Improved Key Recovery Attack on Round-reduced Hierocrypt-L1 in the Single-Key Setting

Hierocrypt-L1 is a 64-bit block cipher with a 128-bit key. It was selected among the Japanese e-Government 2003 recommended ciphers list and has been reselected in the 2013 candidate recommended ciphers list. In this work, we cryptanalyze Hierocrypt-L1 in the single-key setting. In particular, we construct a 5 S-box layers distinguisher that we utilize to launch a meet-in-the-middle attack on 8 S-box layers roundreduced Hierocrypt-L1 using the differential enumeration technique. Our attack allows us to recover the master key with data complexity of 2 chosen plaintexts, time complexity of 2 8-Sbox layers HierocryptL1 encryptions and memory complexity of 2 64-bit blocks. Up to the authors’ knowledge, this is the first cryptanalysis result that reaches 8 S-box layers of Hierocrypt-L1 in the single-key setting.