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Unformatted text preview: by the error, so CBC mode is self-recovering. Two blocks are affected by an error, but the system recovers and continues to work correctly for all subsequent blocks. CBC is an example of a block cipher being used in a self-synchronizing manner, but only at the block level. While CBC mode recovers quickly from bit errors, it doesn’t recover at all from synchronization errors. If a bit is added or lost from the ciphertext stream, then all subsequent blocks are shifted one bit out of position and decryption will generate garbage indefinitely. Any cryptosystem that uses CBC mode must ensure that the block structure remains intact, either by framing or by storing data in multiple-block-sized chunks. Security Problems
Some potential problems are caused by the structure of CBC. First, because a ciphertext block affects the following block in a simple way, Mallory can add blocks to the end of an encrypted message without being detected. Sure, it will probably decrypt to gibberish, but in some situations this is undesirable. If you are using CBC, you should structure your plaintext so that you know where the message ends and can detect the addition of extra blocks. Second, Mallory can alter a ciphertext block to introduce controlled changes in the following decrypted plaintext block. For example, if Mallory toggles a single ciphertext bit, the entire block will decrypt incorrectly, but the following block will have a 1-bit error in the corresponding bit position. There are situations where this is desirable. The entire plaintext message should include some kind of controlled redundancy or authentication. Finally, although plaintext patterns are concealed by chaining, very long messages will still have patterns. The birthday paradox predicts that there will be identical blocks after 2m/2 blocks, where m is the block size. For a 64-bit block size, that’s about 34 gigabytes. A message has to be pretty long before this is a problem. Previous Table of Contents Next Products | Contact Us | About Us | Privacy | Ad Info | Home Use of this site is subject to certain Terms & Conditions,...
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- Fall '10