Improved Meet-in-the-Middle Attacks on 7 and 8-Round ARIA-192 and ARIA-256

The ARIA block cipher has been established as a Korean encryption standard by Korean government since 2004. In this work, we re-evaluate the security bound of reduced round ARIA-192 and ARIA-256 against meet-in-the-middle (MITM) key recovery attacks in the single key model. We present a new 4-round distinguisher to demonstrate the best 7 & 8 round MITM attacks on ARIA-192/256. Our 7-round attack on ARIA-192 has data, time and memory complexity of \(2^{113}\), \(2^{135.1}\) and \(2^{130}\) respectively. For our 7-round attack on ARIA-256, the data/time/memory complexities are \(2^{115}\), \(2^{136.1}\) and \(2^{130}\) respectively. These attacks improve upon the previous best MITM attack on the same in all the three dimensions. Our 8-round attack on ARIA-256 requires \(2^{113}\) cipher calls and has time and memory complexity of \(2^{245.9}\) and \(2^{138}\) respectively. This improves upon the previous best MITM attack on ARIA-256 in terms of time as well as memory complexity. Further, in our attacks, we are able to recover the actual secret key unlike the previous cryptanalytic attacks existing on ARIA-192/256. To the best of our knowledge, this is the first actual key recovery attack on ARIA so far. We apply multiset attack - a variant of meet-in-the-middle attack to achieve these results.