Design og implementasjon av verifikasjon av digitale objekter

01.06.2005

Abstract
Today more and more trade is done over the internet. Most of this trade deals with commodities were the seller gets the money when the customer receives the article. But more and more business is about selling digital items. When trading digital items one meets new problems. It is not difficult for one party to cheat the other party by misbehaving. For instance by not sending the item after receiving the other item. A proposal which solves this problem is fair exchange protocols. Here a trusted third party obtains both items from the other parties and validates them. If the items turn out to be in order, he completes the exchange, if not, nobody gets anything. The requirement here is that the third party can validate the items, and this is in general not possible if the items are images, audio or video.

In this project we have a solution which lets the buyer validate the item without giving it to him. This is achieved by giving the customer a low-quality copy of the item, and then the customer gets a preview. When the preview is taken from the object both the customer and the seller influence where it should be taken from. The customer most likely wants it to come from the most important part of the object, while the seller most likely wants it to come from the less important part. The customers choice is based on the fact that he wants to validate the quality of the important parts, and the seller want to keep the most important parts protected in case of a brute-force attack. We have designed, implemented and tested such a trusted third party.