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Unformatted text preview: te each other. Each of them has a password that the other knows: Alice has PA and Bob has PB. Here’s a protocol that will not work: (1) Alice and Bob trade public keys. (2) Alice encrypts PA with Bob’s public key and sends it to him. (3) Bob encrypts PB with Alice’s public key and sends it to her. (4) Alice decrypts what she received in step (2) and verifies that it is correct. (5) Bob decrypts what he received in step (3) and verifies that it is correct. Mallory can launch a successful maninthemiddle attack (see Section 3.1): (1) Alice and Bob trade public keys. Mallory intercepts both messages. He substitutes his public key for Bob’s and sends it to Alice. Then he substitutes his public key for Alice’s and sends it to Bob. (2) Alice encrypts PA with “Bob’s” public key and sends it to him. Mallory intercepts the message, decrypts PA with his private key, reencrypts it with Bob’s public key and sends it on to him. (3) Bob encrypts PB with “Alice’s” public key and sends it to her. Mallory intercepts the message, decrypts PB with his private key, reencrypts it with Alice’s public key, and sends it on to her. (4) Alice decrypts PB and verifies that it is correct. (5) Bob decrypts PA and verifies that it is correct. Alice and Bob see nothing different. However, Mallory knows both PA and PB. Donald Davies and Wyn Price describe how the interlock protocol (described in Section 3.1) can defeat this attack [435]. Steve Bellovin and Michael Merritt discuss ways to attack this protocol [110]. If Alice is a user and Bob is a host, Mallory can pretend to be Bob, complete the beginning steps of the protocol with Alice, and then drop the connection. True artistry demands Mallory do this by simulating line noise or network failure, but the final result is that Mallory has Alice’s password. He can then connect with Bob and complete the protocol, thus getting Bob’s password, too. The protocol can be modified so that Bob gives his password before Alice, under the assumpti...
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 Fall '10
 ALIULGER
 Cryptography

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