Internal Integrity with External Blockchain Attestation of all the data in the vault.
First, all messages get a SHA256 integrity hash of the main envelope/body and another
for any attachments, plus a Merkle-chain hash of those two hashes and the chain hash of the
previous message. These provide guarantees of integrity for all message, attachment, and
recipient ID and label data received, as well as guarantees of no insertion, deletion, or
reordering of messages. Second, periodically (maybe 1/hr), a message's "chain" hash is
given to an external blockchain-based timestamp facility (opentimestamps.org) to provide
prove that hashes/chains have not been regenerated.