Function pq_handshake_finish 

pub fn pq_handshake_finish(
    hs: &PqHandshake,
    ct: &[u8],
) -> Result<(PqSuite, PqSuite), KemError>

Initiator step 2: decapsulate the responder’s ciphertext, yielding the matching directional suites (initiator→responder, responder→initiator). A ciphertext of the wrong length is refused; a well-formed but forged one implicit-rejects into suites that open nothing.