Improvement of a Cryptosystem Using Two Chaotic Maps

A chaotic map has various features. A chaotic map has sensitivity to a change in initial conditions and parameters, and a long-term forecast becomes impossible by the iterations of a chaotic map. These features looks similar to the properties of the cryptology. For that reason, it is effective to use chaotic maps for cryptosystems. The chaotic cryptosystem is researched as an application of chaos in an engineering field [1-4]. On the other hand, the security of the chaotic cryptosystem is not investigated in detail. It is necessary to investigate the security and to clarify the vulnerability of the chaotic cryptosystem. A cryptosystem using the tent map has been proposed [5]. And a cryptosystem using two chaotic maps has been proposed [6]. This system performed encryption and decryption by using two chaotic maps, a skew tent map and a logistic map, and their inverse maps. In this research, we consider a chosen plaintext attack (CPA) for this cryptosystem. And we investigate vulnerability of the cryptosystem using two chaotic maps to CPA. We propose an effective method against this attack. The proposed method repeats the encryption process twice. We confirm that eavesdroppers cannot select the intended plaintext in the second encryption process.