13 Stream ciphers

Stream ciphers are the opposite of block ciphers. Characters in stream ciphers are enciphered as they are encountered, usually one at a time. The basic concept is to take a stream of message characters and combine them with a stream of key characters to produce a stream of ciphertext characters. This paradigm is well suited for continuous operation where messages are continuously enciphered and transmitted at one end, and continuously received and deciphered at the other end, with no pauses, or only momentary pauses to change keys.

We have already seen a few stream ciphers. The autokey and running key ciphers in section 5.9, the rotor machines of section 5.10, Huffman substitution in section 10.4, and the ciphers based on text compression in section 10.7 are all examples of stream ciphers.

13.1 Combining functions

13.2 Random numbers

13.3 Multiplicative congruential generator

13.4 Linear congruential generator

13.5 Chained exclusive-OR generator

13.6 Chained addition generator

13.7 Shift and XOR generator

13.8 FRand

13.9 Mersenne Twister

13.10 Linear feedback shift registers

13.11 Estimating the period

13.12 Strengthening a generator

13.13 Combining generators