Paper 2026/1147

FATT Chance: On the Robustness of Standalone and Hybrid ML-KEM Key Exchange in TLS 1.3

Abstract

Two post-quantum upgrades to TLS 1.3 are being standardized in parallel: a hybrid key exchange (already deployed) that runs an elliptic-curve Diffie-Hellman exchange alongside ML-KEM, and a standalone mode that uses ML-KEM on its own. The Internet-Draft draft-usama-tls-risks-of-mlkem points out that the machine-checked symbolic proofs of TLS 1.3 rely on the commutativity of Diffie-Hellman, which ML-KEM does not share: a key encapsulation mechanism is asymmetric, one endpoint generating a key pair and the other encapsulating against it. The existing proofs therefore no longer apply, a new one is needed, and the draft argues that hybrids should be preferred. We supply that proof. We extend the reftls ProVerif models with a faithful, non-commutative KEM and analyze classical (EC)DHE, standalone ML-KEM, and the hybrid together, as unbounded concurrent sessions against a single active attacker free to break any cryptographic component. The central result is a sharp and tight contrast in robustness: standalone ML-KEM is a single point of failure, secure only while ML-KEM itself is unbroken, whereas the hybrid stays secure as long as either of its components survives: an attacker must break both, in one session, to learn anything. This single point of failure reaches authentication as well as confidentiality: with the sole key-exchange secret exposed and no secret pre-shared key salting the key schedule, the server Finished message becomes forgeable, so a client can complete a handshake that no server completed, while the hybrid stays safe unless both components break. The three modes also interoperate without ever confusing one another's keys, so migrating from (EC)DHE to a hybrid is a strict improvement. Two further experiments address the draft's remaining concerns: reusing an ML-KEM key forfeits the forward secrecy that an ephemeral key preserves, and a principal acting as both initiator and responder exposes no role-confusion attack arising from the asymmetry. At the symbolic level, and under stated assumptions, the analysis substantiates the draft's case for preferring hybrid key exchange.