Unformatted text preview: ime a cryptanalyst will have breaking it. Figure 9.6 Stream cipher. If, however, the keystream generator produces the same bit stream every time it is turned on, the resulting cryptosystem will be trivial to break. An example will show why. If Eve has a ciphertext and associated plaintext, she can XOR the plaintext and the ciphertext to recover the keystream. Or, if she has two different ciphertexts encrypted with the same keystream, she can XOR them together and get two plaintext messages XORed with each other. This is easy to break, and then she can XOR one of the plaintexts with the ciphertext to get the keystream. Now, whenever she intercepts another ciphertext message, she has the keystream bits necessary to decrypt it. In addition, she can decrypt and read any old ciphertext messages she has previously intercepted. When Eve gets a single plaintext/ciphertext pair, she can read everything. This is why all stream ciphers have keys. The output of the keystream generator is a function of the key. Now, if Eve gets a plaintext/ciphertext pair, she can only read messages encrypted with a single key. Change the key, and the adversary is back to square one. Stream ciphers are especially useful to encrypt neverending streams of communications traffic: a T1 link between two computers, for example. A keystream generator has three basic parts (see Figure 9.7). The internal state describes the current state of the keystream generator. Two keystream generators, with the same key and the same internal state, will produce the same keystream. The output function takes the internal state and generates a keystream bit. The nextstate function takes the internal state and generates a new internal state. 9.5 SelfSynchronizing Stream Ciphers
For a selfsynchronizing stream cipher, each keystream bit is a function of a fixed number of previous ciphertext bits [1378]. The military calls this ciphertext auto key (CTAK). The basic idea was patented in 1946 [667]. Figure 9.7 Inside a ke...
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 Fall '10
 ALIULGER
 Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips

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