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Unformatted text preview: nge with each bit in the plaintext or key. This type of substitution is not necessarily enough; the German Enigma is a complex substitution algorithm that was broken during World War II. Diffusion dissipates the redundancy of the plaintext by spreading it out over the ciphertext. A cryptanalyst looking for those redundancies will have a harder time finding them. The simplest way to cause diffusion is through transposition (also called permutation). A simple transposition cipher, like columnar transposition, simply rearranges the letters of the plaintext. Modern ciphers do this type of permutation, but they also employ other forms of diffusion that can diffuse parts of the message throughout the entire message. Stream ciphers rely on confusion alone, although some feedback schemes add diffusion. Block algorithms use both confusion and diffusion. As a general rule, diffusion alone is easily cracked (although double transposition ciphers hold up better than many other pencilandpaper systems). 11.2 Complexity Theory
Complexity theory provides a methodology for analyzing the computational complexity of different cryptographic techniques and algorithms. It compares cryptographic algorithms and techniques and determines their security. Information theory tells us that all cryptographic algorithms (except onetime pads) can be broken. Complexity theory tells us whether they can be broken before the heat death of the universe. 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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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.
 Fall '10
 ALIULGER
 Cryptography

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