neqo — Hybrid key share whose length does not match the group named with it

Pass

Responded correctly: aborted with illegal_parameter (alert 47) over a server share of the wrong length for X25519MLKEM768, which is the check §3.1.2 asks the client to make and the code it names.

The client did what the clause requires.

What was measured

Client

neqo — neqo (Firefox), 0.31.1

Test

Hybrid key share whose length does not match the group named with it — t-hybrid-share-length

Clause

draft-kwiatkowski-tls-ecdhe-mlkem §3.1.2, RFC 8446 §6.2, RFC 9001 §4.8 (MUST)

Class

correctness — Rejected something invalid, with the code the RFC names.

Required behaviour

Abort rather than proceed. §3.1.2 is explicit: "For all groups, the client MUST check if the ciphertext length matches the selected group, and abort with an illegal_parameter alert if it fails." The port sends 1,088 bytes where X25519MLKEM768 fixes the server share at 1,120 — the ML-KEM ciphertext whole and the 32-byte X25519 tail removed. It parses cleanly as an opaque vector, so the only thing wrong with it is its length for the group it is named with. Neither outcome lets a client derive our keys, because the shared secret needs both halves. What separates them is where it notices: a client that checks the length rejects this at the ServerHello, while one that does not carries a truncated share into decapsulation and fails later and less clearly. What is judged is the abort, not the alert value. RFC 9001 §4.8 lets a QUIC endpoint replace any alert with a generic one, so the code a client chooses is reported and not scored — and a handshake that ends with no CONNECTION_CLOSE at all is inconclusive rather than a failure, because a close that was never sent cannot be told from one that was lost.

Measured

2026-09-18

Reproduce it

The suite is the judge, so the reproduction is to point the same client at the same test and let the server report what it saw.

SESSION=$(curl -sX POST https://conformance.pqcrypta.com/session | jq -r .id)
# then drive neqo at the test URL and read the verdict:
curl -s https://conformance.pqcrypta.com/report/$SESSION.json | jq '.results["t-hybrid-share-length"]'