[Cryptography] Proposal of a fair contract signing protocol

[Allen]

> At any moment, both correspondents have a complete record of everything
> the other has acknowledged "on the record"

Alice signs the contract and sends it to Bob for his signature.  A few
months later, Bob claims he signed the contract and returned it the same
day.  How do you propose proving the time that Bob signed the contract, if
his email client can simply forge the time sent header, and Alice's can
simply forge the time received, or forge the unacknowledged messages saying
"Hey, where's the signed contract?"
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