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

Simultaneous contract signing without an arbitrator

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Unformatted text preview: rom Alice and Alice does not know which one he was able to read successfully. Unfortunately, if the protocol stopped here it would be possible for Alice to cheat. Another step is necessary. (6) After the protocol is complete and both possible results of the transfer are known, Alice must give Bob her private keys so that he can verify that she did not cheat. After all, she could have encrypted the same message with both keys in step (4). At this point, of course, Bob can figure out the second message. The protocol is secure against an attack by Alice because she has no way of knowing which of the two DES keys is the real one. She encrypts them both, but Bob only successfully recovers one of them—until step (6). It is secure against an attack by Bob because, before step (6), he cannot get Alice’s private keys to determine the DES key that the other message was encrypted in. This may still seem like nothing more than a more complicated way to flip coins over a modem, but it has extensive implications when used in more complicated protocols. Of course, nothing stops Alice from sending Bob two completely useless messages: “Nyah Nyah” and “You sucker.” This protocol guarantees that Alice sends Bob one of two messages; it does nothing to ensure that Bob wants to receive either of them. Other oblivious transfer protocols are found in the literature. Some of them are noninteractive, meaning that Alice can publish her two messages and Bob can learn only one of them. He can do this on his own; he doesn’t have to communicate with Alice [105]. No one really cares about being able to do oblivious transfer in practice, but the notion is an important building block for other protocols. Although there are many types of oblivious transfer—I have two secrets and you get one; I have n secrets and you get one; I have one secret which you get with probability 1/2; and so on—they are all equivalent [245,391,395]. 5.6 Oblivious Signatures Honestly, I can’t think of a good use for these, but there are two kinds [346]: 1. Ali...
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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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