Related-Key Attacks on DDP Based Ciphers: CIKS-128 and CIKS-128H

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CIKS-128 and CIKS-128H are 128-bit block ciphers with a 256-bit key sizes based on data-dependent operations, respectively. They are also fast hardware-oriented ciphers and improvements of block cipher CIKS-1 introduced in [14]. This paper presents related-key differential attacks on full-round CIKS-128 and CIKS-128H. In result, using full-round related-key differential characteristics with probability 2− − 36 and 2− − 35.4, these attacks can recover the partial subkey bits for CIKS-128 and CIKS-128H with about 240 plaintexts, respectively. These works suggests that the greatest possible care has to be taken when proposing improvements of the existing block ciphers.