[Cryptography] Proposal of a fair contract signing protocol

Adrian McCullagh amccullagh at live.com
Mon Jun 13 23:06:29 EDT 2016

Hi Allen,

You wrote:
> 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?"

Firstly, when Alice signs the email/document there is no contract.  She actually signs an offer which if accepted by Bob will convert into a contract as then and only then is there a meeting of minds to form an agreement.  For there to be an acceptance at law it must in most cases (Postal rule is an exception) be communicated to and received by the offeror.

So there appears to be an impass as Alice claims she did not receive the acceptance, which at law is difficult.  It is from an evidential perspective difficult to prove the negative which in this case is that Alice did not receive the fully signed document thus forming the contract.  On the otherhand, Bob has the easier task in proving the positive as he only has to prove that he signed the document and sent it but also that Alice received the signed document.

>From a fraud perspective, Bob could alter the headers showing that he sent the document but how does he prove that Alice received the signed document?  Does he set the receipt acknowledgement flag when sending the email/document?

Obviously, a three way handshake could prove this but I do not know where this has been implemented for document signing.

Kind Regards

Dr. Adrian McCullagh
Ph.D. LL.B. (Hons) B.App. Sc. (Computing)

ODMOB Lawyers
Email: ajmccullagh57 at gmail.com
Email: amccullagh at live.com
MOB: +61 401 646 486
SKYPE: admac57

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