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Unformatted text preview: For g = 0 to 63 A = X(4g)<<<2r B = X(4g+1)<<<2r C = X(4g+2)<<<2r D = X(4g+3)<<<2r For step s = 0 to 7 A = A • (B + fr(B,C,D) + S512r+8 g+s) TEMP = D D=C C=B B = A <<< 5 A = TEMP X(4g)<<<2r = A X(4g+1)<<<2r = B X(4g+2)<<<2r = C X(4g+3)<<<2r = D (4) Recombine X0, X1, X2,..., X255 to form the ciphertext. The functions fr(B,C,D) are similar to those used in MD5: f0(B,C,D) = (B › C) ½ ((¬ B) › D) f1(B,C,D) = (B › D) ½ (C › (¬ D)) f2(B,C,D) = B • C • D f3(B,C,D) = C • (B ½ (¬ D)) Decryption is the reverse process. Generating the subkeys is a large task. Here is how the permutation array, P, could be generated from an 80bit key, K. (1) Initialize K0, K1, K2,..., K9 with the 10 bytes of K. (2) For i = 10 to 255 Ki = Ki  2 • Ki  6 • Ki  7 • Ki  10 (3) For i = 0 to 255, Pi = i (4) m = 0 (5) For j = 0 to 1 For i = 256 to 1 step 1 m = (K256  i + K257  i) mod i K257  i = K257  i <<< 3 Swap Pi and Pi  1 The Sarray of 2048 32bit words could be generated in a similar manner, either from the same 80bit key or from another key. The authors caution that these details should “be viewed as motivational; there may very well be alternative schemes which are both more efficient and offer improved security” [810]. Crab was proposed as a testbed of new ideas and not as a working algorithm. It uses many of the same techniques as MD5. Biham has argued that a very large block size makes an algorithm easier to cryptanalyze [160]. On the other hand, Crab may make efficient use of a very large key. In such a case, “easier to cryptanalyze” might not mean much. 14.7 SXAL8/MBAL
This is a 64bit block algorithm from Japan [769]. SXAL8 is the basic algorithm; MBAL is an expanded version with a variable block length. Since MBAL does some clever things internally, the authors claim that they can get adequate security with only a few rounds. With a block length of 1024 bytes, MBAL is about 70 times fast...
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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.
 Fall '10
 ALIULGER
 Cryptography

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