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

Speed of idea current software implementations of

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Unformatted text preview: sults they find. I don’t know of any cryptanalyst outside the employ of the company who studied it, since it would amount to doing their analysis work for them. Still, Ron Rivest is not the usual snake-oil peddler. He’s a respected and competent cryptographer. I would put a fair degree of trust in the algorithm, even though I haven’t personally inspected the code. RC4, once the proprietary intellectual property of RSADSI, was posted to the Internet (see Section 17.1), and it’s probably just a matter of time before RC2 is posted as well. An agreement between the Software Publishers Association (SPA) and the U.S. government gave RC2 and RC4 (see Section 17.1) special export status (see Section 25.14). Products that implement one of these two algorithms have a much simpler export approval process, provided that the keys are no more than 40 bits long. Is a 40-bit key enough? There are a total of one trillion possible keys. Assuming that brute force is the most efficient method of cryptanalysis (a big assumption, considering that the algorithm has never been published), and assuming that a brute-force cryptanalysis chip can test one million keys per second, it will take him 12.7 days to find the correct key. One thousand machines working in parallel can produce the key in twenty minutes. RSA Data Security, Inc., maintains that while encryption and decryption are quick, exhaustive key search is not. A significant amount of time is spent setting up the key schedule. While this time is negligible when encrypting and decrypting messages, it is not when trying every possible key. The U.S. government would never allow export of any algorithm it couldn’t, at least in theory, break. They could create a magnetic tape or CD of a specific plaintext block encrypted with every possible key. To break a given message, they could just run the tape and compare the ciphertext blocks in the message with the ciphertext blocks on the tape. If there is a match, they could try the candidate key and see if the message makes any sense. If they choose...
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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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