Fault Analysis on the Stream Ciphers LILI-128 and Achterbahn

LILI-128 is a clock controlled stream cipher based on two LFSRs with one clock control function and one non-linear filter function. The clocking of the second LFSR is controlled by the first LFSR. In this paper we propose a fault algebraic attack on LILI-128 stream cipher. We first recover the state bits of the first LFSR by injecting a single bit fault in the first LFSR. After that we recover the second LFSR state bits by following algebraic cryptanalysis technique. We also propose fault attack on Achterbahn stream cipher, which is based on 8 NLFSRs, 8 LFSRs and one non-linear combining function. We first inject a single bit fault into the NLFSR-A then observe the normal and faulty keystream bits to recover almost all the state bits of the NLFSR-A after key initialization phase. One can apply our technique to other NLFSR-B, C, D to recover their state bits also.