Fish: A Fast Software Stream Cipher

Coppersmith, Krawczyk, and Mansour ([CKM93]) presented at Crypto '93 a promising stream cipher, the shrinking generator. It is based on linear shift registers with linear feedback. The output bits of one shift register decide which of the output bits of the other shift registers are used and which are discarded. The design is well suited for hardware implementation. In software shift registers are not very efficient because each machine instruction operates on a single bit only. The remaining bits in the registers of the processor are unused. In this paper we suggest an algorithm called Fish. We apply the shrinking principle to a stream cipher based on the lagged Fibonacci generator ([KnuS1]) (Fish F ibonacci shrinking). We use the full 32 bit wordlength of popular processors in order to achieve a high data rate.