Cryptanalysis of the ISDB Scrambling Algorithm (MULTI2)

MULTI2 is the block cipher used in the ISDB standard for scrambling digital multimedia content. MULTI2 is used in Japan to se- cure multimedia broadcasting, including recent applications like HDTV and mobile TV. It is the only cipher specified in the 2007 Japanese ARIB standard for conditional access systems. This paper presents a theoretical break of MULTI2 (not relevant in practice), with shortcut key recovery attacks for any number of rounds. We also describe equivalent keys and linear attacks on reduced versions with up 20 rounds (out of 32), improv- ing on the previous 12-round attack by Matsui and Yamagishi. Practical attacks are presented on up to 16 rounds.