mirror of https://github.com/ospab/ostp.git
ospab
29554a71f1
CRITICAL forward-secrecy fix. Session transport keys were derived as
SHA256(get_handshake_hash() || label). The Noise handshake hash `h` only
ever absorbs PUBLIC transcript data (ephemeral pubkeys + on-wire
ciphertexts, via MixHash); the ephemeral ee DH result is mixed via MixKey
into the chaining key `ck` ONLY, never into `h` (confirmed in snow 0.9.6
symmetricstate.rs). So the data-transport keys depended on the PSK and the
public transcript but NOT on the DH secret, meaning:
- zero forward secrecy: anyone who later learns the access-key PSK can
decrypt all recorded past sessions from the observed handshake alone;
- any PSK holder can passively decrypt any other session on that key;
- the ephemeral Diffie-Hellman was cryptographically wasted.
Fix: take the two directional keys from Noise's Split() over the final `ck`
via snow's dangerously_get_raw_split (risky-raw-split feature). These keys
depend on ee, restoring forward secrecy. The custom out-of-order AEAD,
explicit nonces, session_id AAD, framing and reordering are all unchanged
- only the key SOURCE moved. The dead into_transport()/handshake_hash()
paths and the unreachable NoiseSession::Transport variant are removed.
Wire-breaking: PROTOCOL_VERSION 4 -> 5 so pre-fix peers derive different
keys and cannot interop (version gate is invisible on the wire).
Added noise unit tests for the .0/.1 -> send/recv role mapping and the
not-finished guard.
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