When a contract in digital from is to be signed online by Alice and Bob, an issue concerning the fairness of the signing process crops up as follows: If Alice first signs the document and sends it to Bob, it means she has committed to something (e.g. ready to purchase an article from Bob at a certain price), Bob can however, if he desires, to some extent delay giving his digital signature and thus have a certain finite time interval during which he has no corresponding commitment. This is obviously unfair and hence to be avoided, if possible.
Noting that with visual cryptography a document can be separated into two pieces such that they jointly can reproduce the original but neither piece alone provides any information of the document, we propose the following protocol which well fulfills the requirements of fairness in the present context.
In the following the convention is that signed(A, U) denotes U (as a single piece) digitally signed by A and that A thereby commits to U and that nothing else, e.g. simply a V in a message which as a whole is signed, has the meaning of a commitment.
Step 1: Alice formulates a contract document C, generates with visual cryptography a pair (X, Y), sends a message containing signed(Alice,X) and Y to Bob and asks him to accept C before a certain day T in the future and promises to complete the contract formality within a certain time period TP in case Bob commits to C in step 2.
Step 2: Bob obtains C from (X, Y). If he can't accept C, he informs Alice and the protocol begins again at step 1. Otherwise he sends a message containing signed(Bob,X) and signed(Bob,Y) to Alice and asks her to release C. (If Bob does nothing before T is reached, the protocol begins again at step 1.)
Step 3: Alice examines whether Bob has signed the correct stuff, i.e. whether he hadn't e.g. by mistake sent signed(Bob,Z) in place of signed(Bob,X) with Z != X. If Bob had signed the wrong stuff, she informs Bob and the protocol begins again at step 1. Otherwise she releases C, signed(Alice,X), signed(Alice,Y), signed(Bob,X) and signed(Bob,Y) to the public. (Alice is responsible to complete step 3 within TP.)
The messages of step 1 and 2 are to be sent with signcryption, i.e. encrypted with reciever's public key and signed by the sender, and with authentication (integrity check). Receipt acknowledgments are to be requested for resolving eventual timing issues.
(a) In step 1 Alice has not committed to C. Thus there can be no unfairness of the nature mentioned above.
(b) If Bob commits to C in step 2, then Alice is immediatly obliged to commit to C as well, since the pair (X, Y) stems from herself (hence she couldn't eventually claim that C corresponds to (X, Z) with Z != Y). That is, C is virtually already a valid document. Hence there can be also here no unfairness of the nature mentioned above.
(c) Our protocol doesn't involve/need any trusted third party, which is an advantage.
[Addendum 19.06.2016] There are literatures wich claim (if I have not misinterpreted) that protocols of our genre are impossible. My humble knowledge is unfortunately insufficient to study them in details so as to resolve the apparent contradiction between our result and the impossibility claims. Readers interested in probing the causes of that contradiction may eventually desire to read a paper by H. Pagnia and F. C. Gaertner of 1999 entitled "On the Impossibility of Fair Exchange without a Trusted Third Party" which is however currently not online accessible from the institution where the paper was originally published. In that case I could send over a copy. (My address: email@example.com)