fn message_representative(message: &[u8]) -> [u8; 127]Expand description
The composite message representative M' = PREFIX ‖ LABEL ‖ len(ctx) ‖ ctx ‖ SHA-512(message).
M' is not a hash we take but the message both components sign (each hashes it internally), so
it must be passed whole. The application context is empty, so len(ctx) ‖ ctx is a single
0x00, making M' a fixed M_PRIME_LEN bytes. See
draft-ietf-lamps-pq-composite-sigs-19 §2.2.