporter is empty (zero length) for all four <t>The context_value used for the exporter is empty (zero length) for al
l four
values. There is no need to include additional context values. There is no need to include additional context
information at this stage since the application-supplied context information at this stage because the application-supplied context
is included in the authenticator itself. The length of the exported 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 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 the selected ciphersuite (for TLS 1.3) or the hash function used
for the pseudorandom function (PRF) (for (D)TLS 1.2). Exported authenticators c for the pseudorandom function (PRF) (for (D)TLS 1.2). Exported Authenticators c
annot be annot be
used with (D)TLS 1.2 cipher suites that do not use the TLS PRF and with TLS 1.3 used with (D)TLS 1.2 ciphersuites that do not use the TLS PRF and with TLS 1.3 c
cipher suites that do not have an associated iphersuites that do not have an associated
hash function. This hash is referred to as the authenticator hash.</t> hash function. This hash is referred to as the "authenticator hash".</t>
<t>To avoid key synchronization attacks, Exported Authenticators MUST NOT be gen erated or <t>To avoid key synchronization attacks, Exported Authenticators <bcp14>MUST NOT </bcp14> be generated or
accepted on (D)TLS 1.2 connections that did not negotiate accepted on (D)TLS 1.2 connections that did not negotiate
the extended master secret extension <xref target="RFC7627"/>.</t> the extended master secret extension <xref target="RFC7627" format="default"/>.<
/t>
</section> </section>
<section anchor="authenticator-construction" title="Authenticator Construction"> <section anchor="authenticator-construction" numbered="true" toc="default"
>
<t>An authenticator is formed from the concatenation of TLS 1.3 <xref target="RF <name>Authenticator Construction</name>
C8446"/> <t>An authenticator is formed from the concatenation of TLS 1.3
Certificate, CertificateVerify, and Finished messages. These messages are Certificate, CertificateVerify, and Finished messages <xref target="RFC8446" for
mat="default"/>. These messages are
encoded as TLS handshake messages, including length and type fields. 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 ha They do not include any TLS record-layer framing and are not encrypted with a ha
ndshake or application-data key.</t> ndshake or application-data key.</t>
<t>If the peer populating the certificate_request_context field in an au
<t>If the peer populating the certificate_request_context field in an authentica thenticator's Certificate message has already created or
tor’s Certificate message has already created or
correctly validated an authenticator with the same value, then no correctly validated an authenticator with the same value, then no
authenticator should be constructed. If there is no authenticator request, authenticator should be constructed. If there is no authenticator request,
the extensions are chosen from those presented in the (D)TLS handshake’s ClientH ello. the extensions are chosen from those presented in the (D)TLS handshake's ClientH ello.
Only servers can provide an authenticator without a corresponding request.</t> Only servers can provide an authenticator without a corresponding request.</t>
<t>ClientHello extensions are used to determine permissible extensions <t>ClientHello extensions are used to determine permissible extensions
in the server’s unsolicited Certificate message in order to follow the general m in the server's unsolicited Certificate message in order to follow the general m
odel for odel for
extensions in (D)TLS in which extensions can only be included extensions in (D)TLS in which extensions can only be included
as part of a Certificate message if they were previously sent as 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 part of a CertificateRequest message or ClientHello message. This ensures that the recipient
will be able to process such extensions.</t> will be able to process such extensions.</t>
<section anchor="certificate" numbered="true" toc="default">
<section anchor="certificate" title="Certificate"> <name>Certificate</name>
<t>The Certificate message contains the identity to be used for authen
<t>The Certificate message contains the identity to be used for authentication, tication, such as the
such as the
end-entity certificate and any supporting certificates in the chain. This struct ure is end-entity certificate and any supporting certificates in the chain. This struct ure is
defined in <xref target="RFC8446"/>, Section 4.4.2.</t> defined in <xref target="RFC8446" sectionFormat="of" section="4.4.2"/>.</t>
<t>The Certificate message contains an opaque string called
<t>The Certificate message contains an opaque string called "certificate_request_context", which is extracted from the authenticator request
certificate_request_context, which is extracted from the authenticator request i , if
f
present. If no authenticator request is provided, the certificate_request_conte xt present. If no authenticator request is provided, the certificate_request_conte xt
can be chosen arbitrarily but MUST be unique within the scope of the connection can be chosen arbitrarily; however, it <bcp14>MUST</bcp14> be unique within the scope of the connection
and be unpredictable to the peer.</t> and be unpredictable to the peer.</t>
<t>Certificates chosen in the Certificate message <bcp14>MUST</bcp14>
<t>Certificates chosen in the Certificate message MUST conform to the conform to the
requirements of a Certificate message in the negotiated version of (D)TLS. In 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 algo particular, the entries of certificate_list <bcp14>MUST</bcp14> be valid for the
rithms signature algorithms
indicated by the peer in the “signature_algorithms” and “signature_algorithms_ce indicated by the peer in the "signature_algorithms" and "signature_algorithms_ce
rt” rt"
extension, as described in Section 4.2.3 of <xref target="RFC8446"/> for (D)TLS extensions, as described in <xref target="RFC8446" sectionFormat="of" section="4
1.3 or .2.3"/> for (D)TLS 1.3 or
from Sections 7.4.2 and 7.4.6 of <xref target="RFC5246"/> for (D)TLS 1.2.</t> in Sections <xref target="RFC5246" section="7.4.2" sectionFormat="bare"/> and <x
ref target="RFC5246" section="7.4.6" sectionFormat="bare"/> of <xref target="RFC
<t>In addition to “signature_algorithms” and “signature_algorithms_cert”, 5246"/> for (D)TLS 1.2.</t>
the “server_name” <xref target="RFC6066"/>, “certificate_authorities” <t>In addition to "signature_algorithms" and "signature_algorithms_cer
(Section 4.2.4. of <xref target="RFC8446"/>), and “oid_filters” t",
(Section 4.2.5. of <xref target="RFC8446"/>) extensions are used to guide certif the "server_name" <xref target="RFC6066" format="default"/>, "certificate_author
icate ities"
(<xref target="RFC8446" sectionFormat="of" section="4.2.4"/>), and "oid_filters"
(<xref target="RFC8446" sectionFormat="of" section="4.2.5"/>) extensions are use
d to guide certificate
selection.</t> selection.</t>
<t>Only the X.509 certificate type defined in <xref target="RFC8446" f
<t>Only the X.509 certificate type defined in <xref target="RFC8446"/> is suppor ormat="default"/> is supported.
ted. Alternative certificate formats such as Raw Public Keys as described in <xref ta
Alternative certificate formats such as <xref target="RFC7250"/> Raw Public Keys rget="RFC7250" format="default"/> are
are
not supported in this version of the specification and their use in this context not supported in this version of the specification and their use in this context
has not yet been analysed.</t> has not yet been analyzed.</t>
<t>If an authenticator request was provided, the Certificate message <
<t>If an authenticator request was provided, the Certificate message MUST contai bcp14>MUST</bcp14> contain
n
only extensions present in the authenticator request. Otherwise, the only extensions present in the authenticator request. Otherwise, the
Certificate message MUST contain only extensions present in the (D)TLS ClientHel Certificate message <bcp14>MUST</bcp14> contain only extensions present in the (
lo. D)TLS ClientHello.
Unrecognized extensions in the authenticator request MUST be ignored.</t> Unrecognized extensions in the authenticator request <bcp14>MUST</bcp14> be igno
red.</t>
</section> </section>
<section anchor="certificateverify" title="CertificateVerify"> <section anchor="certificateverify" numbered="true" toc="default">
<name>CertificateVerify</name>
<t>This message is used to provide explicit proof that an endpoint possesses the <t>This message is used to provide explicit proof that an endpoint pos
sesses the
private key corresponding to its identity. The format of this message is taken from TLS 1.3:</t> private key corresponding to its identity. The format of this message is taken from TLS 1.3:</t>
<artwork name="" type="" align="left" alt=""><![CDATA[
<figure><artwork><![CDATA[
struct { struct {
SignatureScheme algorithm; SignatureScheme algorithm;
opaque signature<0..2^16-1>; opaque signature<0..2^16-1>;
} CertificateVerify; } CertificateVerify;
]]></artwork></figure> ]]></artwork>
<t>The algorithm field specifies the signature algorithm used (see <xr
<t>The algorithm field specifies the signature algorithm used (see Section 4.2.3 ef target="RFC8446" sectionFormat="of" section="4.2.3"/>
of <xref target="RFC8446"/>
for the definition of this field). The signature is a digital signature for the definition of this field). The signature is a digital signature
using that algorithm.</t> using that algorithm.</t>
<t>The signature scheme <bcp14>MUST</bcp14> be a valid signature schem
<t>The signature scheme MUST be a valid signature scheme for TLS 1.3. This e for TLS 1.3. This
excludes all RSASSA-PKCS1-v1_5 algorithms and combinations of ECDSA and hash excludes all RSASSA-PKCS1-v1_5 algorithms and combinations of Elliptic Curve Dig
ital Signature Algorithm (ECDSA) and hash
algorithms that are not supported in TLS 1.3.</t> algorithms that are not supported in TLS 1.3.</t>
<t>If an authenticator request is present, the signature algorithm <bc
<t>If an authenticator request is present, the signature algorithm MUST be chose p14>MUST</bcp14> be chosen
n from one of the signature schemes present in the "signature_algorithms"
from one of the signature schemes present in the “signature_algorithms” extension of the authenticator request.
extensino of the authenticator request. Otherwise, with spontaneous server authentication, the signature algorithm used
Otherwise, with spontaneous server authentication, the signature algorithm used <bcp14>MUST</bcp14> be chosen
MUST be chosen from the "signature_algorithms" sent by the peer in the ClientHello of the (D)TL
from the “signature_algorithms” sent by the peer in the ClientHello of the (D)TL S
S
handshake. If there are no available signature algorithms, then no handshake. If there are no available signature algorithms, then no
authenticator should be constructed.</t> authenticator should be constructed.</t>
<t>The signature is computed using the chosen signature scheme over the concaten ation of:</t> <t>The signature is computed using the chosen signature scheme over the concaten ation of:</t>
<t><list style="symbols"> <ul spacing="normal">
<t>A string that consists of octet 32 (0x20) repeated 64 times</t> <li>a string that consists of octet 32 (0x20) repeated 64 times,</li
<t>The context string “Exported Authenticator” (which is not NUL-terminated)</ >
t> <li>the context string "Exported Authenticator" (which is not NUL-te
<t>A single 0 octet which serves as the separator</t> rminated),</li>
<t>The hashed authenticator transcript</t> <li>a single 0 octet that serves as the separator, and</li>
</list></t> <li>the hashed authenticator transcript.</li>
</ul>
<t>The authenticator transcript is the hash of the concatenated Handshake Contex <t>The authenticator transcript is the hash of the concatenated Handsh
t, ake Context,
authenticator request (if present), and Certificate message:</t> authenticator request (if present), and Certificate message:</t>
<artwork name="" type="" align="left" alt=""><![CDATA[
<figure><artwork><![CDATA[
Hash(Handshake Context || authenticator request || Certificate) Hash(Handshake Context || authenticator request || Certificate)
]]></artwork></figure> ]]></artwork>
<t>Where Hash is the authenticator hash defined in section 4.1. If the authenti cator request <t>Where Hash is the authenticator hash defined in <xref target="authe nticator-keys"/>. If the authenticator request
is not present, it is omitted from this construction, i.e., it is zero-length.</ t> is not present, it is omitted from this construction, i.e., it is zero-length.</ t>
<t>If the party that generates the authenticator does so with a differ
<t>If the party that generates the exported authenticator does so with a differe ent
nt
connection than the party that is validating it, then the Handshake Context will connection than the party that is validating it, then the Handshake Context will
not match, resulting in a CertificateVerify message that does not validate. not match, resulting in a CertificateVerify message that does not validate.
This includes situations in which the application data is sent via TLS-terminati ng This includes situations in which the application data is sent via TLS-terminati ng
proxy. Given a failed CertificateVerify validation, it may be helpful for proxy. Given a failed CertificateVerify validation, it may be helpful for
the application to confirm that both peers share the same connection the application to confirm that both peers share the same connection
using a value derived from the connection secrets (such as the Handshake Context ) using a value derived from the connection secrets (such as the Handshake Context )
before taking a user-visible action.</t> before taking a user-visible action.</t>
</section>
</section> <section anchor="finished" numbered="true" toc="default">
<section anchor="finished" title="Finished"> <name>Finished</name>
<t>An HMAC <xref target="RFC2104" format="default"/> over the hashed a
<t>An HMAC <xref target="HMAC"/> over the hashed authenticator transcript, which uthenticator transcript is the
is the
concatenation of the Handshake Context, authenticator request (if present), concatenation of the Handshake Context, authenticator request (if present),
Certificate, and CertificateVerify. The HMAC is computed using the authenticato r hash, using the Finished MAC Key as Certificate, and CertificateVerify. The HMAC is computed using the authenticato r hash, using the Finished MAC Key as
a key.</t> a key.</t>
<artwork name="" type="" align="left" alt=""><![CDATA[
<figure><artwork><![CDATA[
Finished = HMAC(Finished MAC Key, Hash(Handshake Context || Finished = HMAC(Finished MAC Key, Hash(Handshake Context ||
authenticator request || Certificate || CertificateVerify)) authenticator request || Certificate || CertificateVerify))
]]></artwork></figure> ]]></artwork>
</section>
</section> <section anchor="authenticator-creation" numbered="true" toc="default">
<section anchor="authenticator-creation" title="Authenticator Creation"> <name>Authenticator Creation</name>
<t>An endpoint constructs an authenticator by serializing the Certific
<t>An endpoint constructs an authenticator by serializing the Certificate, Certi ate, CertificateVerify, and Finished as TLS handshake messages and concatenating
ficateVerify, and Finished as TLS handshake messages and concatenating the octet the octets:</t>
s:</t> <artwork name="" type="" align="left" alt=""><![CDATA[
<figure><artwork><![CDATA[
Certificate || CertificateVerify || Finished Certificate || CertificateVerify || Finished
]]></artwork></figure> ]]></artwork>
<t>An authenticator is valid if the CertificateVerify message is correctly const ructed given the authenticator request (if <t>An authenticator is valid if the CertificateVerify message is corre ctly constructed given the authenticator request (if
used) and the Finished message matches the expected value. When validating an a uthenticator, constant-time used) and the Finished message matches the expected value. When validating an a uthenticator, constant-time
comparisons SHOULD be used for signature and MAC validation.</t> comparisons <bcp14>SHOULD</bcp14> be used for signature and MAC validation.</t>
</section>
</section> </section>
</section> </section>
</section> <section anchor="empty-authenticator" numbered="true" toc="default">
<section anchor="empty-authenticator" title="Empty Authenticator"> <name>Empty Authenticator</name>
<t>If, given an authenticator request, the endpoint does not have an appro
<t>If, given an authenticator request, the endpoint does not have an appropriate priate
identity or does not want to return one, it constructs an authenticated identity or does not want to return one, it constructs an authenticated
refusal called an empty authenticator. This is a Finished refusal called an "empty authenticator". This is a Finished
message sent without a Certificate or CertificateVerify. This message is an message sent without a Certificate or CertificateVerify. This message is an
HMAC over the hashed authenticator transcript with a Certificate message HMAC over the hashed authenticator transcript with a Certificate message
containing no CertificateEntries and the CertificateVerify message omitted. containing no CertificateEntries and the CertificateVerify message omitted.
The HMAC is computed using the authenticator hash, using the Finished MAC Key as a key. 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 fi eld. This message is encoded as a TLS handshake message, including length and type fi eld.
It does not include TLS record layer framing and is not encrypted with a handsha It does not include TLS record-layer framing and is not encrypted with a handsha
ke or application-data key.</t> ke or application-data key.</t>
<artwork name="" type="" align="left" alt=""><![CDATA[
<figure><artwork><![CDATA[
Finished = HMAC(Finished MAC Key, Hash(Handshake Context || Finished = HMAC(Finished MAC Key, Hash(Handshake Context ||
authenticator request || Certificate)) authenticator request || Certificate))
]]></artwork></figure> ]]></artwork>
</section>
</section> <section anchor="api-considerations" numbered="true" toc="default">
<section anchor="api-considerations" title="API considerations"> <name>API Considerations</name>
<t>The creation and validation of both authenticator requests and authenti
<t>The creation and validation of both authenticator requests and authenticators cators
SHOULD be implemented inside the (D)TLS library even if it is possible to implem <bcp14>SHOULD</bcp14> be implemented inside the (D)TLS library even if it is pos
ent sible to implement
it at the application layer. (D)TLS implementations supporting the use of expor it at the application layer. (D)TLS implementations supporting the use of Expor
ted ted
authenticators SHOULD provide application programming interfaces by which client Authenticators <bcp14>SHOULD</bcp14> provide application programming interfaces
s by which clients
and servers may request and verify exported authenticator messages.</t> and servers may request and verify Exported Authenticator messages.</t>
<t>Notwithstanding the success conditions described below, all APIs <bcp14
<t>Notwithstanding the success conditions described below, all APIs MUST fail if >MUST</bcp14> fail if:</t>
:</t> <ul spacing="normal">
<li>the connection uses a (D)TLS version of 1.1 or earlier, or</li>
<t><list style="symbols"> <li>the connection is (D)TLS 1.2 and the extended master secret extensio
<t>the connection uses a (D)TLS version of 1.1 or earlier, or</t> n <xref target="RFC7627" format="default"/> was not
<t>the connection is (D)TLS 1.2 and the extended master secret extension <xref negotiated</li>
target="RFC7627"/> was not </ul>
negotiated</t> <t>The following sections describe APIs that are considered necessary to
</list></t> implement Exported Authenticators. These are informative only.</t>
<section anchor="the-request-api" numbered="true" toc="default">
<t>The following sections describe APIs that are considered necessary to <name>The "request" API</name>
implement exported authenticators. These are informative only.</t> <t>The "request" API takes as input:</t>
<ul spacing="normal">
<section anchor="the-request-api" title="The “request” API"> <li>certificate_request_context (from 0 to 255 octets)</li>
<li>the set of extensions to include (this <bcp14>MUST</bcp14> include
<t>The “request” API takes as input:</t> signature_algorithms) and the
contents thereof</li>
<t><list style="symbols"> </ul>
<t>certificate_request_context (from 0 to 255 octets)</t> <t>It returns an authenticator request, which is a sequence of octets
<t>set of extensions to include (this MUST include signature_algorithms) and t
he
contents thereof</t>
</list></t>
<t>It returns an authenticator request, which is a sequence of octets
that comprises a CertificateRequest or ClientCertificateRequest message.</t> that comprises a CertificateRequest or ClientCertificateRequest message.</t>
</section>
<section anchor="the-get-context-api" numbered="true" toc="default">
<name>The "get context" API</name>
<t>The "get context" API takes as input:</t>
<ul spacing="normal">
<li>authenticator or authenticator request</li>
</ul>
<t>It returns the certificate_request_context.</t>
</section>
<section anchor="the-authenticate-api" numbered="true" toc="default">
<name>The "authenticate" API</name>
<t>The "authenticate" API takes as input:</t>
<ul spacing="normal">
<li>a reference to the initial connection</li>
<li>an identity, such as a set of certificate chains and associated ex
tensions
(OCSP <xref target="RFC6960" format="default"/>, SCT <xref target="RFC6962" form
at="default"/> (obsoleted by <xref target="RFC9162"/>), etc.)</li>
</section> <li>a signer (either the private key associated with the identity or t
<section anchor="the-get-context-api" title="The “get context” API"> he interface to perform private key operations) for each chain</li>
<li>an authenticator request or certificate_request_context (from 0 to
<t>The “get context” API takes as input:</t> 255 octets)</li>
</ul>
<t><list style="symbols">
<t>authenticator or authenticator request</t>
</list></t>
<t>It returns the certificate_request_context.</t>
</section>
<section anchor="the-authenticate-api" title="The “authenticate” API">
<t>The “authenticate” API takes as input:</t>
<t><list style="symbols">
<t>a reference to the initial connection</t>
<t>an identity, such as a set of certificate chains and associated extensions
(OCSP <xref target="RFC6960"/>, SCT <xref target="RFC6962"/>, etc.)</t>
<t>a signer (either the private key associated with the identity, or interface
to perform private key operations) for each chain</t>
<t>an authenticator request or certificate_request_context (from 0 to 255 octe
ts)</t>
</list></t>
<t>It returns either the exported authenticator or an empty authenticator <t>It returns either the authenticator or an empty authenticator
as a sequence of octets. It is recommended that as a sequence of octets. It is <bcp14>RECOMMENDED</bcp14> that
the logic for selecting the certificates and extensions to include the logic for selecting the certificates and extensions to include
in the exporter is implemented in the TLS library. Implementing this in the exporter be implemented in the TLS library. Implementing this
in the TLS library lets the implementer take advantage of existing in the TLS library lets the implementer take advantage of existing
extension and certificate selection logic and more easily remember extension and certificate selection logic, and the implementer can more easily r emember
which extensions were sent in the ClientHello.</t> which extensions were sent in the ClientHello.</t>
<t>It is also possible to implement this API outside of the TLS library
<t>It is also possible to implement this API outside of the TLS library using using
TLS exporters. This may be preferable in cases where the application 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 does not have access to a TLS library with these APIs or when TLS is
handled independently of the application layer protocol.</t> handled independently of the application-layer protocol.</t>
</section>
</section> <section anchor="the-validate-api" numbered="true" toc="default">
<section anchor="the-validate-api" title="The “validate” API"> <name>The "validate" API</name>
<t>The "validate" API takes as input:</t>
<t>The “validate” API takes as input:</t> <ul spacing="normal">
<li>a reference to the initial connection</li>
<t><list style="symbols"> <li>an optional authenticator request</li>
<t>a reference to the initial connection</t> <li>an authenticator</li>
<t>an optional authenticator request</t> <li>a function for validating a certificate chain</li>
<t>an authenticator</t> </ul>
<t>a function for validating a certificate chain</t> <t>It returns a status to indicate whether or not the authenticator is v
</list></t> alid after
<t>It returns a status to indicate whether the authenticator is valid or not aft
er
applying the function for validating the certificate chain to the chain applying the function for validating the certificate chain to the chain
contained in the authenticator. If validation is successful, it also returns contained in the authenticator. If validation is successful, it also returns
the identity, such as the certificate chain and its extensions.</t> the identity, such as the certificate chain and its extensions.</t>
<t>The API should return a failure if the certificate_request_context of
<t>The API should return a failure if the certificate_request_context of the the
authenticator was used in a different authenticator that was previously validate d. authenticator was used in a different authenticator that was previously validate d.
Well-formed empty authenticators are returned as invalid.</t> Well-formed empty authenticators are returned as invalid.</t>
<t>When validating an authenticator, constant-time comparison should be
used.</t>
</section>
</section>
<section anchor="iana-considerations" numbered="true" toc="default">
<name>IANA Considerations</name>
<section anchor="update-of-the-tls-extensiontype-registry" numbered="true"
toc="default">
<name>Update of the TLS ExtensionType Registry</name>
<t>IANA has updated the entry for server_name(0) in the "TLS ExtensionTy
pe Values" registry <xref target="IANA-TLS"/> (defined in <xref target="RFC8446"
format="default"/>) by replacing the value in the "TLS 1.3"
column with the value "CH, EE, CR" and listing this document in the "Reference"
column.</t>
<t>IANA has also added the following note to the registry:</t>
<blockquote> <t>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 ClientCertificat
eRequest created as part of client-generated authenticator requests.</t></blockq
uote>
</section>
<section anchor="update-of-the-tls-exporter-labels-registry" numbered="tru
e" toc="default">
<name>Update of the TLS Exporter Labels Registry</name>
<t>When validating an authenticator, constant-time comparison should be used.</t <t>IANA has added the following entries to the "TLS Exporter Labels" reg
> istry <xref target="IANA-EXPORT"/> (defined in <xref target="RFC5705" format="de
fault"/>): "EXPORTER-client authenticator handshake context",
</section> "EXPORTER-server authenticator handshake context",
</section> "EXPORTER-client authenticator finished key" and
<section anchor="iana-considerations" title="IANA Considerations"> "EXPORTER-server authenticator finished key" with "DTLS-OK" and "Recommended" se
t to "Y" and
<section anchor="update-of-the-tls-extensiontype-registry" title="Update of the this document listed as the reference.</t>
TLS ExtensionType Registry"> </section>
<section anchor="update-of-the-tls-handshaketype-registry" numbered="true"
<t>IANA is requested to update the entry for server_name(0) in the registry for toc="default">
ExtensionType (defined in <xref target="RFC8446"/>) by replacing the value in th <name>Update of the TLS HandshakeType Registry</name>
e “TLS 1.3”
column with the value “CH, EE, CR” and adding this document in the “Reference” c
olumn.</t>
<t>IANA is also requested to add the following note to the registry:</t>
<t>The addition of the “CR” to the “TLS 1.3” column for the server_name(0) exten
sion only marks the extension as valid in a ClientCertificateRequest created as
part of client-generated authenticator requests.</t>
</section>
<section anchor="update-of-the-tls-exporter-labels-registry" title="Update of th
e TLS Exporter Labels Registry">
<t>IANA is requested to add the following entries to the registry for Exporter
Labels (defined in <xref target="RFC5705"/>): “EXPORTER-client authenticator han
dshake context”,
“EXPORTER-server authenticator handshake context”, “EXPORTER-client authenticato
r
handshake context”, “EXPORTER-client authenticator finished key” and “EXPORTER-s
erver
authenticator finished key” with “DTLS-OK” and “Recommended” set to “Y” and
this document added to the “Reference” column.</t>
</section>
<section anchor="update-of-the-tls-handshaketype-registry" title="Update of the
TLS HandshakeType Registry">
<t>IANA is requested to add the following entry to the registry for
HandshakeType (defined in <xref target="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.”</t>
</section> <t>IANA has added the following entry to the "TLS HandshakeType" registr
</section> y <xref target="IANA-HANDSHAKE"/> (defined in <xref target="RFC8446" format="def
<section anchor="security" title="Security Considerations"> ault"/>): "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:</
t>
<blockquote><t>Used in TLS versions prior to 1.3.</t></blockquote>
<t>The Certificate/Verify/Finished pattern intentionally looks like the </section>
TLS 1.3 pattern which now has been analyzed several times. For example, </section>
<xref target="SIGMAC"/> presents a relevant framework for analysis, and section <section anchor="security" numbered="true" toc="default">
10. of <xref target="RFC8446"/> <name>Security Considerations</name>
contains a conprehensive set of references.</t> <t>The Certificate/Verify/Finished pattern intentionally looks like the
TLS 1.3 pattern that now has been analyzed several times. For example,
<xref target="SIGMAC" format="default"/> presents a relevant framework for analy
sis, and <xref target="RFC8446" sectionFormat="of" section="E.1.6"/>
contains a comprehensive set of references.</t>
<t>Authenticators are independent and unidirectional. There is no explicit stat e change <t>Authenticators are independent and unidirectional. There is no explici t state change
inside TLS when an authenticator is either created or validated. The applicatio n in inside TLS when an authenticator is either created or validated. The applicatio n in
possession of a validated authenticator can rely on any semantics associated wit h data possession of a validated authenticator can rely on any semantics associated wit h data
in the certificate_request_context.</t> in the certificate_request_context.</t>
<ul spacing="normal">
<t><list style="symbols"> <li>This property makes it difficult to formally prove that a server is
<t>This property makes it difficult to formally prove that a server is jointly jointly authoritative
authoritative over multiple identities, rather than individually authoritative over each.</li>
over multiple identities, rather than individually authoritative over each.</t> <li>There is no indication in (D)TLS about which point in time an authen
<t>There is no indication in (D)TLS about which point in time an authenticator ticator was
was
computed. Any feedback about the time of creation or validation of the authenti cator computed. Any feedback about the time of creation or validation of the authenti cator
should be tracked as part of the application layer semantics if required.</t> should be tracked as part of the application-layer semantics if required.</li>
</list></t> </ul>
<t>The signatures generated with this API cover the context string
<t>The signatures generated with this API cover the context string "Exported Authenticator"; therefore, they cannot be transplanted into other
“Exported Authenticator” and therefore cannot be transplanted into other
protocols.</t> protocols.</t>
<t>In TLS 1.3, the client cannot explicitly learn from the TLS layer wheth
<t>In TLS 1.3 the client can not explicitly learn from the TLS layer whether its er its
Finished message was accepted. Because the application traffic keys are not Finished message was accepted. Because the application traffic keys are not
dependent on the client’s final flight, receiving messages from the server 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 disagree does not prove that the server received the client's Finished message.
ment between the client and server To avoid disagreement between the client and server
on the authentication status of EAs, servers MUST verify the client Finished on the authentication status of Exported Authenticators, servers <bcp14>MUST</bc
before sending an EA or processing a received EA.</t> p14> verify the client Finished message
before sending an EA or processing a received Exported Authenticator.</t>
</section> </section>
<section anchor="ack" title="Acknowledgements">
<t>Comments on this proposal were provided by Martin Thomson. Suggestions for
<xref target="security"/> were provided by Karthikeyan Bhargavan.</t>
</section>
</middle> </middle>
<back> <back>
<references title='Normative References'> <displayreference target="RFC2104" to="HMAC"/>
<reference anchor='RFC8446' target='https://www.rfc-editor.org/info/rfc8446'>
<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.3</title>
<author fullname='E. Rescorla' initials='E.' surname='Rescorla'><organization/><
/author>
<date month='August' year='2018'/>
<abstract><t>This document specifies version 1.3 of the Transport Layer Security
(TLS) protocol. TLS allows client/server applications to communicate over the
Internet in a way that is designed to prevent eavesdropping, tampering, and mess
age forgery.</t><t>This document updates RFCs 5705 and 6066, and obsoletes RFCs
5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2
implementations.</t></abstract>
</front>
<seriesInfo name='RFC' value='8446'/>
<seriesInfo name='DOI' value='10.17487/RFC8446'/>
</reference>
<reference anchor='RFC6347' target='https://www.rfc-editor.org/info/rfc6347'>
<front>
<title>Datagram Transport Layer Security Version 1.2</title>
<author fullname='E. Rescorla' initials='E.' surname='Rescorla'><organization/><
/author>
<author fullname='N. Modadugu' initials='N.' surname='Modadugu'><organization/><
/author>
<date month='January' year='2012'/>
<abstract><t>This document specifies version 1.2 of the Datagram Transport Layer
Security (DTLS) protocol. The DTLS protocol provides communications privacy fo
r datagram protocols. The protocol allows client/server applications to communi
cate in a way that is designed to prevent eavesdropping, tampering, or message f
orgery. The DTLS protocol is based on the Transport Layer Security (TLS) protoc
ol and provides equivalent security guarantees. Datagram semantics of the under
lying transport are preserved by the DTLS protocol. This document updates DTLS
1.0 to work with TLS version 1.2. [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='6347'/>
<seriesInfo name='DOI' value='10.17487/RFC6347'/>
</reference>
<reference anchor='RFC5246' target='https://www.rfc-editor.org/info/rfc5246'>
<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
<author fullname='T. Dierks' initials='T.' surname='Dierks'><organization/></aut
hor>
<author fullname='E. Rescorla' initials='E.' surname='Rescorla'><organization/><
/author>
<date month='August' year='2008'/>
<abstract><t>This document specifies Version 1.2 of the Transport Layer Security
(TLS) protocol. The TLS protocol provides communications security over the Int
ernet. The protocol allows client/server applications to communicate in a way t
hat is designed to prevent eavesdropping, tampering, or message forgery. [STAND
ARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='5246'/>
<seriesInfo name='DOI' value='10.17487/RFC5246'/>
</reference>
<reference anchor='RFC2119' target='https://www.rfc-editor.org/info/rfc2119'>
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels</title>
<author fullname='S. Bradner' initials='S.' surname='Bradner'><organization/></a
uthor>
<date month='March' year='1997'/>
<abstract><t>In many standards track documents several words are used to signify
the requirements in the specification. These words are often capitalized. This
document defines these words as they should be interpreted in IETF documents.
This document specifies an Internet Best Current Practices for the Internet Comm
unity, and requests discussion and suggestions for improvements.</t></abstract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='2119'/>
<seriesInfo name='DOI' value='10.17487/RFC2119'/>
</reference>
<reference anchor='RFC8174' target='https://www.rfc-editor.org/info/rfc8174'>
<front>
<title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
<author fullname='B. Leiba' initials='B.' surname='Leiba'><organization/></autho
r>
<date month='May' year='2017'/>
<abstract><t>RFC 2119 specifies common key words that may be used in protocol s
pecifications. This document aims to reduce the ambiguity by clarifying that on
ly UPPERCASE usage of the key words have the defined special meanings.</t></abs
tract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='8174'/>
<seriesInfo name='DOI' value='10.17487/RFC8174'/>
</reference>
<reference anchor='RFC8447' target='https://www.rfc-editor.org/info/rfc8447'>
<front>
<title>IANA Registry Updates for TLS and DTLS</title>
<author fullname='J. Salowey' initials='J.' surname='Salowey'><organization/></a
uthor>
<author fullname='S. Turner' initials='S.' surname='Turner'><organization/></aut
hor>
<date month='August' year='2018'/>
<abstract><t>This document describes a number of changes to TLS and DTLS IANA re
gistries that range from adding notes to the registry all the way to changing th
e registration policy. These changes were mostly motivated by WG review of the
TLS- and DTLS-related registries undertaken as part of the TLS 1.3 development p
rocess.</t><t>This document updates the following RFCs: 3749, 5077, 4680, 5246,
5705, 5878, 6520, and 7301.</t></abstract>
</front>
<seriesInfo name='RFC' value='8447'/>
<seriesInfo name='DOI' value='10.17487/RFC8447'/>
</reference>
<reference anchor='RFC6066' target='https://www.rfc-editor.org/info/rfc6066'> <references>
<front> <name>References</name>
<title>Transport Layer Security (TLS) Extensions: Extension Definitions</title> <references>
<author fullname='D. Eastlake 3rd' initials='D.' surname='Eastlake 3rd'><organiz <name>Normative References</name>
ation/></author> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
<date month='January' year='2011'/> FC.8446.xml"/>
<abstract><t>This document provides specifications for existing TLS extensions. <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
It is a companion document for RFC 5246, &quot;The Transport Layer Security (TL FC.6347.xml"/>
S) Protocol Version 1.2&quot;. The extensions specified are server_name, max_fr <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
agment_length, client_certificate_url, trusted_ca_keys, truncated_hmac, and stat FC.5246.xml"/>
us_request. [STANDARDS-TRACK]</t></abstract> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
</front> FC.2119.xml"/>
<seriesInfo name='RFC' value='6066'/> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
<seriesInfo name='DOI' value='10.17487/RFC6066'/> FC.8174.xml"/>
</reference> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
FC.8447.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
FC.6066.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
FC.5705.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
FC.7627.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
FC.2104.xml"/>
</references>
<references>
<name>Informative References</name>
<reference anchor='RFC5705' target='https://www.rfc-editor.org/info/rfc5705'> <reference anchor="SIGMAC" target="https://eprint.iacr.org/2016/711.pdf">
<front> <front>
<title>Keying Material Exporters for Transport Layer Security (TLS)</title> <title>A Unilateral-to-Mutual Authentication Compiler for Key Exchange
<author fullname='E. Rescorla' initials='E.' surname='Rescorla'><organization/>< (with Applications to Client Authentication in TLS 1.3)</title>
/author> <author initials="H" surname="Krawczyk" fullname="Hugo Krawczyk">
<date month='March' year='2010'/> <organization/>
<abstract><t>A number of protocols wish to leverage Transport Layer Security (TL </author>
S) to perform key establishment but then use some of the keying material for the <date month="August" year="2016"/>
ir own purposes. This document describes a general mechanism for allowing that. </front>
[STANDARDS-TRACK]</t></abstract> <refcontent>Proceedings of the 2016 ACM SIGSAC Conference on Computer an
</front> d Communications Security</refcontent>
<seriesInfo name='RFC' value='5705'/> <seriesInfo name="DOI" value="10.1145/2976749.2978325"/>
<seriesInfo name='DOI' value='10.17487/RFC5705'/>
</reference> </reference>
<reference anchor='RFC7627' target='https://www.rfc-editor.org/info/rfc7627'> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
<front> FC.9113.xml"/>
<title>Transport Layer Security (TLS) Session Hash and Extended Master Secret Ex <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
tension</title> FC.9001.xml"/>
<author fullname='K. Bhargavan' initials='K.' role='editor' surname='Bhargavan'> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
<organization/></author> FC.7250.xml"/>
<author fullname='A. Delignat-Lavaud' initials='A.' surname='Delignat-Lavaud'><o <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.R
rganization/></author> FC.6960.xml"/>
<author fullname='A. Pironti' initials='A.' surname='Pironti'><organization/></a
uthor>
<author fullname='A. Langley' initials='A.' surname='Langley'><organization/></a
uthor>
<author fullname='M. Ray' initials='M.' surname='Ray'><organization/></author>
<date month='September' year='2015'/>
<abstract><t>The Transport Layer Security (TLS) master secret is not cryptograph
ically bound to important session parameters such as the server certificate. Co
nsequently, it is possible for an active attacker to set up two sessions, one wi
th a client and another with a server, such that the master secrets on the two s
essions are the same. Thereafter, any mechanism that relies on the master secre
t for authentication, including session resumption, becomes vulnerable to a man-
in-the-middle attack, where the attacker can simply forward messages back and fo
rth between the client and server. This specification defines a TLS extension t
hat contextually binds the master secret to a log of the full handshake that com
putes it, thus preventing such attacks.</t></abstract>
</front>
<seriesInfo name='RFC' value='7627'/>
<seriesInfo name='DOI' value='10.17487/RFC7627'/>
</reference>
<reference anchor='HMAC' target='https://www.rfc-editor.org/info/rfc2104'> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RF
<front> C.6962.xml"/>
<title>HMAC: Keyed-Hashing for Message Authentication</title>
<author fullname='H. Krawczyk' initials='H.' surname='Krawczyk'><organization/><
/author>
<author fullname='M. Bellare' initials='M.' surname='Bellare'><organization/></a
uthor>
<author fullname='R. Canetti' initials='R.' surname='Canetti'><organization/></a
uthor>
<date month='February' year='1997'/>
<abstract><t>This document describes HMAC, a mechanism for message authenticatio
n using cryptographic hash functions. HMAC can be used with any iterative crypto
graphic hash function, e.g., MD5, SHA-1, in combination with a secret shared key
. The cryptographic strength of HMAC depends on the properties of the underlyin
g hash function. This memo provides information for the Internet community. Th
is memo does not specify an Internet standard of any kind</t></abstract>
</front>
<seriesInfo name='RFC' value='2104'/>
<seriesInfo name='DOI' value='10.17487/RFC2104'/>
</reference>
</references> <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RF C.9162.xml"/>
<references title='Informative References'> <reference anchor="IANA-TLS"
target="https://www.iana.org/assignments/tls-extensiontype-values/">
<reference anchor="SIGMAC" target="https://eprint.iacr.org/2016/711.pdf">
<front> <front>
<title>A Unilateral-to-Mutual Authentication Compiler for Key Exchange (with <title>TLS ExtensionType Values</title>
Applications to Client Authentication in TLS 1.3)</title> <author><organization>IANA</organization></author>
<author initials="H." surname="Krawczyk">
<organization></organization>
</author>
<date year="2016"/>
</front> </front>
</reference> </reference>
<reference anchor='RFC7540' target='https://www.rfc-editor.org/info/rfc7540'> <reference anchor="IANA-EXPORT"
<front> target="https://www.iana.org/assignments/tls-parameters/">
<title>Hypertext Transfer Protocol Version 2 (HTTP/2)</title> <front>
<author fullname='M. Belshe' initials='M.' surname='Belshe'><organization/></aut <title>TLS Exporter Labels</title>
hor> <author><organization>IANA</organization></author>
<author fullname='R. Peon' initials='R.' surname='Peon'><organization/></author> </front>
<author fullname='M. Thomson' initials='M.' role='editor' surname='Thomson'><org
anization/></author>
<date month='May' year='2015'/>
<abstract><t>This specification describes an optimized expression of the semanti
cs of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTT
P/2). HTTP/2 enables a more efficient use of network resources and a reduced pe
rception of latency by introducing header field compression and allowing multipl
e concurrent exchanges on the same connection. It also introduces unsolicited p
ush of representations from servers to clients.</t><t>This specification is an a
lternative to, but does not obsolete, the HTTP/1.1 message syntax. HTTP's exist
ing semantics remain unchanged.</t></abstract>
</front>
<seriesInfo name='RFC' value='7540'/>
<seriesInfo name='DOI' value='10.17487/RFC7540'/>
</reference>
<reference anchor='RFC9001' target='https://www.rfc-editor.org/info/rfc9001'>
<front>
<title>Using TLS to Secure QUIC</title>
<author fullname='M. Thomson' initials='M.' role='editor' surname='Thomson'><org
anization/></author>
<author fullname='S. Turner' initials='S.' role='editor' surname='Turner'><organ
ization/></author>
<date month='May' year='2021'/>
<abstract><t>This document describes how Transport Layer Security (TLS) is used
to secure QUIC.</t></abstract>
</front>
<seriesInfo name='RFC' value='9001'/>
<seriesInfo name='DOI' value='10.17487/RFC9001'/>
</reference>
<reference anchor='RFC7250' target='https://www.rfc-editor.org/info/rfc7250'>
<front>
<title>Using Raw Public Keys in Transport Layer Security (TLS) and Datagram Tran
sport Layer Security (DTLS)</title>
<author fullname='P. Wouters' initials='P.' role='editor' surname='Wouters'><org
anization/></author>
<author fullname='H. Tschofenig' initials='H.' role='editor' surname='Tschofenig
'><organization/></author>
<author fullname='J. Gilmore' initials='J.' surname='Gilmore'><organization/></a
uthor>
<author fullname='S. Weiler' initials='S.' surname='Weiler'><organization/></aut
hor>
<author fullname='T. Kivinen' initials='T.' surname='Kivinen'><organization/></a
uthor>
<date month='June' year='2014'/>
<abstract><t>This document specifies a new certificate type and two TLS extensio
ns for exchanging raw public keys in Transport Layer Security (TLS) and Datagram
Transport Layer Security (DTLS). The new certificate type allows raw public ke
ys to be used for authentication.</t></abstract>
</front>
<seriesInfo name='RFC' value='7250'/>
<seriesInfo name='DOI' value='10.17487/RFC7250'/>
</reference>
<reference anchor='RFC6960' target='https://www.rfc-editor.org/info/rfc6960'>
<front>
<title>X.509 Internet Public Key Infrastructure Online Certificate Status Protoc
ol - OCSP</title>
<author fullname='S. Santesson' initials='S.' surname='Santesson'><organization/
></author>
<author fullname='M. Myers' initials='M.' surname='Myers'><organization/></autho
r>
<author fullname='R. Ankney' initials='R.' surname='Ankney'><organization/></aut
hor>
<author fullname='A. Malpani' initials='A.' surname='Malpani'><organization/></a
uthor>
<author fullname='S. Galperin' initials='S.' surname='Galperin'><organization/><
/author>
<author fullname='C. Adams' initials='C.' surname='Adams'><organization/></autho
r>
<date month='June' year='2013'/>
<abstract><t>This document specifies a protocol useful in determining the curren
t status of a digital certificate without requiring Certificate Revocation Lists
(CRLs). Additional mechanisms addressing PKIX operational requirements are spec
ified in separate documents. This document obsoletes RFCs 2560 and 6277. It al
so updates RFC 5912.</t></abstract>
</front>
<seriesInfo name='RFC' value='6960'/>
<seriesInfo name='DOI' value='10.17487/RFC6960'/>
</reference> </reference>
<reference anchor='RFC6962' target='https://www.rfc-editor.org/info/rfc6962'> <reference anchor="IANA-HANDSHAKE"
<front> target="https://www.iana.org/assignments/tls-parameters/">
<title>Certificate Transparency</title> <front>
<author fullname='B. Laurie' initials='B.' surname='Laurie'><organization/></aut <title>TLS HandshakeType</title>
hor> <author><organization>IANA</organization></author>
<author fullname='A. Langley' initials='A.' surname='Langley'><organization/></a </front>
uthor>
<author fullname='E. Kasper' initials='E.' surname='Kasper'><organization/></aut
hor>
<date month='June' year='2013'/>
<abstract><t>This document describes an experimental protocol for publicly loggi
ng the existence of Transport Layer Security (TLS) certificates as they are issu
ed or observed, in a manner that allows anyone to audit certificate authority (C
A) activity and notice the issuance of suspect certificates as well as to audit
the certificate logs themselves. The intent is that eventually clients would re
fuse to honor certificates that do not appear in a log, effectively forcing CAs
to add all issued certificates to the logs.</t><t>Logs are network services that
implement the protocol operations for submissions and queries that are defined
in this document.</t></abstract>
</front>
<seriesInfo name='RFC' value='6962'/>
<seriesInfo name='DOI' value='10.17487/RFC6962'/>
</reference> </reference>
</references>
</references> </references>
<section anchor="ack" numbered="false" toc="default">
<name>Acknowledgements</name>
<t>Comments on this proposal were provided by <contact fullname="Martin Th
omson"/>. Suggestions for
<xref target="security" format="default"/> were provided by <contact fullname="K
arthikeyan Bhargavan"/>.</t>
</section>
</back> </back>
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