New Differential Cryptanalytic Results for Reduced-Round CAST-128

抄録

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 F₁ and F₃ in CAST-128, and identified differential characteristics for F₁ round function and F₃ 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 72bits and 9-round CAST-128 with key sizes greater than or equal to 104bits. We give the summary of attacks on reduced-round CAST-128 in Table 10.