Transcript-Bound Combiners for Downgrade-Resilient Hybrid Post-Quantum Key Establishment: Definition, Proof, and Embedded-Device Cost
Researchers prove transcript-bound hybrid KEM combiners stop downgrade attacks, adding one hash and no extra messages.
Hybrid key establishment with ML-KEM and classical Diffie-Hellman protects the session key but not negotiation integrity when deployed without a handshake transcript. The paper defines a combiner-layer security game and proves that a combiner ignoring the transcript can be downgraded with certainty, while binding the session key and confirmation tag to a transcript hash blocks such attempts up to a negligible term for a 256-bit hash. On Cortex-M4, transcript binding adds one hash per party—about 11.8% of handshake computation and 1.5% of radio-inclusive energy—with no extra messages or bytes. Measurements come from a released harness that passes a 30-check validation gate.
- Hybrid KEM alone does not authenticate which primitives were negotiated.
- Transcript binding blocks downgrade up to a negligible 256-bit hash term.
- Cortex-M4 cost is one extra hash: 11.8% compute, 1.5% energy.
- Binding adds no messages or bytes on the wire.
- Design targets hybrid ML-KEM use in TLS, SSH, and IKE.
Full article249 words · extracted from arxiv.org · click to collapse
Hybrid key establishment runs a post-quantum key-encapsulation mechanism (KEM) alongside a classical Diffie-Hellman primitive, so that the session key stays secure while either component resists attack. This design is now standardized in the Transport Layer Security protocol, Secure Shell, and the Internet Key Exchange, with the standardized module-lattice KEM (ML-KEM) as the post-quantum component. A hybrid KEM secures the derived key, but not the integrity of the negotiation that selects which primitives are used. Full protocols authenticate that negotiation through a handshake transcript; a hybrid KEM deployed as a standalone drop-in primitive, or inside a minimal handshake without transcript authentication, inherits no such guarantee, and an active attacker can strip the post-quantum option. We ask what the key schedule alone must contain to make downgrade resilience a local property of the combiner. We give a game-based definition at the combiner layer and prove a two-sided separation: a combiner that ignores the transcript is downgraded with certainty, whereas one that binds the session key and the confirmation tag to a hash of the transcript blocks every such attempt, up to a term negligible for a 256-bit transcript hash. We also give an explicit strongest-link security bound. Using a calibrated cost model composed from published Cortex-M4 measurements, transcript binding adds one hash per party - about 11.8% of handshake computation but only 1.5% of radio-inclusive energy - and adds no messages or bytes on the wire. Every reported number is produced by a released harness that passes a 30-check validation gate.
Text extracted automatically; images, tables and formatting may be missing. Original: https://arxiv.org/abs/2609.21273