A stream cipher is a symmetric cryptosystem (see key) 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, \(m_0 m_1 \ldots\), is encrypted into a sequence of ciphertext digits \(c_0 c_1 \ldots\) as follows: a pseudorandom sequence \(s_0 s_1 \ldots\), called the running-key or the keystream, is produced by a finite state automaton whose initial state is determined by a secret key. The ith keystream digit only depends on the secret key and on the \((i-1)\) previous plaintext digits. Then, the ith ciphertext digit is obtained by combining the ith plaintext digit with the ith keystream digit.
Stream ciphers are classified into two types: synchronous stream ciphers and asynchronous stream ciphers. The most famous stream cipher is the Vernam cipher, also called one-time pad, that leads to perfect...
References
Rueppel, R.A. (1986). Analysis and Design of Stream Ciphers. Springer-Verlag, Berlin.
Vernam, G.S. (1926). “Cipher printing telegraph systems for secret wire and radio telegraphic communications.” Journal of the American Institute of Electrical Engineers, 55, 109–115.
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Canteaut, A. (2005). Stream Cipher. In: van Tilborg, H.C.A. (eds) Encyclopedia of Cryptography and Security. Springer, Boston, MA . https://doi.org/10.1007/0-387-23483-7_412
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