Unformatted text preview: This operation has two purposes: It makes the right half the same size as the key for the XOR operation and it provides a longer result that can be compressed during the substitution operation. However, neither of those is its main cryptographic purpose. By allowing one bit to affect two substitutions, the dependency of the output bits on the input bits spreads faster. This is called an avalanche effect. DES is designed to reach the condition of having every bit of the ciphertext depend on every bit of the plaintext and every bit of the key as quickly as possible. Figure 12.3 defines the expansion permutation. This is sometimes called the Ebox. For each 4bit input block, the first and fourth bits each represent two bits of the output block, while the second and third bits each represent one bit of the output block. Table 12.5 shows which output positions correspond to which input positions. For example, the bit in position 3 of the input block moves to position 4 of the output block, and the bit in position 21 of the input block moves to positions 30 and 32 of the output block. Although the output block is larger than the input block, each input block generates a unique output block. Table 12.3 Number of Key Bits Shifted per Round Round 1 Number 1 2 1 3 2 4 2 5 2 6 2 7 2 8 2 9 1 10 2 11 2 12 2 13 2 14 2 15 2 16 1 Table 12.4 Compression Permutation 14, 23, 41, 44, 17, 19, 52, 49, 11, 12, 31, 39, 24, 4, 37, 56, 1, 26, 47, 34, 5, 8, 55, 53, 3, 16, 30, 46, 28, 7, 40, 42, 15, 27, 51, 50, 6, 20, 45, 36, 21, 13, 33, 29, 10, 2, 48, 32 Previous Table of Contents Next Products  Contact Us  About Us  Privacy  Ad Info  Home Use of this site is subject to certain Terms & Conditions, Copyright © 19962000 EarthWeb Inc. All rights reserved. Reproduction whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Read EarthWeb's privacy statement. To access the contents, click the chapter and section titles. Applied Cryptography, Second Edition: Protocols, Algorthms, and Source Co...
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