Post-Quantum Enhancements to TLS-Based EAP Methods

Introduction The emergence of a Cryptographically Relevant Quantum Computer (CRQC) would break the mathematical assumptions that underpin widely deployed public-key algorithms, rendering them insecure and obsolete. As a result, there is an urgent need to update protocols and infrastructure with post-quantum cryptographic (PQC) algorithms designed to resist attacks from both quantum and classical adversaries. The cryptographic primitives requiring replacement are discussed in , and the NIST PQC Standardization process has initially selected algorithms such as ML-KEM , ML-DSA , and SLH-DSA for usage in security protocols. To mitigate the risks posed by a CRQC, such as the potential compromise of encrypted data and the forging of digital signatures, existing security protocols must be upgraded to support PQC. These risks include "Harvest Now, Decrypt Later" (HNDL) attacks, where adversaries capture encrypted traffic today with the intent to decrypt it once CRQCs become available. TLS-based EAP methods are widely used for network access authentication in enterprise and wireless environments. This document applies to all EAP methods that use TLS as their underlying transport, including EAP-TLS , EAP-TTLS , PEAP , and TEAP . The use of TLS 1.3 with EAP-TTLS and PEAP is specified in . To continue providing long-term confidentiality and authentication guarantees, these methods must evolve to incorporate post-quantum algorithms. However, transitioning these protocols to support PQC introduces practical challenges. highlights issues related to large certificates and certificate chains in EAP-TLS, which can lead to session failures due to round-trip limitations. PQC certificates and certificate chains tend to be significantly larger than their traditional counterparts, further exacerbating these issues by increasing TLS handshake sizes and the likelihood of session failures. To address these challenges, this document specifies post-quantum key agreement and authentication requirements for TLS-based EAP methods, together with a mechanism that reduces TLS handshake size for use in constrained network environments.

Conventions and Definitions 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 when, and only when, they appear in all capitals, as shown here. This document adopts terminology defined in . For the purposes of this document, it is useful to categorize cryptographic algorithms into three distinct classes:

Traditional Algorithm: An asymmetric cryptographic algorithm based on integer factorization, finite field discrete logarithms, or elliptic curve discrete logarithms. In the context of TLS, an example of a traditional key exchange algorithm is Elliptic Curve Diffie-Hellman (ECDH), which is almost exclusively used in its ephemeral mode, referred to as Elliptic Curve Diffie-Hellman Ephemeral (ECDHE).
Post-Quantum Algorithm: An asymmetric cryptographic algorithm designed to be secure against attacks from both quantum and classical computers. An example of a post-quantum key exchange algorithm is the Module-Lattice Key Encapsulation Mechanism (ML-KEM).
Hybrid Algorithm: We distinguish between key exchanges and signature algorithms:
- Hybrid Key Exchange: A key exchange mechanism that combines two component algorithms
  - one traditional algorithm and one post-quantum algorithm. The resulting shared secret remains secure as long as at least one of the component key exchange algorithms remains unbroken.
- PQ/T Hybrid Digital Signature: A multi-algorithm digital signature scheme composed of two or more component signature algorithms, where at least one is a post-quantum algorithm and at least one is a traditional algorithm.

Digital signature algorithms play a critical role in X.509 certificates, Certificate Transparency Signed Certificate Timestamps, Online Certificate Status Protocol (OCSP) statements, and any other mechanism that contributes signatures during a TLS handshake or in the context of a secure communication establishment.

Data Confidentiality in TLS-Based EAP Methods One of the primary threats to TLS-based EAP methods is the HNDL attack. In this scenario, adversaries can passively capture EAP-TLS handshakes such as those transmitted over the air in Wi-Fi networks and store them for future decryption once CRQCs become available. While EAP-TLS 1.3 provides forward secrecy through ephemeral key exchange and improves privacy by encrypting client identity and reducing exposure of session metadata, these protections rely on the security of the underlying key exchange algorithm. In the presence of a CRQC, traditional key exchange mechanisms (e.g., ECDHE) would no longer provide long-term confidentiality. In such cases, an adversary could mount an HNDL attack by passively recording EAP-TLS handshakes and decrypting the captured traffic once quantum-capable cryptanalysis becomes feasible. This could retroactively expose information that TLS 1.3 is otherwise designed to protect, including:

The identity of the authenticated client.
Client credentials used in certificate-based authentication (e.g., usernames, device or organization identifiers).

In the case of EAP-TTLS, PEAP, and TEAP, HNDL attacks present an additional threat. These methods typically carry legacy inner authentication protocols within the outer TLS tunnel, such as MS-CHAPv2. If a CRQC is used to break the outer TLS tunnel, the exposed inner authentication exchange could enable offline password attacks, potentially allowing an adversary to recover user credentials. To protect against HNDL attacks, TLS-based EAP deployments that require long-term confidentiality MUST use TLS 1.3 and MUST negotiate a post-quantum or PQ/T hybrid key agreement group. Traditional key agreement groups alone do not provide long-term confidentiality against an adversary equipped with a CRQC. EAP peers and EAP servers MUST support at least one post-quantum or PQ/T hybrid key agreement group registered in the "TLS Supported Groups" registry. PQ/T hybrid groups combining ML-KEM with ECDHE are constructed as described in and registered in . This document does not mandate a specific group. The Recommended column of the "TLS Supported Groups" registry, defined in , records current IETF guidance on the use of registered groups. At the time of writing, X25519MLKEM768 (code point 4588) is the only PQ/T hybrid group marked Recommended. PQ/T hybrid key agreement is generally preferred, because the shared secret remains secure as long as either component algorithm remains unbroken . Standalone ML-KEM key agreement may be required for deployments subject to regulatory or compliance mandates that require exclusive use of post-quantum cryptography. The choice is a deployment decision and is out of scope for this document.

Post-Quantum Authentication in TLS-Based EAP Methods Although a CRQC would primarily impact the confidentiality of recorded TLS sessions, it could also pose risks to authentication mechanisms that rely on traditional public-key algorithms with long-lived credentials. In particular, if quantum-capable cryptanalysis were to become practical within the validity period of a certificate, an adversary could recover the private key corresponding to a traditionally signed certificate and subsequently impersonate the certificate holder in real time. The feasibility and impact of such attacks depend on several factors, including certificate lifetimes and key management practices. TLS-based EAP deployments rely on X.509 certificates issued by CAs, and the transition to PQ certificate authentication is constrained by the long lifecycle associated with distributing, deploying, and validating new trust anchors. If CRQCs arrive sooner than anticipated, deployed authentication systems may lack the agility to transition credentials and trust anchors in a timely manner. As a result, deployments that rely on long-lived certificates or that require resistance to future quantum-capable adversaries face an increased risk of authentication compromise. In such scenarios, an on-path attacker that is able to recover a server's private key within the certificate validity period could impersonate access points (APs) in real time, potentially deceiving users into revealing credentials or connecting to rogue networks. To mitigate these risks, TLS-based EAP deployments that require resistance to future CRQCs MUST support at least one post-quantum or PQ/T hybrid signature scheme registered in the "TLS SignatureScheme" registry for authentication. ML-DSA is specified for use in X.509 certificates in and for authentication in TLS 1.3 in . This document does not mandate a specific signature scheme or parameter set. The Recommended column of the "TLS SignatureScheme" registry, defined in , records current IETF guidance on the use of registered schemes. PQ/T hybrid authentication using composite signatures may be preferred by deployments seeking defense in depth during the transition, so that authentication remains secure as long as either component algorithm remains unbroken. Composite ML-DSA is specified for X.509 in and for TLS 1.3 in . The choice between pure post-quantum and PQ/T hybrid authentication is a deployment decision and is out of scope for this document. The relatively large SLH-DSA signatures may make SLH-DSA less suitable for TLS-based EAP deployments, particularly where handshake size and fragmentation are significant constraints. Signature sizes are given in . The end-entity certificate and the CertificateVerify each carry a signature in every handshake, and neither can be avoided by the mechanism in , which removes only intermediate certificates. A post-quantum or PQ/T hybrid end-entity certificate does not by itself provide post-quantum authentication. Every signature in the path up to the trust anchor has to use a post-quantum or PQ/T hybrid scheme, and since the trust anchor is provisioned out of band, it has to be in place before the certificates that chain to it are issued. The use of PQ or PQ/T hybrid certificates increases the size of individual certificates, certificate chains, and signatures, resulting in significantly larger handshake messages. These larger payloads can lead to packet fragmentation, retransmissions, and handshake delays, issues that are particularly disruptive in constrained or lossy network environments. describes mitigations.

EST Integration The EAP client is expected to validate the certificate presented by the EAP server using a trust anchor that is provisioned out-of-band prior to authentication (e.g., using EST). The intermediate certificates are provided by the EAP server during the TLS handshake. The EAP client relies solely on the pre-provisioned trust anchor to build and validate the certificate chain. This model assumes a managed deployment environment with explicitly configured trust relationships between the EAP client and EAP server. Certificate compression provides limited benefit for certificates containing large high-entropy post-quantum public keys and signatures, and session resumption requires a prior full handshake. Out-of-band provisioning of the intermediate chain can avoid transmitting it during the first TLS authentication of a newly provisioned device. To further reduce handshake overhead, particularly in deployments using large certificate chains due to post-quantum (PQ) or composite certificates, this document specifies an optimization that leverages the Enrollment over Secure Transport (EST) protocol , extended by . Specifically, it allows intermediate certificates to be retrieved in advance by using EST, thereby avoiding the need to transmit them during each TLS handshake. The optimization applies only when every EAP client that authenticates to the EAP server has retrieved its intermediate certificates as part of provisioning. For EAP methods that use TLS as an outer tunnel (e.g., PEAP and TEAP), the EST optimization described in this section applies to the certificates used in the outer TLS tunnel. The EST pre-fetching of client intermediate certificates is relevant only when mutual TLS authentication is used. This is always the case for EAP-TLS, and optionally the case for EAP-TTLS and TEAP when client certificate authentication is used in the outer tunnel. This section defines extensions to EST to support retrieval of the certificate chain used by an EAP server and EAP clients. The first extension enables EAP clients to retrieve the intermediate certificates required to build a certification path to the EAP server's end-entity certificate. A new path component is defined under the EST well-known URI: The '/eapservercertchain' is intended for informational retrieval only and does not require client authentication. It allows clients to retrieve the intermediate certificate chain that the EAP server presents during TLS handshakes. This request is performed using the HTTPS protocol. The EST server MUST support requests without requiring client authentication. The EST server MUST provide the intermediate certificates of the CAs that issue EAP server certificates. The second extension enables EAP servers to retrieve the intermediate certificates required to build a certification path to the EAP clients' end-entity certificates. Rather than relying on static configuration, the EAP server can dynamically fetch the client's intermediate certificate chain from a trusted EST server within the same administrative domain. A new path component is defined under the EST well-known URI: The '/eapclientcertchain' is intended for informational retrieval only and does not require client authentication. It allows the EAP server to retrieve the intermediate certificate chain that the EAP clients present during TLS handshakes. This request is performed using the HTTPS protocol. The EST server MUST support requests without requiring client authentication. The EST server MUST provide the intermediate certificates of the CAs that issue EAP client certificates. The response to '/eapservercertchain' and '/eapclientcertchain' uses the format of the '/cacerts' response defined in , as updated by , and carries only intermediate CA certificates. Retrieved intermediate certificates are used, together with any certificates received in the TLS handshake, to construct a certification path from the end-entity certificate to the pre-provisioned trust anchor. Where an EAP server has certificates issued by more than one CA, the retrieved intermediate certificates cover all of them, and the EAP client selects those needed to construct a path for the certificate presented in the handshake. If no valid path can be constructed, authentication fails as for any other certificate validation failure. EAP clients and servers MUST authenticate the EST server using a trust anchor obtained via a suitable bootstrapping mechanism before retrieving intermediate certificate chains via HTTPS. Various bootstrapping mechanisms exist for establishing this trust, such as BRSKI , EST , or out-of-band provisioning. The choice of bootstrapping mechanism is a deployment decision and is out of scope for this document. Certificate chains retrieved from an unauthenticated or untrusted EST server MUST NOT be used for TLS chain validation. EAP servers and clients are RECOMMENDED to cache retrieved certificate chains. They SHOULD re-fetch them periodically, following HTTP caching rules , and before cached intermediate certificates expire. EAP clients MAY omit intermediate certificates from the TLS handshake only if they have been explicitly configured by the administrator to do so. Such configuration is recommended only in deployments where both the EAP client and EAP server support this specification and have completed EST pre-fetching as part of provisioning. If no such configuration is present, the EAP client MUST include the full certificate chain in the TLS handshake. Similarly, an EAP server MAY omit intermediate certificates from the TLS handshake only if it has been explicitly configured by the administrator to do so. Administrators are advised to ensure that clients in the deployment have retrieved the server's intermediate certificates via EST as part of their provisioning process before enabling this configuration. EST is one transport for retrieving intermediate certificates out of band. Device management systems can deliver the same chains, and TEAP could be extended to provision them within the tunnel it establishes. Note: A TLS extension could be used to explicitly signal support for intermediate certificate omission between peers, avoiding the need for administrator configuration. Such a mechanism is considered a possible future solution but is out of scope for this document.

Security Considerations The security considerations of and apply to the EST extensions defined in . If an intermediate CA certificate is renewed or replaced and the peer omits it from the TLS handshake, path construction fails until the cached intermediate certificates are refreshed. The security considerations outlined in and must be carefully evaluated and taken into account for all TLS-based EAP deployments.

IANA Considerations This document defines two new path components under the EST well-known URI '/.well-known/est/', following the extension mechanism established by : '/eapservercertchain' and '/eapclientcertchain'. As these are sub-paths under the already-registered '/.well-known/est/' prefix defined in , no new IANA registry entries are required.

Acknowledgements Thanks to John Mattsson, Hannes Tschofenig, Alan DeKok, Michael Richardson, Junjun Chen, and Guilin Wang for the discussion and comments.