Stream ciphers based on NLFSR

Stream ciphering devices seem to be one of the best alternatives in order to provide confidentiality to high-speed transmissions. Several indices on the security of stream ciphers have been proposed for guaranteeing their strength. According to the literature, the linear complexity of the key stream, randomness and correlation-immune attacks are of great importance. But the proposed indices are not sufficient to guarantee the security of stream ciphers. It is possible that sequences with a high linear complexity have a very bad "linear complexity stability", i.e., after changing a few bits of the original sequence, its linear complexity decreases or increases fast. In this case, a BAA attack (best affine approximation) could be very successful; the sequences may be very well approximated by another one with very much lower linear unpredictability. This problem is especially important when linear feedback shift registers are used. To solve this problem, nonlinear next state functions are a possible solution. This paper shows stream ciphering alternatives based on nonlinear feedback shift registers (NLFSR), and some techniques in order to analyse them. Finally, we compare a structure based on NLFSR with the best-known stream ciphering schemes.