[Cryptography] Proposal of a fair contract signing protocol

Jerry Leichter leichter at lrw.com
Tue Jun 28 16:59:41 EDT 2016

> On Jun 13, 2016, at 11:06 PM, Adrian McCullagh <amccullagh at live.com> wrote:
> 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....
You're actually displaying the race condition that's lurking at the heart of the traditional view.

Alice as offeror controls the offer, and can revoke at any time - until there's a "meeting of minds".  Except that we don't want to say there's a "meeting of minds" until Alice "actually knows" that Bob has accepted.  Is that when the letter (or email) arrives at Alice's mailbox?  When she opens it?  When she reads it?  If it's in a foreign language she doesn't understand (or if its encrypted), does it wait until it's been translated/decrypted, or does it occur immediately?

You could probably make an argument for any of these in the right circumstances I suppose, but in practice the evidentiary issues would be so difficult that what actually happened in the real world becomes irrelevant.  (Shock, horror:  The reality doesn't matter!)  Here the legal discussions stop and questions about who the judge or jury, as the case may be, finds more believable take over - the ultimate fallback in the legal system, and something that's always missing from these attempts at technological solutions that have no edge cases.

The Postal Rule is one (almost) extreme, making the postal service a third party and defining the "meeting of minds" to have occurred upon delivery to that third party - even though Bob himself is completely unaware of it, and the third party has no idea what is actually being communicated.  Seems very odd - but it does have exactly one significant advantage:  It's easy for Bob to prove what he did!  (Oh, sure, you can gen up oddball examples - Bob claims the registered letter he sent was an acceptance, Alice claims it simply contained blank sheets of paper.  Back to believability....)  
                                                        -- Jerry

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