applied cryptography - protocols, algorithms, and source code in c

A running key ciphersometimes called a book cipherin

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Unformatted text preview: ion ciphers. Polyalphabetic substitution ciphers have multiple one-letter keys, each of which is used to encrypt one letter of the plaintext. The first key encrypts the first letter of the plaintext, the second key encrypts the second letter of the plaintext, and so on. After all the keys are used, the keys are recycled. If there were 20 one-letter keys, then every twentieth letter would be encrypted with the same key. This is called the period of the cipher. In classical cryptography, ciphers with longer periods were significantly harder to break than ciphers with short periods. There are computer techniques that can easily break substitution ciphers with very long periods. A running-key cipher—sometimes called a book cipher—in which one text is used to encrypt another text, is another example of this sort of cipher. Even though this cipher has a period the length of the text, it can also be broken easily [576,794]. 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 Source Code in C (cloth) Go! Keyword 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: Go! Previous Table of Contents Next ----------- Transposition Ciphers In a transposition cipher the plaintext remains the same, but the order of characters is shuffled around. In a simple columnar transposition cipher, the plaintext is written horizontally onto a piece of graph paper of fixed width and the ciphertext is read off vertically (see Figure 1.4). Decryption is a matter of writing the ciphertext vertically onto a p...
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