A Known Plaintext Attack of FEAL-4 and FEAL-6

We present new results on the cryptanalysis of the FEAL-N blockcipher. As a matter of fact, almost all the attacks of this cryptosystem published so far are chosen plaintext attacks [3,4,5,7], except the announcement in [7] of a non-differential known plaintext attack of FEAL-4 which requires about 100000 plaintext blocks. We describe known plaintext attacks of FEAL-4 and FEAL-6, which require about 1000 and 20000 plaintext blocks respectively and are based on correlations with linear functions. Using similar methods, we have also found more recently an improved attack on FEAL-4, which requires only 200 knwon plaintext blocks.