wdiff rfc9973.original rfc9973.txt

Network Working Group

Internet Engineering Task Force (IETF) R. Housley
Internet-Draft
Request for Comments: 9973 Vigil Security
Obsoletes: 8773 (if approved) 5 September 2025
Intended status: April 2026
Category: Standards Track
Expires: 9 March 2026
ISSN: 2070-1721

TLS 1.3 Extension for Using Certificates with an External Pre-Shared Key
draft-ietf-tls-8773bis-13

Abstract

This document specifies a TLS 1.3 extension that allows TLS clients
and servers to authenticate with certificates and provide
confidentiality based on encryption with a symmetric key from the
usual key agreement algorithm and an external pre-shared key (PSK).
This Standards Track RFC (once approved) obsoletes RFC 8773, which was an
Experimental RFC.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

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(IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid the IETF community. It has
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Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 7841.

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This Internet-Draft will expire on 9 March 2026.
https://www.rfc-editor.org/info/rfc9973.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 Requirements Language
3. Motivation and Design Rationale . . . . . . . . . . . . . . . 3
4. Extension Overview . . . . . . . . . . . . . . . . . . . . . 4
5. Certificate with External PSK Extension . . . . . . . . . . . 5
5.1. Companion Extensions . . . . . . . . . . . . . . . . . . 6
5.2. Authentication . . . . . . . . . . . . . . . . . . . . . 8
5.3. Keying Material . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Changes Since RFC 8773 . . . . . . . . . . . . . . . 15
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16

1. Introduction

The TLS 1.3 [I-D.ietf-tls-rfc8446bis] [RFC9846] handshake protocol provides two mutually
exclusive forms of server authentication. First, the server can be
authenticated by providing a signature certificate and creating a
valid digital signature to demonstrate that it possesses the
corresponding private key. Second, the server can be authenticated
by demonstrating that it possesses a pre-shared key (PSK) that was
established by a previous handshake. A PSK that is established in
this fashion is called a resumption PSK. A PSK that is established
by any other means is called an external PSK.

A TLS 1.3 server that is authenticating with a certificate may
optionally request a certificate from the TLS 1.3 client for
authentication
authentication, as described in Section 4.3.2 of
[I-D.ietf-tls-rfc8446bis]. [RFC9846].

This document specifies a TLS 1.3 extension permitting certificate-
based authentication and providing an external PSK to be input to the
TLS 1.3 key schedule.

Please see Appendix A for a list of changes since the publication of
RFC 8773.
[RFC8773].

2. Terminology Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Motivation and Design Rationale

There are two motivations for using a certificate with an external
PSK.

One motivation is confidentiality protection against the future
invention of a Cryptographically Relevant Quantum Computer (CRQC),
which would pose a serious challenge for the asymmetric cryptographic
algorithms that are widely deployed today, including the digital
signature algorithms that are used to authenticate the server in the
TLS 1.3 handshake protocol and key agreement algorithm used to
establish a pairwise shared secret between the client and server. It
is an open question whether or not it is feasible to build such a
quantum computer, and computer and, if so, when that might happen. However, if
such a quantum computer is invented, many of the cryptographic
algorithms and the security protocols that use them would become
vulnerable. In particular, The the TLS 1.3 handshake protocol employs
key agreement algorithms that could be broken by the invention of a
CRQC [I-D.ietf-pquip-pqc-engineers]. [PQC]. Including a strong external PSK in the TLS 1.3 key
schedule offers confidentiality protection against the long-term
quantum computing threat; it requires the attacker to learn the
external PSK as well as the shared secret produced by the key
agreement algorithm to break confidentiality.

The term strong "strong external PSK PSK" is used to mean that the PSK that has
been generated and distributed in such a way that the invention of a
CRQC will not allow the owner of that quantum computer to learn the
PSK. While generation and distribution of the PSK are outside the
scope of this document, in the context of a CRQC, security of the TLS
1.3 session using the strong external PSK relies on and is implicitly
tied to the confidentiality, entropy, and authenticity of the PSK.
Thus, the security claims in this document depend on generation and
distribution of the strong external PSK.

When a certificate is used for authentication, the authentication is
provided by the existing certificate and digital signature
mechanisms. This authentication cannot be relied upon if a CRQC is
ever invented. The addition of a strong external PSK in the TLS 1.3
key schedule does not offer improvement against the long-term quantum
computing threat regarding authentication.

Likewise, a raw public key can be provided provided, as described in
[RFC7250].

Quantum-resistant public-key cryptographic algorithms are becoming
standards, but it will take many years for TLS 1.3 ciphersuites that
use these algorithms to be developed and deployed. In some
environments, deployment of a strong external PSK provides protection
until these quantum-resistant algorithms are deployed.

Another motivation is the use of a public key with a factory-
provisioned secret value for the initial enrollment of a device in an
enterprise network [I-D.ietf-emu-bootstrapped-tls]. [TLS-POK].

4. Extension Overview

This section provides a brief overview of the
"tls_cert_with_extern_psk" extension.

The client includes the "tls_cert_with_extern_psk" extension in the
ClientHello message. The "tls_cert_with_extern_psk" extension MUST
be accompanied by the "key_share", "supported_groups",
"psk_key_exchange_modes", and "pre_shared_key" extensions. Since the
"tls_cert_with_extern_psk" extension is intended to be used only with
initial handshakes, it MUST NOT be sent alongside the "early_data"
extension. These extensions are all described in Section 4.2 of
[I-D.ietf-tls-rfc8446bis],
[RFC9846], which also requires the "pre_shared_key" extension to be
the last extension in the ClientHello message.

If the client includes both the "tls_cert_with_extern_psk" extension
and the "early_data" extension, then the server MUST terminate the
connection with an "illegal_parameter" alert.

If the server is willing to use one of the external PSKs listed in
the "pre_shared_key" extension and perform certificate-based
authentication, then the server includes the
"tls_cert_with_extern_psk" extension in the ServerHello message. The
"tls_cert_with_extern_psk" extension MUST be accompanied by the
"key_share" and "pre_shared_key" extensions. If none of the external
PSKs in the list provided by the client is acceptable to the server,
then the "tls_cert_with_extern_psk" extension is omitted from the
ServerHello message.

When the "tls_cert_with_extern_psk" extension is successfully
negotiated, the TLS 1.3 key schedule processing includes both the
selected external PSK and the (EC)DHE shared secret value. (EC)DHE
refers to Diffie-Hellman over either finite fields or elliptic
curves. As a result, the Early Secret, Handshake Secret, and Main
Secret (previously known as the Master Secret) values all depend upon
the value of the selected external PSK. Of course, the Early Secret
does not depend upon the (EC)DHE shared secret.

The authentication of the server and optional authentication of the
client depend upon the ability to generate a signature that can be
validated with the public key in their certificates. The
authentication processing is not changed in any way by the selected
external PSK. As a result, if a CRQC is ever invented, the digital
signature algorithm will need to be replaced with a quantum resistant quantum-resistant
one. Failure to do so will result in vulnerable entity and data
authentication mechanisms.

Each external PSK is associated with a single hash algorithm, which
is required by Section 4.2.11 of [I-D.ietf-tls-rfc8446bis]. [RFC9846]. The hash algorithm MUST
be set when the PSK is established, with a default of SHA-256.

5. Certificate with External PSK Extension

This section specifies the "tls_cert_with_extern_psk" extension,
which MAY appear in the ClientHello message and ServerHello message.
It MUST NOT appear in any other messages. The
"tls_cert_with_extern_psk" extension MUST NOT appear in the
ServerHello message unless the "tls_cert_with_extern_psk" extension
appeared in the preceding ClientHello message. If an implementation
recognizes the "tls_cert_with_extern_psk" extension and receives it
in any other message, then the implementation MUST abort the
handshake with an "illegal_parameter" alert.

The general extension mechanisms enable clients and servers to
negotiate the use of specific extensions. Clients request extended
functionality from servers with the extensions field in the
ClientHello message. If the server responds with a HelloRetryRequest
message, then the client sends another ClientHello message as
described in Section 4.1.2 of [I-D.ietf-tls-rfc8446bis], [RFC9846], including the same
"tls_cert_with_extern_psk" extension as the original ClientHello
message, or aborts the handshake.

Many server extensions are carried in the EncryptedExtensions
message; however, the "tls_cert_with_extern_psk" extension is carried
in the ServerHello message. Successful negotiation of the
"pre_shared_key" extension enables certificate verification to take
place in addition to the inclusion of the external PSK in the key
schedule. The external PSK is identified by the "key_share"
extension, and the inclusion of the external PSK in the key schedule
affects the key used for encryption. The "tls_cert_with_extern_psk"
extension is only present in the ServerHello message if the server
recognizes the "tls_cert_with_extern_psk" extension and the server
possesses one of the external PSKs offered by the client in the
"pre_shared_key" extension in the ClientHello message.

The Extension structure is defined in [I-D.ietf-tls-rfc8446bis]; [RFC9846]; it is repeated here
for convenience.

struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;

The "extension_type" identifies the particular extension type, and
the "extension_data" contains information specific to the particular
extension type.

This document specifies the "tls_cert_with_extern_psk" extension,
adding one new type to ExtensionType:

enum {
tls_cert_with_extern_psk(33), (65535)
} ExtensionType;

The "tls_cert_with_extern_psk" extension is relevant when the client
and server possess an external PSK in common that can be used as an
input to the TLS 1.3 key schedule. The "tls_cert_with_extern_psk"
extension is essentially a flag to use the external PSK in the key
schedule, and it has the following syntax:

struct {
select (Handshake.msg_type) {
case client_hello: Empty;
case server_hello: Empty;
};
} CertWithExternPSK;

5.1. Companion Extensions

Section 4 lists the extensions that are required to accompany the
"tls_cert_with_extern_psk" extension. Most of those extensions are
not impacted in any way by this specification. However, this section
discusses the extensions that require additional consideration.

The "psk_key_exchange_modes" extension is defined in Section 4.2.9 of
[I-D.ietf-tls-rfc8446bis].
[RFC9846]. The "psk_key_exchange_modes" extension restricts the use
of both the PSKs offered in this ClientHello and those that the
server might supply via a subsequent NewSessionTicket. As a result,
when the "psk_key_exchange_modes" extension is included in the
ClientHello message, clients MUST include psk_dhe_ke mode. In
addition, clients MAY also include psk_ke mode to support a
subsequent NewSessionTicket. When the "psk_key_exchange_modes"
extension is included in the ClientHello message, servers MUST select
the psk_dhe_ke mode for the initial handshake. Servers MUST select a
key exchange mode that is listed by the client for subsequent
handshakes that include the resumption PSK from the initial
handshake.

The "pre_shared_key" extension is defined in Section 4.2.11 of
[I-D.ietf-tls-rfc8446bis].
[RFC9846]. The syntax is repeated below for convenience. All of the
listed PSKs MUST be external PSKs. If a resumption PSK is listed
along with the "tls_cert_with_extern_psk" extension, the server MUST
abort the handshake with an "illegal_parameter" alert.

struct {
opaque identity<1..2^16-1>;
uint32 obfuscated_ticket_age;
} PskIdentity;

opaque PskBinderEntry<32..255>;

struct {
PskIdentity identities<7..2^16-1>;
PskBinderEntry binders<33..2^16-1>;
} OfferedPsks;

struct {
select (Handshake.msg_type) {
case client_hello: OfferedPsks;
case server_hello: uint16 selected_identity;
};
} PreSharedKeyExtension;

"OfferedPsks" contains the list of PSK identities and associated
binders for the external PSKs that the client is willing to use with
the server.

The identities are a list of external PSK identities that the client
is willing to negotiate with the server. Each external PSK has an
associated identity that is known to the client and the server; the
associated identities may be known to other parties as well. In
addition, the binder validation (see below) confirms that the client
and server have the same key associated with the identity.

The "obfuscated_ticket_age" is not used for external PSKs. As stated
in Section 4.2.11 of [I-D.ietf-tls-rfc8446bis], [RFC9846], clients SHOULD set this value to 0,
and servers MUST ignore the value.

The binders are a series of HMAC [RFC2104] values, one for each
external PSK offered by the client, in the same order as the
identities list. The HMAC value is computed using the binder_key,
which is derived from the external PSK, and a partial transcript of
the current handshake. Generation of the binder_key from the
external PSK is described in Section 7.1 of
[I-D.ietf-tls-rfc8446bis]. [RFC9846]. The partial
transcript of the current handshake includes a partial ClientHello up
to and including the PreSharedKeyExtension.identities field, as
described in Section 4.2.11.2 of [I-D.ietf-tls-rfc8446bis]. [RFC9846].

The "selected_identity" contains the index of the external PSK
identity that the server selected from the list offered by the
client. As described in Section 4.2.11 of [I-D.ietf-tls-rfc8446bis], [RFC9846], the server MUST
validate the binder value that corresponds to the selected external
PSK, and if the binder does not validate, the server MUST abort the
handshake with an "illegal_parameter" alert.

5.2. Authentication

When the "tls_cert_with_extern_psk" extension is successfully
negotiated, authentication of the server depends upon the ability to
generate a signature that can be validated with the public key. When
the server uses a certificate, this is accomplished by the server
sending the Certificate and CertificateVerify messages, as described
in Sections 4.4.2 and 4.4.3 of [I-D.ietf-tls-rfc8446bis]. [RFC9846]. Alternatively, the server
can use a raw public key key, as described in [RFC7250].

TLS 1.3 does not permit the server to send a CertificateRequest
message when a PSK is being used. This restriction is removed when
the "tls_cert_with_extern_psk" extension is negotiated, allowing
certificate-based authentication for both the client and the server.
If certificate-based client authentication is desired, this is
accomplished by the client sending the Certificate and
CertificateVerify messages as described in Sections 4.4.2 and 4.4.3
of [I-D.ietf-tls-rfc8446bis]. [RFC9846].

5.3. Keying Material

Section 7.1 of [I-D.ietf-tls-rfc8446bis] [RFC9846] specifies the TLS 1.3 key schedule. The
successful negotiation of the "tls_cert_with_extern_psk" extension
requires the key schedule processing to include both the external PSK
and the (EC)DHE shared secret value.

If the client and the server have different values associated with
the selected external PSK identifier, then the client and the server
will compute different values for every entry in the key schedule,
which will lead to the client aborting the handshake with a
"decrypt_error" alert.

6. IANA Considerations

Once this document is approved,

IANA is asked to update has updated the "TLS ExtensionType Values" registry [IANA] entry
for the "tls_cert_with_extern_psk" extension to reference this
document.

7. Security Considerations

The Security Considerations in [I-D.ietf-tls-rfc8446bis] [RFC9846] remain relevant.

TLS 1.3 [I-D.ietf-tls-rfc8446bis] [RFC9846] does not permit the server to send a
CertificateRequest message when a PSK is being used. This
restriction is removed when the "tls_cert_with_extern_psk" extension
is offered by the client and accepted by the server. However, TLS
1.3 does not permit an external PSK to be used in the same fashion as
a resumption PSK, and this extension preserves that restriction.

Implementations must protect the external pre-shared key (PSK).
Compromise of the external PSK will make the encrypted session
content vulnerable to the future development of a Cryptographically
Relevant Quantum Computer (CRQC). However, the generation,
distribution, and management of the external PSKs is out of scope for
this specification.

Implementers should not transmit the same content on a connection
that is protected with an external PSK and a connection that is not.
Doing so may allow an eavesdropper to correlate the connections,
making the content vulnerable to the future invention of a CRQC.

Implementations must generate external PSKs with a secure key-
management technique, such as pseudorandom generation of the key or
derivation of the key from one or more other secure keys. The use of
inadequate pseudorandom number generators (PRNGs) to generate
external PSKs can result in little or no security. An attacker may
find it much easier to reproduce the PRNG environment that produced
the external PSKs and search the resulting small set of
possibilities, rather than brute-force searching the whole key space.
The generation of quality random numbers is difficult. [RFC4086]
offers important guidance in this area.

Implementations must use a ciphersuite that includes a symmetric
encryption algorithm with sufficiently large keys. For protection
against the future invention of a CRQC, the symmetric key needs to be
at least 128 bits. While Grover’s Grover's algorithm (described in
Section 2.1 3.1 of [I-D.ietf-pquip-pqc-engineers]) [PQC]) allows a quantum computer to perform a brute
force key search using quadratically fewer steps than would be
required with classical computers, there are a number of mitigating
factors suggesting that Grover’s Grover's algorithm will not speed up a brute
force symmetric key search as dramatically as one might suspect.
First, quantum computing hardware will likely be more expensive to
build and use than classical hardware. Second, to obtain the full
quadratic speedup, all the steps of Grover’s Grover's algorithm must be
performed in series. However, attacks on cryptography use massively
parallel processing, processing; the advantage of Grover's algorithm will be
smaller.

Implementations must use sufficiently large external PSKs. For
protection against the future invention of a CRQC, the external PSK
needs to be at least 128 bits.

TLS 1.3 [I-D.ietf-tls-rfc8446bis] [RFC9846] has received careful security analysis, and the
following informal reasoning shows that the addition of this
extension does not introduce any security defects in the threat model
of a traditional adversary, that is is, an adversary that does not have
access to a CRQC. This extension requires the use of certificates
for authentication, but the processing of certificates is unchanged
by this extension. This extension requires an external PSK in the
key schedule as part of the computation of the Early Secret. In the
initial handshake without an external PSK in
[I-D.ietf-tls-rfc8446bis], [RFC9846], the Early
Secret is computed as:

Early Secret = HKDF-Extract(0, 0)

With this extension, the Early Secret is computed as:

Early Secret = HKDF-Extract(0, External PSK)

Any entropy contributed by the external PSK can only make the Early
Secret better; the External external PSK cannot make it worse. Thus, TLS 1.3
continues to meet well-studied confidentiality goals when this
extension is used.

Even when the external PSK is not known to any party other than the
client and the server, then the external PSK MUST NOT be the sole basis
for authentication. The reasoning is explained in Section 4.2 of
[K2016]. The authentication of the server and optional
authentication of the client depend upon the ability to generate a
signature that can be validated with the public key in their
certificates. The authentication processing is not changed in any
way by the selected external PSK.

This external PSK preserves some confidentiality and authentication
even if the (EC)DH key agreement is broken by cryptanalysis or the
future invention of a CRQC. As long as the attacker does not know
the PSK and the key derivation algorithm remains unbroken, the
attacker cannot derive the session secrets, even if the attacker is
able to compute the (EC)DH shared secret. While the ephemeral (EC)DH
private key used during a given TLS 1.3 session is destroyed before
the end of a session, the (EC)DH private key would nevertheless be
recoverable due to the break of the (EC)DH algorithm. However, a
more general notion of "secrecy after key material is destroyed"
would still be achievable using external PSKs, if they are managed in
a way that ensures their destruction when they are no longer needed,
and with the assumption that the symmetric algorithms remain safe
against the invention of a CRQC.

The forward-secrecy advantages traditionally associated with
ephemeral (EC)DH keys are not easily replaced by external PSKs. The
confidentially and authentication provided by the external PSK depend
on whether the external PSK is used for more than one TLS 1.3 session
and the parties that know the external PSK. Assuming the (EC)DH key
agreement is broken:

* If the external PSK is used for a single TLS 1.3 session and it is
known only by the client and server, then the usual TLS 1.3
confidentially and authentication is provided, including the
cryptographic separation between TLS 1.3 sessions. Of course,
this places a significant burden on the generation and
distribution of external PSKs.

* If the external PSK is used for more than one TLS 1.3 session and
it is known only by the client and server, then the confidentially
is limited to the client and server, but there is no cryptographic
separation between TLS 1.3 sessions.

* If the external PSK is used for more than one TLS 1.3 session and
it is known by the client, server server, and others, then the
confidentially is limited to the group that knows the external
PSK, but there is no cryptographic separation between TLS 1.3
sessions.

This specification does not require that the external PSK is known
only by the client and server. The external PSK may be known to a
group. Since authentication depends on the public key in a
certificate, knowledge of the external PSK by other parties does not
enable impersonation. The authentication of the server and optional
authentication of the client depend upon the ability to generate a
signature that can be validated with the public key in their
certificates. The authentication processing is not changed in any
way by the selected external PSK.

Confidentiality depends on the shared secret from (EC)DH, so
knowledge of the external PSK by other parties does not enable
eavesdropping. However, group members can record the traffic of
other members and then decrypt that traffic if they ever gain access
to a CRQC. Also, when many parties know the external PSK, there are
many opportunities for theft of the external PSK by an attacker.
Once an attacker has the external PSK, they can decrypt stored
traffic if they ever gain access to a CRQC, in the same manner as a
legitimate group member.

TLS 1.3 key derivation makes use of the HMAC-based Key Derivation
Function (HKDF) algorithm, which depends upon the HMAC [RFC2104]
construction and a hash function. This extension provides the
desired protection for the session secrets, as long as HMAC with the
selected hash function is a pseudorandom function (PRF) [GGM1986].

TLS 1.3 [I-D.ietf-tls-rfc8446bis] [RFC9846] takes a conservative approach to PSKs; they are
bound to a specific hash function and KDF. By contrast, TLS 1.2
[RFC5246] allows PSKs to be used with any hash function and the TLS
1.2 PRF. Thus, the safest approach is to use a PSK exclusively with
TLS 1.2 or exclusively with TLS 1.3. Given one PSK, one can derive a
PSK for exclusive use with TLS 1.2 and derive another PSK for
exclusive use with TLS 1.3 using the mechanism specified in
[RFC9258].

8. Privacy Considerations

Appendix F.6 of [I-D.ietf-tls-rfc8446bis] [RFC9846] discusses identity-exposure attacks on
PSKs. Also, Appendix C.4 of [I-D.ietf-tls-rfc8446bis] [RFC9846] discusses tracking prevention.
The guidance in these sections remain relevant.

If an external PSK identity is used for multiple connections, then an
observer will generally be able track clients and/or servers across
connections. The rotation of the external PSK identity or the use of
the Encrypted Client Hello extension [I-D.ietf-tls-esni] [RFC9849] can mitigate this
risk.

This extension makes use of external PSKs to improve resilience
against attackers that gain access to a CRQC in the future and
provides authentication for initial enrollment of devices in an
enterprise network. This extension is always accompanied by the
"pre_shared_key" extension to provide the PSK identities in plaintext
in the ClientHello message. Passive observation of the these PSK
identities will aid an attacker in tracking users or devices that
make use of this extension.

9. References

9.1. Normative References

[I-D.ietf-tls-rfc8446bis]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", Work in Progress, Internet-Draft, draft-
ietf-tls-rfc8446bis-13, 21 July 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
rfc8446bis-13>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC9846] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 9846, DOI 10.17487/RFC9846, January
2026, <https://www.rfc-editor.org/info/rfc9846>.

9.2. Informative References

[Err7598] RFC Editor, "RFC Errata 7598", Errata, Erratum ID 7598, RFC 8773,
<https://www.rfc-editor.org/errata/eid7598>.

[GGM1986] Goldreich, O., Goldwasser, S., and S. Micali, "How to
construct random functions", Journal of the ACM, Vol. ACM (JACM),
vol. 33,
No. no. 4, pp. 792-807, DOI 10.1145/6490.6503, August
1986, <https://dl.acm.org/doi/10.1145/6490.6503>.

[I-D.ietf-emu-bootstrapped-tls]
Friel, O. and D. Harkins, "Bootstrapped TLS

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

[K2016] Krawczyk, H., "A Unilateral-to-Mutual Authentication
Compiler for Key Exchange (with Applications to Client
Authentication
with Proof of Knowledge (TLS-POK)", Work in Progress,
February 2025, <https://datatracker.ietf.org/doc/html/
draft-ietf-emu-bootstrapped-tls-08>.

[I-D.ietf-pquip-pqc-engineers] TLS 1.3)", Cryptology ePrint Archive,
Paper 2016/711, 2016, <https://eprint.iacr.org/2016/711>.

[PQC] Banerjee, A., Reddy.K, T., Schoinianakis, D., Hollebeek,
T., and M. Ounsworth, "Post-Quantum Cryptography for
Engineers", Work in Progress, Internet-Draft, draft-ietf-
pquip-pqc-engineers-14, 25 August 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-pquip-
pqc-engineers-14>.

[I-D.ietf-tls-esni]
Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", Work in Progress, Internet-Draft,
draft-ietf-tls-esni-25, 14 June 2025,
<https://datatracker.ietf.org/doc/html/draft-ietf-tls-
esni-25>.

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

[K2016] Krawczyk, H., "A Unilateral-to-Mutual Authentication
Compiler for Key Exchange (with Applications to Client
Authentication in TLS1.3)", cryptoeprint 2016/711, 1
September 2016, <https://eprint.iacr.org/2016/711>.

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[RFC8773] Housley, R., "TLS 1.3 Extension for Certificate-Based
Authentication with an External Pre-Shared Key", RFC 8773,
DOI 10.17487/RFC8773, March 2020,
<https://www.rfc-editor.org/info/rfc8773>.

[RFC9258] Benjamin, D. and C. A. Wood, "Importing External Pre-
Shared Keys (PSKs) for TLS 1.3", RFC 9258,
DOI 10.17487/RFC9258, July 2022,
<https://www.rfc-editor.org/info/rfc9258>.

[RFC9849] Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
Encrypted Client Hello", RFC 9849, DOI 10.17487/RFC9849,
March 2026, <https://www.rfc-editor.org/info/rfc9849>.

[TLS-POK] Friel, O. and D. Harkins, "Bootstrapped TLS Authentication
with Proof of Knowledge (TLS-POK)", Work in Progress,
October 2025, <https://datatracker.ietf.org/doc/html/
draft-ietf-emu-bootstrapped-tls-11>.

Appendix A. Changes Since RFC 8773

The status elevation from Experimental RFC to Standards Track RFC is
the most significant change in this document.

In addition to minor editorial updates, which include a change to the
title, the following changes were made:

* Correct the order of the arguments to HKDF-Extract when an
external PSK is present.

* The client must include the "supported_groups" extension in the
ClientHello message.

* Expand the motivation discussion to talk about protection against
the future development of a Cryptographically Relevant Quantum
Computer (CRQC) and enrollment in enterprise networks.

* Separate the discussion of confidentiality and authentication.
The inclusion of the external PSK offers some confidentiality
protection against the future invention of a CRQC, but the
external PSK does not improve authentication.

* Correct RFC Errata Erratum 7598 [Err7598].

* Add a discussion of TLS Encrypted Client Hello to the Privacy
Considerations.
Considerations section.

* Adopt terminology that has become widely accepted, such as CRQC
and Main Secret (instead of Master Secret).

* Provide URLs for all references.

Acknowledgments

Many thanks to Liliya Akhmetzyanova, Roman Danyliw, Christian
Huitema, Ben Kaduk, Geoffrey Keating, Hugo Krawczyk, Mirja Kühlewind,
Nikos Mavrogiannopoulos, Nick Sullivan, Martin Thomson, and Peter Yee
for their review and comments on the Internet-Drafts that eventually
became RFC 8773; their efforts have improved the document.

Many thanks to Dan Harkins, Owen Friel, John Preuß Mattsson,
Christian Huitema, Joe Salowey, Britta Hale, Peter Yee, Joe Mandel,
Muhammad Usama Sardar, Paul Wouters, Mike Bishop, Deb Cooley, and
Eric Rescorla for their review and comments on the updates to RFC
8773 that became this document; their efforts have improved the
document.

Author's Address

Russ Housley
Vigil Security, LLC
516 Dranesville Road
Herndon, VA 20170
United States of America
Email: housley@vigilsec.com