Or the flash. Or Silverlite. Or something else, but on the client side. Happen?
The full question sounds like this: there are 10,000 shops. For practical purposes they want to share information about purchases, "and you have the same buyer?". There is a single center, which will conduct the comparison. But none of the network members do not trust neither the center nor each other. To pass specific data about your customers, nobody wants. Then there is the idea: each store separately considers the hash all fields (customer name, card number, address,...) and merges them into one single center, where he reported and that there is someone else in the network. This, in theory, the trust problem solved.
To calculate the hash on the server side is possible, this problem will be removed, but the implementation of such a mechanism requires the intervention of programmers. And out of 10,000 participating stores, the vast majority of such opportunity is not present or it seems to them too gemoroyno. They would like to have something like Google Analitichnogo code for checking out code, which is relatively easy to build, but still would not transfer customer data in the clear center, because they did not believe.
Here it seems to me that in principle it can be done (not taken into account while the complexity of the script). Total of the first part of the question, for form's sake asked: am I right, is it possible?
What about the authorization? If all the hashing is done on the client side, how to make so that an outsider attacker can shove the left data into the system on behalf of one of the shops? While not touching (or minimizing) executable scripts on the server side of the store.
Of home explanation: now you have a store on Yahoo Store. Does not allow you Yakha touch your server. Java script unable to enter in the page of checking out code. But no more. And to live as it is necessary.