Hybrid KEM combiner design feedback: SHA3-256 concat vs HKDF for X25519 + ML-KEM-1024

[cleitonaugusto]

Context

I've been building a post-quantum authenticated command-and-control layer for MAVLink v2 drones using crates from this org: ml-kem for session key encapsulation, ml-dsa for per-command signing, hmac + sha3 for the session MAC. The library is at https://github.com/cleitonaugusto/CleitonQ if the code is useful context.

One thing I'd value feedback on is the hybrid KDF combiner.

The combiner pattern

For session establishment, I combine X25519 and ML-KEM-1024 shared secrets using a SHA3-256 concatenation combiner (NIST SP 800-227 §4.3.1):

pub fn derive_hybrid_session_key(x25519_ss: &[u8; 32], ml_kem_ss: &[u8; 32]) -> [u8; 32] {
    let mut h = Sha3_256::new();
    h.update(x25519_ss);
    h.update(ml_kem_ss);
    h.update(b"cleitonq-hybrid-v1");  // domain separation
    h.finalize().into()
}

The rationale: secure as long as either component is pseudo-random (so a broken X25519 or a broken ML-KEM-1024 doesn't compromise the session key in isolation), and SHA3-256 as a random oracle gives this property.

The question: would you prefer HKDF-SHA3-256(salt=x25519_ss, ikm=ml_kem_ss, info=label) over the concatenation form here? HKDF has the extract-then-expand structure that seems designed for exactly this kind of multi-source key material, but SP 800-227 lists both as valid combiners for hybrid KEMs. Is there a reason to prefer one over the other when both inputs are fixed-length (32 bytes) and the output is used directly as a MAC key?

The relay finding (as context for the use case)

During testing I found something that might be worth noting for anyone else building authenticated protocols over MAVLink: any authentication material appended after a MAVLink v2 frame boundary — after the CRC — is silently stripped by any MAVLink-aware relay (MAVProxy, mavlink-router, QGC) during normal parse-and-forward. The relay reads exactly 10 + LEN + 2 bytes per spec and discards the rest.

This affects any signature scheme (classical or post-quantum) that uses the "append-after-frame" pattern. The fix is to carry authentication material as first-class MAVLink messages so relays treat them as valid but opaque frames. A GitHub Security Advisory (GHSA-f5rj-mrxh-r7vm) documents the class of affected schemes.

The reason I mention it: if anyone else is evaluating ml-kem or ml-dsa for similar protocol-level authenticated channels, this constraint shapes the wire format significantly.