Masao Nonaka

Cryptanalysis of the Reduced-Round RC6

In this paper, we propose the efficient and feasible key recovery algorithm against the reduced-round RC6 without whitening, called RC6W. Our attack applies to a rather large number of rounds. RC6W with r rounds can be broken in a success probability of 90% by using 28.1r-13.8 plaintexts. Therefore, our attack can break RC6W with 17 rounds by using 2123.9 plaintexts in a probability of 90%.

Known Plaintext Correlation Attack against RC5

We investigate a known plaintext attack on RC5 based on correlations. Compared with the best previous known-plaintext attack on RC5-32, a linear cryptanalysis by Borst, Preneel, and Vandewalle, our attack applies to a larger number of rounds. RC5-32 with r rounds can be broken with a success probability of 90% by using 26.14r+2.27 plaintexts. Therefore, our attack can break RC5-32 with 10 rounds (20 half-rounds) with 263.67 plaintexts with a probability of 90%. With a success probability of 30%, our attack can break RC5-32 with 21 half-rounds by using 263.07 plaintexts.