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

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Unformatted text preview: with the right key in the pair. (8) Bob sends his pile of message pairs to Alice, making sure that Alice knows which messages are which halves of which pairs. (9) Alice and Bob send each other every key pair using the oblivious transfer protocol. That is, Alice sends Bob either the key used to encrypt the left message or the key used to encrypt the right message, for each of the n pairs. Bob does the same. They can either alternate sending halves or one can send n and then the other—it doesn’t matter. Now both Alice and Bob have one key in each key pair, but neither knows which halves the other has. (10) Both Alice and Bob decrypt the halves they can and make sure that the decrypted messages are valid. (11) Alice and Bob send each other the first bits of all 2n DES keys. (If they are worried about Eve being able to read these mail messages, then they should encrypt their transmissions to each other.) (12) Alice and Bob repeat step (11) for the second bits of all 2n DES keys, the third bits, and so on, until all the bits of all the DES keys have been transferred. (13) Alice and Bob decrypt the remaining halves of the message pairs. Alice has a valid receipt from Bob, and Bob can XOR any key pair to get the original message encryption key. (14) Alice and Bob exchange the private keys used during the oblivious transfer protocol and each verifies that the other did not cheat. Steps (5) through (8) for Bob, and steps (9) through (12) for both Alice and Bob, are the same as the contract-signing protocol. The twist is all of Alice’s dummy messages. They give Bob some way of checking the validity of her oblivious transfer in step (10), which forces her to stay honest during steps (11) through (13). And, as with the simultaneous contract-signing protocol, both a left and a right half of one of Alice’s message pairs are required to complete the protocol. 5.9 Simultaneous Exchange of Secrets Alice knows secret A; Bob knows secret B. Alice is willing to tell Bob A, if Bob tells her B. Bob is willing to tell...
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