Abstract: ARIA cipher is a new block cipher proposed by some South Korean experts in 2003. The design principle of ARIA is similar to the AES, and it has relatively high security. ARIA was established as a Korean Standard block cipher algorithm by Korean Agency for Technology and Standards in 2004. Combining the features of ARIA algorithm, a new impossible differential attack on 7-round ARIA is proposed by adding 2-round at the beginning and 1-round at the end. It is shown that this new impossible differential attack requires a data complexity of about 2\+\{119\}chosen plaintexts and a time complexity of about 2\+\{218\}7-round ARIA encryptions. Compared with the previous impossible differential attacks, this attack efficiently reduces the data complexity and time complexity. Similar to the attack of 7-round, a new impossible differential attack on 8-round ARIA is proposed first time by adding 2-round at the beginning and 2-round at the end. It is shown that this new impossible differential attack requires a data complexity of about 2\+\{207\} chosen plaintexts and a time complexity of about 2\+\{346\}8-round ARIA encryptions. It has exceeded the attack complexity of exhaustive search attack, so we can believe that ARIA cryptographic algorithm is safe in this path of 8-round impossible differential attack.

Key words: block cipher, ARIA cipher, impossible differential, time complexity, data complexity