== Transcript hashes ==
Every message specifies its parent's sequence number. Some Encrypted messages also specify include a "transcript hash " of that their parent and all prior messagesas "additional authenticated data". The hash also covers the sender_key for each message (set to zeros for cleartext messages):
The hash also covers the sender_key for each message (set to zeros for cleartext messages): H(parent) = SHA256(sender_key[parent] || ciphertextmsg[parent] || H(parent-1))
== Timing ==