applied cryptography - protocols, algorithms, and source code in c

Adjudicated protocols because of the high cost of

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Unformatted text preview: he ideal. This ideal can translate to the computer world, but there are several problems with computer arbitrators: — It is easier to find and trust a neutral third party if you know who the party is and can see his face. Two parties suspicious of each other are also likely to be suspicious of a faceless arbitrator somewhere else on the network. — The computer network must bear the cost of maintaining an arbitrator. We all know what lawyers charge; who wants to bear that kind of network overhead? — There is a delay inherent in any arbitrated protocol. — The arbitrator must deal with every transaction; he is a bottleneck in large-scale implementations of any protocol. Increasing the number of arbitrators in the implementation can mitigate this problem, but that increases the cost. — Since everyone on the network must trust the arbitrator, he represents a vulnerable point for anyone trying to subvert the network. Even so, arbitrators still have a role to play. In protocols using a trusted arbitrator, the part will be played by Trent. Adjudicated Protocols Because of the high cost of hiring arbitrators, arbitrated protocols can be subdivided into two lower-level subprotocols. One is a nonarbitrated subprotocol, executed every time parties want to complete the protocol. The other is an arbitrated subprotocol, executed only in exceptional circumstances—when there is a dispute. This special type of arbitrator is called an adjudicator (see Figure 2.1b). An adjudicator is also a disinterested and trusted third party. Unlike an arbitrator, he is not directly involved in every protocol. The adjudicator is called in only to determine whether a protocol was performed fairly. Judges are professional adjudicators. Unlike a notary public, a judge is brought in only if there is a dispute. Alice and Bob can enter into a contract without a judge. A judge never sees the contract until one of them hauls the other into court. This contract-signing protocol can be formalized in this way:...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.

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