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/rfc9973.¶

Copyright Notice

Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶

1. Introduction

The TLS 1.3 [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, as described in Section 4.3.2 of [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 [RFC8773].¶

2. 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, 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 TLS 1.3 handshake protocol employs key agreement algorithms that could be broken by the invention of a CRQC [RFC9958]. 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 external 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, 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 [RFC9966].¶

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 [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 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 [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 [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 [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;

  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;

6. IANA Considerations

IANA 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 [RFC9846] remain relevant.¶

TLS 1.3 [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 PSK. Compromise of the external PSK will make the encrypted session content vulnerable to the future development of a 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 algorithm (described in Section 3.1 of [RFC9958]) 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 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 algorithm must be performed in series. However, attacks on cryptography use massively parallel 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 [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, 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 [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 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, 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 confidentiality 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 confidentiality 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 confidentiality 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, and others, then the confidentiality 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, if they ever gain access to a CRQC, they can decrypt stored traffic 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 [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 [RFC9846] discusses identity-exposure attacks on PSKs. Also, Appendix C.4 of [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 [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.¶

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 [Err8888].¶
• 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 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 Erratum 7598 [Err7598].¶
• Add a discussion of TLS Encrypted Client Hello to the Privacy Considerations section.¶
• Adopt terminology that has become widely accepted, such as CRQC and Main Secret (instead of Master Secret).¶
• Provide URLs for all references.¶

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