Improved Meet-in-the-Middle Attacks on Round-Reduced Crypton-256

The meet-in-the-middle (MITM) attack has prove to be efficient in analyzing the AES block cipher. Its efficiency has been increasing with the introduction of various techniques such as differential enumeration, key-dependent sieve, super-box etc. The recent MITM attack given by Li and Jin has successfully mounted to 10-round AES-256. Crypton is an AES-like block cipher. In this paper, we apply the MITM method to the cryptanalysis of Crypton-256. Following Li and Jin’s idea, we give the first 6-round distinguisher for Crypton. Based on the distinguisher as well as the properties of Crypton’s simple key schedule, we successfully launch MITM attacks on Crypton-256 reduced to 9 and 10 rounds. For 9-round Crypton-256, our MITM attack can recover the 256-bit key with a time complexity 2, a memory complexity 2. For the 10-round version, we give two MITM attacks. The basic attack requires a time complexity 2 and memory complexity 2. The time/memory complexity of the advanced MITM attack on 10-round Crypton is 2/2. Our MITM attacks share the same data complexity 2 and their error rates are negligible.