Unformatted text preview: bit subkeys. These are the first eight subkeys for the algorithm (the six for the first round, and the first two for the second round). Then, the key is rotated 25 bits to the left and again divided into eight subkeys. The first four are used in round 2; the last four are used in round 3. The key is rotated another 25 bits to the left for the next eight subkeys, and so on until the end of the algorithm. 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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 Decryption is exactly the same, except that the subkeys are reversed and slightly different. The decryption subkeys are either the additive or multiplicative inverses of the encryption subkeys. (For the purposes of IDEA, the allzero subblock is considered to represent 216 = – 1 for multiplication modulo 216 + 1; thus the multiplicative inverse of 0 is 0.) Calculating these takes some doing, but you only have to do it once for each decryption key. Table 13.4 shows the encryption subkeys and the corresponding decryption subkeys. Speed of IDEA
Current software implementations of IDEA are about twice as fast as DES. IDEA on a 33 megahertz 386 machine encrypts data at 880 kilobits per second, and 2400 kilobits per second on a 66 megahertz 486 machine. You might think IDEA should be faster, but multiplications aren’t cheap. To multiply two 32bit numbers on a 486 requires 40 clock cycles (10 on a Penti...
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 Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips

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