Abstract:
Security of Crypto systems is usually analyzed through different cryptanalytic methods. Since Advance
Encryption Standard (AES) is one of the most widely used and popular block cipher, a number of attacks have already
been proposed on it. Lots of reduced round attacks on AES are available in the literature. In this paper, two efficient
reduced round impossible differential attacks are introduced against AES - 256.The attacks proposed here show how an
attack can modified for betterment. The first one is a new 8th round attack, which shows the data complexity and time
complexity can get interchanged without affecting the memory requirement, by introducing proper change in the attack
procedure.The second cryptanalysis is carried out in which four round impossible differential begins from third round
only, wherein conventional attacks it starts from second round itself. This difference in attack procedure leads to
reduction in data as well as time complexities. Moreover, the interchange of Add Round Key and Mix Column
operations done in the 7th round of conventional impossible attacks can be avoided here. A conventional attack
appeared in the literature is taken as the main reference. Comparison of the complexities is also given.