Related-Cipher Attacks

We formally introduce the concept of related-cipher attack. In this paper, we consider the related ciphers as block ciphers with the same round function but with different round numbers. If their key schedules do not depend on the total round number, then related-cipher attack could be applied if the same key is used. We applied this attack to block cipher SQUARE and show that SQUARE is vulnerable to this attack. We also show that a new AES key schedule proposed at ACISP02 is weaker than the original one under this attack. We then classify the differential attacks into three categories: related-message attack (the original differential cryptanalysis), related-key attack and relatedcipher attack. These attacks should be taken into consideration in cipher design.