Internet Engineering Task Force (IETF) N. Sullivan
Request for Comments: 9261
Category: Standards Track July 2022
Exported Authenticators in TLS
This document describes a mechanism that builds on Transport Layer
Security (TLS) or Datagram Transport Layer Security (DTLS) and
enables peers to provide proof of ownership of an identity, such as
an X.509 certificate. This proof can be exported by one peer,
transmitted out of band to the other peer, and verified by the
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9261
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents 1.
Conventions and Terminology 3.
Message Sequences 4.
Authenticator Request 5.
Authenticator Keys 5.2.
Authenticator Construction 5.2.1.
Authenticator Creation 6.
Empty Authenticator 7.
API Considerations 7.1.
The "request" API 7.2.
The "get context" API 7.3.
The "authenticate" API 7.4.
The "validate" API 8.
IANA Considerations 8.1.
Update of the TLS ExtensionType Registry 8.2.
Update of the TLS Exporter Labels Registry 8.3.
Update of the TLS HandshakeType Registry 9.
Security Considerations 10.
Normative References 10.2.
This document provides a way to authenticate one party of a Transport
Layer Security (TLS) or Datagram Transport Layer Security (DTLS)
connection to its peer using authentication messages created after
the session has been established. This allows both the client and
server to prove ownership of additional identities at any time after
the handshake has completed. This proof of authentication can be
exported and transmitted out of band from one party to be validated
by its peer.
This mechanism provides two advantages over the authentication that
TLS and DTLS natively provide:
multiple identities: Endpoints that are authoritative for multiple
identities, but that do not have a single certificate that
includes all of the identities, can authenticate additional
identities over a single connection.
spontaneous authentication: After a connection is established,
endpoints can authenticate in response to events in a higher-layer
protocol; they can also integrate more context (such as context
from the application).
Versions of TLS prior to TLS 1.3 used renegotiation as a way to
enable post-handshake client authentication given an existing TLS
connection. The mechanism described in this document may be used to
replace the post-handshake authentication functionality provided by
renegotiation. Unlike renegotiation, Exported Authenticator-based
post-handshake authentication does not require any changes at the TLS
Post-handshake authentication is defined in TLS 1.3 Section 4.6.2 of
], but it has the disadvantage of requiring additional state
to be stored as part of the TLS state machine. Furthermore, the
authentication boundaries of TLS 1.3 post-handshake authentication
align with TLS record boundaries, which are often not aligned with
the authentication boundaries of the higher-layer protocol. For
example, multiplexed connection protocols like HTTP/2 [RFC9113
not have a notion of which TLS record a given message is a part of.
Exported Authenticators are meant to be used as a building block for
application protocols. Mechanisms such as those required to
advertise support and handle authentication errors are not handled by
TLS (or DTLS).
The minimum version of TLS and DTLS required to implement the
mechanisms described in this document are TLS 1.2 [RFC5246
] and DTLS
]. (These were obsoleted by TLS 1.3 [RFC8446
] and DTLS
2. Conventions and Terminology
The key words "MUST
", "MUST NOT
", "SHALL NOT
", "SHOULD NOT
", "NOT RECOMMENDED
" in this document are to be interpreted as described in
BCP 14 [RFC2119
] when, and only when, they appear in all
capitals, as shown here.
This document uses terminology such as client, server, connection,
handshake, endpoint, and peer that are defined in Section 1.1 of
]. The term "initial connection" refers to the (D)TLS
connection from which the Exported Authenticator messages are
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:
* Server generates authenticator request
* Client generates Authenticator from Server's authenticator request
* Server validates Client's authenticator
* 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
its underlying transport to keep the request confidential. The
use the existing (D)TLS connection to transport the
An authenticator request message can be constructed by either the
client or the server. Server-generated authenticator requests use
the CertificateRequest message from Section 4.3.2 of [RFC8446
Client-generated authenticator requests use a new message, called the
"ClientCertificateRequest", that uses the same structure as
CertificateRequest. (Note that the latter is not a request for a
client certificate, but rather a certificate request generated by the
client.) These message structures are used even if the connection
protocol is TLS 1.2 or DTLS 1.2.
The CertificateRequest and ClientCertificateRequest messages are used
to define the parameters in a request for an authenticator. These
are encoded as TLS handshake messages, including length and type
fields. They do not include any TLS record-layer framing and are not
encrypted with a handshake or application-data key.
The structures are defined to be:
certificate_request_context: An opaque string that identifies the
authenticator request and that will be echoed in the authenticator
message. A certificate_request_context value MUST
be unique for
each authenticator request within the scope of a connection
(preventing replay and context confusion). The
be chosen to be unpredictable
to the peer (e.g., by randomly generating it) in order to prevent
an attacker who has temporary access to the peer's private key
from precomputing valid authenticators. For example, the
application may choose this value to correspond to a value used in
an existing data structure in the software to simplify
extensions: The set of extensions allowed in the structures of
CertificateRequest and ClientCertificateRequest is comprised of
those defined in the "TLS ExtensionType Values" IANA registry
containing CR in the "TLS 1.3" column (see [IANA-TLS] and
]). In addition, the set of extensions in the
ClientCertificateRequest structure MAY
include the server_name
The uniqueness requirements of the certificate_request_context apply
across CertificateRequest and ClientCertificateRequest messages that
are used as part of authenticator requests. A
certificate_request_context value used in a ClientCertificateRequest
cannot be used in an authenticator CertificateRequest on the same
connection, and vice versa. There is no impact if the value of a
certificate_request_context used in an authenticator request matches
the value of a certificate_request_context in the handshake or in a
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
a secure transport channel with equivalent security to TLS, such as
], as its underlying transport to keep the authenticator
confidential. The application MAY
use the existing (D)TLS connection
to transport the authenticator.
An authenticator message can be constructed by either the client or
the server given an established (D)TLS connection; an identity, such
as an X.509 certificate; and a corresponding private key. Clients MUST NOT
send an authenticator without a preceding authenticator
request; for servers, an authenticator request is optional. For
authenticators that do not correspond to authenticator requests, the
certificate_request_context is chosen by the server.
5.1. Authenticator Keys
Each authenticator is computed using a Handshake Context and Finished
MAC (Message Authentication Code) Key derived from the (D)TLS
connection. These values are derived using an exporter as described
in Section 4
] (for (D)TLS 1.2) or Section 7.5 of
] (for (D)TLS 1.3). For (D)TLS 1.3, the
be used, not the
early_exporter_master_secret. These values use different labels
depending on the role of the sender:
* The Handshake Context is an exporter value that is derived using
the label "EXPORTER-client authenticator handshake context" or
"EXPORTER-server authenticator handshake context" for
authenticators sent by the client or server, respectively.
* The Finished MAC Key is an exporter value derived using the label
"EXPORTER-client authenticator finished key" or "EXPORTER-server
authenticator finished key" for authenticators sent by the client
or server, respectively.
The context_value used for the exporter is empty (zero length) for
all four values. There is no need to include additional context
information at this stage because the application-supplied context is
included in the authenticator itself. The length of the exported
value is equal to the length of the output of the hash function
associated with the selected ciphersuite (for TLS 1.3) or the hash
function used for the pseudorandom function (PRF) (for (D)TLS 1.2).
Exported Authenticators cannot be used with (D)TLS 1.2 ciphersuites
that do not use the TLS PRF and with TLS 1.3 ciphersuites that do not
have an associated hash function. This hash is referred to as the
To avoid key synchronization attacks, Exported Authenticators MUST
be generated or accepted on (D)TLS 1.2 connections that did not
negotiate the extended master secret extension [RFC7627
5.2. Authenticator Construction
An authenticator is formed from the concatenation of TLS 1.3
Certificate, CertificateVerify, and Finished messages [RFC8446
These messages are encoded as TLS handshake messages, including
length and type fields. They do not include any TLS record-layer
framing and are not encrypted with a handshake or application-data
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
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.
The Certificate message contains the identity to be used for
authentication, such as the end-entity certificate and any supporting
certificates in the chain. This structure is defined in
Section 4.4.2 of [RFC8446
The Certificate message contains an opaque string called
"certificate_request_context", which is extracted from the
authenticator request, if present. If no authenticator request is
provided, the certificate_request_context can be chosen arbitrarily;
however, it MUST
be unique within the scope of the connection and be
unpredictable to the peer.
Certificates chosen in the Certificate message MUST
conform to the
requirements of a Certificate message in the negotiated version of
(D)TLS. In particular, the entries of certificate_list MUST
for the signature algorithms indicated by the peer in the
"signature_algorithms" and "signature_algorithms_cert" extensions, as
described in Section 4.2.3 of [RFC8446
] for (D)TLS 1.3 or in Sections 7.4
.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
"oid_filters" (Section 4.2.5 of [RFC8446
]) extensions are used to
guide certificate selection.
Only the X.509 certificate type defined in [RFC8446
] is supported.
Alternative certificate formats such as Raw Public Keys as described
] are not supported in this version of the specification
and their use in this context has not yet been analyzed.
If an authenticator request was provided, the Certificate message MUST
contain only extensions present in the authenticator request.
Otherwise, the Certificate message MUST
contain only extensions
present in the (D)TLS ClientHello. Unrecognized extensions in the
authenticator request MUST
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:
The algorithm field specifies the signature algorithm used (see
Section 4.2.3 of [RFC8446
] for the definition of this field). The
signature is a digital signature using that algorithm.
The signature scheme MUST
be a valid signature scheme for TLS 1.3.
This excludes all RSASSA-PKCS1-v1_5 algorithms and combinations of
Elliptic Curve Digital Signature Algorithm (ECDSA) and hash
algorithms that are not supported in TLS 1.3.
If an authenticator request is present, the signature algorithm MUST
be chosen from one of the signature schemes present in the
"signature_algorithms" extension of the authenticator request.
Otherwise, with spontaneous server authentication, the signature
algorithm used MUST
be chosen from the "signature_algorithms" sent by
the peer in the ClientHello of the (D)TLS handshake. If there are no
available signature algorithms, then no authenticator should be
The signature is computed using the chosen signature scheme over the
* a string that consists of octet 32 (0x20) repeated 64 times,
* the context string "Exported Authenticator" (which is not NUL-
* a single 0 octet that serves as the separator, and
* the hashed authenticator transcript.
The authenticator transcript is the hash of the concatenated
Handshake Context, authenticator request (if present), and
Hash(Handshake Context || authenticator request || Certificate)
Where Hash is the authenticator hash defined in Section 5.1
. If the
authenticator request is not present, it is omitted from this
construction, i.e., it is zero-length.
If the party that generates the authenticator does so with a
different connection than the party that is validating it, then the
Handshake Context will not match, resulting in a CertificateVerify
message that does not validate. This includes situations in which
the application data is sent via TLS-terminating proxy. Given a
failed CertificateVerify validation, it may be helpful for the
application to confirm that both peers share the same connection
using a value derived from the connection secrets (such as the
Handshake Context) before taking a user-visible action.
An HMAC [HMAC] over the hashed authenticator transcript is the
concatenation of the Handshake Context, authenticator request (if
present), Certificate, and CertificateVerify. The HMAC is computed
using the authenticator hash, using the Finished MAC Key as a key.
Finished = HMAC(Finished MAC Key, Hash(Handshake Context ||
authenticator request || Certificate || CertificateVerify))
5.2.4. Authenticator Creation
An endpoint constructs an authenticator by serializing the
Certificate, CertificateVerify, and Finished as TLS handshake
messages and concatenating the octets:
Certificate || CertificateVerify || Finished
An authenticator is valid if the CertificateVerify message is
correctly constructed given the authenticator request (if used) and
the Finished message matches the expected value. When validating an
authenticator, constant-time comparisons SHOULD
be used for signature
and MAC validation.
6. Empty Authenticator
If, given an authenticator request, the endpoint does not have an
appropriate identity or does not want to return one, it constructs an
authenticated refusal called an "empty authenticator". This is a
Finished message sent without a Certificate or CertificateVerify.
This message is an HMAC over the hashed authenticator transcript with
a Certificate message containing no CertificateEntries and the
CertificateVerify message omitted. The HMAC is computed using the
authenticator hash, using the Finished MAC Key as a key. This
message is encoded as a TLS handshake message, including length and
type field. It does not include TLS record-layer framing and is not
encrypted with a handshake or application-data key.
Finished = HMAC(Finished MAC Key, Hash(Handshake Context ||
authenticator request || Certificate))
7. API Considerations
The creation and validation of both authenticator requests and
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
* 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
] was not negotiated
The following sections describe APIs that are considered necessary to
implement Exported Authenticators. These are informative only.
7.1. The "request" API
The "request" API takes as input:
* certificate_request_context (from 0 to 255 octets)
* the set of extensions to include (this MUST
signature_algorithms) and the contents thereof
It returns an authenticator request, which is a sequence of octets
that comprises a CertificateRequest or ClientCertificateRequest
7.2. The "get context" API
The "get context" API takes as input:
* authenticator or authenticator request
It returns the certificate_request_context.
7.3. The "authenticate" API
The "authenticate" API takes as input:
* a reference to the initial connection
* an identity, such as a set of certificate chains and associated
extensions (OCSP [RFC6960
], SCT [RFC6962
] (obsoleted by
* a signer (either the private key associated with the identity or
the interface to perform private key operations) for each chain
* an authenticator request or certificate_request_context (from 0 to
It returns either the authenticator or an empty authenticator as a
sequence of octets. It is RECOMMENDED
that the logic for selecting
the certificates and extensions to include in the exporter be
implemented in the TLS library. Implementing this in the TLS library
lets the implementer take advantage of existing extension and
certificate selection logic, and the implementer can more easily
remember which extensions were sent in the ClientHello.
It is also possible to implement this API outside of the TLS library
using TLS exporters. This may be preferable in cases where the
application does not have access to a TLS library with these APIs or
when TLS is handled independently of the application-layer protocol.
7.4. The "validate" API
The "validate" API takes as input:
* a reference to the initial connection
* an optional authenticator request
* an authenticator
* a function for validating a certificate chain
It returns a status to indicate whether or not the authenticator is
valid after applying the function for validating the certificate
chain to the chain contained in the authenticator. If validation is
successful, it also returns the identity, such as the certificate
chain and its extensions.
The API should return a failure if the certificate_request_context of
the authenticator was used in a different authenticator that was
previously validated. Well-formed empty authenticators are returned
When validating an authenticator, constant-time comparison should be
8. IANA Considerations
8.1. Update of the TLS ExtensionType Registry
IANA has updated the entry for server_name(0) in the "TLS
ExtensionType Values" registry [IANA-TLS] (defined in [RFC8446
replacing the value in the "TLS 1.3" column with the value "CH, EE,
CR" and listing this document in the "Reference" column.
IANA has also added the following note to the registry:
| The addition of the "CR" to the "TLS 1.3" column for the
| server_name(0) extension only marks the extension as valid in a
| ClientCertificateRequest created as part of client-generated
| authenticator requests.
8.2. Update of the TLS Exporter Labels Registry
IANA has added the following entries to the "TLS Exporter Labels"
registry [IANA-EXPORT] (defined in [RFC5705
authenticator handshake context", "EXPORTER-server authenticator
handshake context", "EXPORTER-client authenticator finished key" and
"EXPORTER-server authenticator finished key" with "DTLS-OK" and
"Recommended" set to "Y" and this document listed as the reference.
8.3. Update of the TLS HandshakeType Registry
IANA has added the following entry to the "TLS HandshakeType"
registry [IANA-HANDSHAKE] (defined in [RFC8446
"client_certificate_request" (17) with "DTLS-OK" set to "Y" and this
document listed as the reference. In addition, the following appears
in the "Comment" column:
| Used in TLS versions prior to 1.3.
9. Security Considerations
The Certificate/Verify/Finished pattern intentionally looks like the
TLS 1.3 pattern that now has been analyzed several times. For
example, [SIGMAC] presents a relevant framework for analysis, and
Appendix E.1.6 of [RFC8446
] contains a comprehensive set of
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
* This property makes it difficult to formally prove that a server
is jointly authoritative over multiple identities, rather than
individually authoritative over each.
* There is no indication in (D)TLS about which point in time an
authenticator was computed. Any feedback about the time of
creation or validation of the authenticator should be tracked as
part of the application-layer semantics if required.
The signatures generated with this API cover the context string
"Exported Authenticator"; therefore, they cannot be transplanted into
In TLS 1.3, the client cannot explicitly learn from the TLS layer
whether its Finished message was accepted. Because the application
traffic keys are not dependent on the client's final flight,
receiving messages from the server does not prove that the server
received the client's Finished message. To avoid disagreement
between the client and server on the authentication status of
Exported Authenticators, servers MUST
verify the client Finished
message before sending an EA or processing a received Exported
10.1. Normative References
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104
, February 1997,
] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
, March 1997,
] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246
, August 2008,
] 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
] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066
, January 2011,
] 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
] 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,
] 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
] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446
, DOI 10.17487/RFC8446
, August 2018,
] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447
, DOI 10.17487/RFC8447
, August 2018,
] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147
, DOI 10.17487/RFC9147
, April 2022,
10.2. Informative References
IANA, "TLS Exporter Labels",
IANA, "TLS HandshakeType",
[IANA-TLS] IANA, "TLS ExtensionType Values",
] 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,
] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962
, DOI 10.17487/RFC6962
, June 2013,
] 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
] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001
, DOI 10.17487/RFC9001
, May 2021,
] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113
, June 2022,
] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162
, DOI 10.17487/RFC9162
December 2021, <https://www.rfc-editor.org/info/rfc9162
[SIGMAC] Krawczyk, H., "A Unilateral-to-Mutual Authentication
Compiler for Key Exchange (with Applications to Client
Authentication in TLS 1.3)", Proceedings of the 2016 ACM
SIGSAC Conference on Computer and Communications Security,
DOI 10.1145/2976749.2978325, August 2016,
Comments on this proposal were provided by Martin Thomson.
Suggestions for Section 9
were provided by Karthikeyan Bhargavan.