Unformatted text preview: s created when it is needed to encrypt communications and destroyed when it is no longer needed. This drastically reduces the risk of compromising the session key. Of course, the private key is vulnerable to compromise, but it is at less risk because it is only used once per communication to encrypt a session key. This is further discussed in Section 3.1. Merkle’s Puzzles
Ralph Merkle invented the first construction of publickey cryptography. In 1974 he registered for a course in computer security at the University of California, Berkeley, taught by Lance Hoffman. His term paper topic, submitted early in the term, addressed the problem of “Secure Communication over Insecure Channels” [1064]. Hoffman could not understand Merkle’s proposal and eventually Merkle dropped the course. He continued to work on the problem, despite continuing failure to make his results understood. Merkle’s technique was based on “puzzles” that were easier to solve for the sender and receiver than for an eavesdropper. Here’s how Alice sends an encrypted message to Bob without first having to exchange a key with him. (1) Bob generates 220, or about a million, messages of the form: “This is puzzle number x. This is the secret key number y,” where x is a random number and y is a random secret key. Both x and y are different for each message. Using a symmetric algorithm, he encrypts each message with a different 20bit key and sends them all to Alice. (2) Alice chooses one message at random and performs a bruteforce attack to recover the plaintext. This is a large, but not impossible, amount of work. (3) Alice encrypts her secret message with the key she recovered and some symmetric algorithm, and sends it to Bob along with x. (4) Bob knows which secret key y he encrypts in message x, so he can decrypt the message. Eve can break this system, but she has to do far more work than either Alice or Bob. To recover the message in step (3), she has to perform a bruteforce attack against each of Bob’s 220 messages in step (1); this attack has a complex...
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 Fall '10
 ALIULGER
 Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips

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