Unformatted text preview: es the constants for each group of 20 rounds. 3. “The function G in round 2 was changed from ((X¥ Y) ¦ (X¥ Z) ¦ (Y¥ Z)) to ((X¥ Z) ¦ (Y¥ ¬ (Z))) to make G less symmetric.” SHA uses the MD4 version: ((X¥ Y) ¦ (X¥ Z) ¦ (Y¥ Z)). 4. “Each step now adds in the result of the previous step. This promotes a faster avalanche effect.” This change has been made in SHA as well. The difference in SHA is that a fifth variable is added, and not b, c, or d, which is already used in ft. This subtle change makes the den Boer-Bosselaers attack against MD5 impossible against SHA. 5. “The order in which message sub-blocks are accessed in rounds 2 and 3 is changed, to make these patterns less alike.” SHA is completely different, since it uses a cyclic error-correcting code. 6. “The left circular shift amounts in each round have been approximately optimized, to yield a faster avalanche effect. The four shifts used in each round are different from the ones used in other rounds.” SHA uses a constant shift amount in each round. This shift amount is relatively prime to the word size, as in MD4. This leads to the following comparison: SHA is MD4 with the addition of an expand transformation, an extra round, and better avalanche effect; MD5 is MD4 with improved bit hashing, an extra round, and better avalanche effect. There are no known cryptographic attacks against SHA. Because it produces a 160-bit hash, it is more resistant to brute-force attacks (including birthday attacks) than 128-bit hash functions covered in this chapter. 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 © 1996-2000 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 So...
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