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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:
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----------- 9.4 Stream Ciphers
Stream ciphers convert plaintext to ciphertext 1 bit at a time. The simplest implementation of a stream cipher is shown in Figure 9.6. A keystream generator (sometimes called a running-key generator) outputs a stream of bits: k1, k2, k3,..., ki. This keystream (sometimes called a running key) is XORed with a stream of plaintext bits, p1, p2, p3,..., pi, to produce the stream of ciphertext bits. ci = pi • ki At the decryption end, the ciphertext bits are XORed with an identical keystream to recover the plaintext bits. pi = ci • ki Since pi • ki • ki = pi this works nicely. The system’s security depends entirely on the insides of the keystream generator. If the keystream generator outputs an endless stream of zeros, the ciphertext will equal the plaintext and the whole operation will be worthless. If the keystream generator spits out a repeating 16-bit pattern, the algorithm will be a simple XOR with negligible security (see Section 1.4). If the keystream generator spits out an endless stream of random (not pseudo-random, but real random—see Section 2.8) bits, you have a one-time pad and perfect security. The reality of stream cipher security lies somewhere between the simple XOR and the one-time pad. The keystream generator generates a bit stream that looks random, but is actually a deterministic stream that can be flawlessly reproduced at decryption time. The closer the keystream generator’s output is to random, the harder t...
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