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

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Unformatted text preview: wn plaintexts. It also improves security against differential and linear cryptanalysis; the attacks require 261 chosen plaintexts and 260 known plaintexts, respectively [1338]. CRYPT(3) CRYPT(3) is a DES variant found on UNIX systems. It is primarily used as a one-way function for passwords, but sometimes can also be used for encryption. The difference between CRYPT(3) and DES is that CRYPT(3) has a key-dependent expansion permutation with 212 possible permutations. This was done primarily so that off-the-shelf DES chips could not be used to construct a hardware password-cracker. Generalized DES Generalized DES (GDES) was designed both to speed up DES and to strengthen the algorithm [1381,1382]. The overall block size increases while the amount of computation remains constant. Figure 12.11 is a block diagram of GDES. GDES operates on variable-sized blocks of plaintext. Encryption blocks are divided up into q 32-bit sub-blocks; the exact number depends on the total block size (this was variable in the design, but must be fixed for each implementation). In general, q equals the block size divided by 32. Function f is calculated once per round on the right-most block. The result is XORed with all the other parts, which are then rotated to the right. GDES has a variable number of rounds, n. There is a slight modification to the last round, so that the encryption and decryption processes differ only in the order of the subkeys (just like DES). In fact, if q = 2 and n = 16, this is DES. Biham and Shamir [167,168] showed that, using differential cryptanalysis, GDES with q = 8 and n = 16 is breakable with only six chosen plaintexts. If independent subkeys are also used, 16 chosen plaintexts are required. GDES with q = 8 and n = 22 is breakable with 48 chosen plaintexts, and GDES with q = 8 and n = 31 requires only 500,000 chosen plaintexts to break. Even GDES with q = 8 and n = 64 is weaker than DES; 249 chosen plaintexts are required to break it. In fact, any GDES scheme that is faster...
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