[Cryptography] Proposal of a fair contract signing protocol

[mok-kong shen]

Am 27.06.2016 um 07:38 schrieb Sidney Markowitz:
> mok-kong shen wrote on 27/06/16 9:54 AM:
>> Am 26.06.2016 um 10:08 schrieb Sidney Markowitz:
>>> This leaves no reason for your protocol to require that contract C is split
>>> into separate halves X and Y that are signed separately. You could just as
>>> easily write the contract to say "Alice and Bob agree to this transaction
>>> subject to this contract being signed by both Alice and Bob" and then have
>>> both Alice and Bob "promise" to sign it conditional on the other person
>>> signing it. If you do that, then a single signature provides the simultaneous
>>> commitment that you require for "fairness", according to your definitions.
>>>
>>> This protocol puts all the power into the "promise" which binds Alice, but you
>>> never define how a binding promise can be made without the promise itself
>>> being a contract that has to be put into effect in a "fair" fashion. That
>>> leads to an endless recursion.
>>
>> I wrote: "The messages of step 1 and 2 are to be sent with
>> signcryption, .....", i.e. that promise is signed by Alice.
>>
>
> You never mentioned that the promise is what is signed in step 1. You said
> that Alice only signs X in step 1 and promises to sign C = X | Y if Bob signs
> it in step 2. How could Alice sign a promise to sign X and Y if what she signs
> only includes X? And if Alice does have to sign "I promise to sign Contract C
> = X | Y within time T1 after Bob signs Contract C if he signs within time T2"
> and she is bound by that, then that promise is an unfair contract by your
> definition.

The message sent by Alice in step 1 contains in it, among others, two
pieces, one is signed(Alice, X), i.e. X digitally signed by her, the
other is Y, i.e. simply Y. She promises to sign Y in step 3, i.e. to
produce signed(Alice, Y), if Bob does step 2. The whole message sent
by her in step 1, which contains besides signed(Alice, X) and Y also
the said promise, is sent to Bob with signcryption, i.e. first
encrypted with Bob's public key and signed with Alice's private key.

M. K. Shen