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Unformatted text preview: o modify the encryption of the next block. Each ciphertext block is dependent not just on the plaintext block that generated it but on all the previous plaintext blocks. In cipher block chaining (CBC) mode, the plaintext is XORed with the previous ciphertext block before it is encrypted. Figure 9.3a shows CBC encryption in action. After a plaintext block is encrypted, the resulting ciphertext is also stored in a feedback register. Before the next plaintext block is encrypted, it is XORed with the feedback register to become the next input to the encrypting routine. The resulting ciphertext is again stored in the feedback register, to be XORed with the next plaintext block, and so on until the end of the message. The encryption of each block depends on all the previous blocks. Decryption is just as straightforward (see Figure 9.3b). A ciphertext block is decrypted normally and also saved in a feedback register. After the next block is decrypted, it is XORed with the results of the feedback register. Then the next ciphertext block is stored in the feedback register, and so on, until the end of the message. Mathematically, this looks like: Ci = EK(Pi • Ci-1) Pi = Ci-1 • DK(Ci) Figure 9.3 Cipher block chaining mode. Initialization Vector
CBC mode forces identical plaintext blocks to encrypt to different ciphertext blocks only when some previous plaintext block is different. Two identical messages will still encrypt to the same ciphertext. Even worse, two messages that begin the same will encrypt in the same way up to the first difference. 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)
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- Fall '10