The benefit of using chaos in key schedule algorithm

Abstract

Abstract Many ciphers suffer from weak-key issues. This problem is due to the simplicity of the key schedule algorithm. Unfortunately, the cipher designers have not paid much attention to the sub-keys generation as the cipher itself. Thus, any attacker can find many attractive properties between sub-keys such as statistical pattern, linearity, low avalanche criteria, etc. Many ciphers use a large key during encryption (e.g. 128 bits). This makes it practically impossible to test all keys, allowing all of today’s ciphers to be potentially vulnerable to the weak-key issue, including AES. Accordingly, this paper sheds light on the advantage of using chaos in sub-keys generation by developing a new key schedule algorithm based on the logistic maps. We name this algorithm CKSA. Compared to several existing key schedule algorithm, CKSA provides a good avalanche effect. It is a one-way function, and its size is flexible, i.e. the size of the sub-keys is not fixed, which allows to many ciphers the possibility of using it. Further, CKSA ensures confusion and diffusion. It resists differential attacks and avoids linear key correlation between sub-keys while ensuring a high degree of randomization among them.