Unformatted text preview: roducing errors in the ciphertext should be difficult, and encryption of more than one message with the same key should be possible. These will be discussed in detail in the next sections. Efficiency is another consideration. The mode should not be significantly less efficient than the underlying cipher. In some circumstances it is important that the ciphertext be the same size as the plaintext. A third consideration is fault-tolerance. Some applications need to parallelize encryption or decryption, while others need to be able to preprocess as much as possible. In still others it is important that the decrypting process be able to recover from bit errors in the ciphertext stream, or dropped or added bits. As we will see, different modes have different subsets of these characteristics. 9.1 Electronic Codebook Mode
Electronic codebook (ECB) mode is the most obvious way to use a block cipher: A block of plaintext encrypts into a block of ciphertext. Since the same block of plaintext always encrypts to the same block of ciphertext, it is theoretically possible to create a code book of plaintexts and corresponding ciphertexts. However, if the block size is 64 bits, the code book will have 264 entries—much too large to precompute and store. And remember, every key has a different code book. This is the easiest mode to work with. Each plaintext block is encrypted independently. You don’t have to encrypt a file linearly; you can encrypt the 10 blocks in the middle first, then the blocks at the end, and finally the blocks in the beginning. This is important for encrypted files that are accessed randomly, like a database. If a database is encrypted with ECB mode, then any record can be added, deleted, encrypted, or decrypted independently of any other record—assuming that a record consists of a discrete number of encryption blocks. And processing is parallizeable; if you have multiple encryption processors, they can encrypt or decrypt different blocks without regard for each other. The problem with ECB mode is that if a cryptanalyst has the plaintext...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips