Derives the per-message msg key from a shared view secret and counter.
msg_key = HMAC-SHA256(key = view_secret, "raven/mailbox/v1" ‖ counter_be32)
32-byte secret shared sender↔recipient
uint32 rotation counter (advance per message)
32-byte msg key
InvalidViewSecretError if the secret is not 32 bytes
InvalidCounterError if the counter is not a uint32
Derives the per-message msg key from a shared view secret and counter.
msg_key = HMAC-SHA256(key = view_secret, "raven/mailbox/v1" ‖ counter_be32)