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Unformatted text preview: Block Ciphers — Tutorial c circlecopyrt Eli Biham  August 18, 2010 145 Block Ciphers — Tutorial (5) A Known Plaintext Attack on 1Round DES After removing the permutations IP and FP we get: F R R L’ L 32 32 32 32 32 K=? 48 c circlecopyrt Eli Biham  August 18, 2010 146 Block Ciphers — Tutorial (5) A Known Plaintext Attack on 1Round DES (cont.) We are given a pair ( M,C ) where M = ( L,R ) and C = ( L ′ ,R ) and we want to find the 48bit key K . We know that: F ( R,K ) = L ⊕ L ′ 1. Why is the output of all Sboxes known? 2. Given the 4 bits output of S 1 how many 6bit combinations are possible as input to S 1 ? 3. How many 6bit combinations are possible as the 6 bit key which takes part in the creation of the input to S 1 ? 4. How many 48bit combinations are possible for K ? c circlecopyrt Eli Biham  August 18, 2010 147 Block Ciphers — Tutorial (5) • A Known Plaintext Attack on 2Round DES F R L 32 32 32 48 F 32 32 32 48 K 1 K 2 R’ L’ c circlecopyrt Eli Biham  August 18, 2010 148 Block Ciphers — Tutorial (5) A Known Plaintext Attack on 2Round DES (cont.) Thus, we have: • F ( R,K 1 ) = L ⊕ R ′ • F ( R ′ ,K 2 ) = L ′ ⊕ R As in the attack on one round, the first expression reduces the number of possibilities for the 48 bits of K 1 to 4 8 = 2 16 (as only a fraction of 2 − 32 of the keys pass the test)....
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 Spring '11
 YanivCarmeli

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