Is there a protocol or best practice for trading information between parties where one, after receiving his share of the information from the others, might refuse to answer or might alter his answer, based on the information he just received? Ideally there should be a system where each put in their information, and the system only reveals it after all have posted theirs.
The point is, there are a number of people, not more than ten, who want to use an online system to share information (which, for the sake on an example, resembles trading information. For example, I sell x for y amount of money). It has to be done at most a few times per month. It would be best to use something like a chat room, private forum, an instant messaging group or a plain old e-mail list, and we need to eliminate the possibility of someone altering their answer upon reading what the others have posted. Even if not expecting malice, one can unconsciously alter their part if reading the others beforehand.
I'm trying to hinder any and all kinds of game theory related dilemmas.
There are multiple solutions to the problem, but all have major disadvantages.
- create a website which handles this: once everyone posted their message, the website will reveal them to all. Problem: if such a public and well-known service existed, it would be the answer to this question. If it doesn't exist, and someone from the group (or someone hired by someone from the group) implements it, how could the others trust that it doesn't leak information to him?
- meet in person, write it on paper, and read them together. Problem: hard to organize, and costly, especially if the members live far away from each other.
- Everyone posts a hash of their message. After all the hashes have been posted, all can post the normal message in any order, because an altered message can be spotted. Problem: too ominous, frustrating and time-consuming to do, might make people paranoid, and as they check every time the hashes of the others, it accentuates that they don't trust each other. A full-body scan when leaving a baker's store to check whether you have stolen a cookie would accentuate too much that the baker distrust you. To understand my point with the baker example: putting the cookies out of easy reach and giving them only on demand to the first customer in line is a sensible security precaution, but making a full-body scan is not.
The information needed to be exchanged can be expressed in basic textual form, and is at most a few hundreds of characters long.