Linear Feedback Shift Register

A stream cipher is a symmetric cipher which operates with a time-varying transformation on individual plaintext digits. By contrast, block ciphers operate with a fixed transformation on large blocks of plaintext digits. More precisely, in a stream cipher a sequence of plaintext digits, m0m1 . . ., is encrypted into a sequence of ciphertext digits c0c1 . . . as follows: a pseudorandom sequence s0s1 . . ., called the running-key or the keystream, is produced by a finite state automaton whose initial state is determined by a secret key. The i-th keystream digit only depends on the secret key and on the (i−1) previous plaintext digits. Then, the i-th ciphertext digit is obtained by combining the i-th plaintext digit with the i-th keystream digit.