I am researching how Masked Authenticated Messaging (MAM) works. The masked_payload is created in the create function in file: https://github.com/iotaledger/MAM/blob/master/mam/src/mam.rs
masked_payload = [ Encoded Index, Encoded Message Length, encrypted [ Message, Nonce, Signature, Encoded Number of Siblings, Siblings ] ]
My question is what is the purpose of this Nonce in the masked_payload?