Stream cipher

From CryptoDox, The Online Encyclopedia on Cryptography and Information Security

A stream cipher is a symmetric cipher where plaintext digits are encrypted one at a time, and in which the transformation of successive digits varies during the encryption. It is also known as a state cipher.

The encrypter generates a pseudorandom "key stream" of digits. Each digit of the key stream is combined with one digit of the plain text, and then sent out.

Most stream ciphers combine 1 bit at a time, using the "xor" function.

Some stream ciphers, such as the Enigma, combine a letter of the plaintext with a letter of the keystream using a Beaufort cipher function.

The difficult part of designing the cipher is generating the "key stream". Both the transmitter and the receiver must use exactly the same "key stream" of digits, but it must be practically impossible for anyone else to guess any digit of the key stream with a success better than chance.

If the "key stream" is generated independently of the plaintext or the ciphertext, and either the "xor" function or the "Beaufort cipher" function is used, then the cipher is a kind of reciprocal cipher.

See Also

• Block cipher
• one-time pad

External Links

• Stream Cipher at Wikipedia
• What is a stream cipher? - RSA Security
• eSTREAM is a project to identify "new stream ciphers that might become suitable for widespread adoption", organised by the EU ECRYPT network.