Unformatted text preview: key is extracted, a different 48bit subkey is generated for each of the 16 rounds of DES. These subkeys, Ki are determined in the following manner. First, the 56bit key is divided into two 28bit halves. Then, the halves are circularly shifted left by either one or two bits, depending on the round. This shift is given in Table 12.3. Table 12.1 Initial Permutation 58, 62, 57, 61, 50, 54, 49, 53, 42, 46, 41, 45, 34, 38, 33, 37, 26, 30, 25, 29, 18, 10, 22, 14, 17, 9, 21, 13, 2, 6, 1, 5, 60, 64, 59, 63, 52, 56, 51, 55, 44, 48, 43, 47, 36, 40, 35, 39, 28, 32, 27, 31, 20, 24, 19, 23, 12, 16, 11, 15, 4, 8, 3, 7 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 Code in C (cloth)
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Brief Full Advanced Search Search Tips (Publisher: John Wiley & Sons, Inc.) Author(s): Bruce Schneier ISBN: 0471128457 Publication Date: 01/01/96 Search this book:
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 Table 12.2 Key Permutation 57, 10, 63, 14, 49, 2, 55, 6, 41, 59, 47, 61, 33, 51, 39, 53, 25, 43, 31, 45, 17, 35, 23, 37, 9, 27, 15, 29, 1, 19, 7, 21, 58, 11, 62, 13, 50, 3, 54, 5, 42, 60, 46, 28, 34, 52, 38, 20, 26, 44, 30, 12, 18, 36, 22, 4 After being shifted, 48 out of the 56 bits are selected. Because this operation permutes the order of the bits as well as selects a subset of bits, it is called a compression permutation. This operation provides a subset of 48 bits. Table 12.4 defines the compression permutation (also called the permuted choice). For example, the bit in position 33 of the shifted key moves to position 35 of the output, and the bit in position 18 of the shifted key is ignored. Because of the shifting, a different subset of key bits is used in each subkey. Each bit is used in approximately 14 of the 16 subkeys, although not all bits are used exactly the same number of times. The Expansion Permutation
This operation expands the right half of the data, Ri, from 32 bits to 48 bits. Because this operation changes the order of the bits as well as repeating certain bits, it is known as an expansion permutation....
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