New Differential Cryptanalytic Results for Reduced-Round CAST-128

摘要

CAST-128 is a block cipher used in a number of products, notably as the default cipher in some versions of GPG and PGP. It has been approved for Canadian government use by the Communications Security Establishment. Haruki Seki et al. found 2-round differential characteristics and they can attack 5-round CAST-128. In this paper, we studied the properties of round functions F and F_3 in CAST-128, and identified differential characteristics for F round function and F_3 round function. So we identified a 6-round differential characteristic with probability 2~(-53) under 2~(-23.8) of the total key space. Then based on 6-round differential characteristic, we can attack 8-round CAST-128 with key sizes greater than or equal to 72 bits and 9-round CAST-128 with key sizes greater than or equal to 104 bits. We give the summary of attacks on reduced-round CAST-128 in Table 10.