| Summary: | 碩士 === 國立成功大學 === 電腦與通信工程研究所 === 98 === Chaotic cryptography has been an important research area during the last two decades. The close relationship between chaos and cryptosystem makes chaos be used in secure communication and cryptosystem. Based on the facts that chaotic signals are usually noise-like and chaotic systems are very sensitive to initial conditions, a number of digital chaotic cryptographic approaches have been studied so far for secure communications.
In this thesis, a block cryptographic scheme based on iterating multiple chaotic maps is proposed. The proposed cryptosystem uses the external 128-bit secret key and four one-dimensional chaotic maps. The proposed scheme is a combination of J. Wei cryptosystem and X. Wang’s. Its kernel is from X. Wang’s algorithm, then the idea of J. Wei’s algorithm is used to speed up the proposed cryptosystem. We keep the merits while avoiding the flaws of the two existing cryptosystems. Furthermore, we modify some parts of the algorithms to enhance the security and speed up the proposed cryptosystem.
The keyspace of our proposed method is which is large enough to resist the brute-force attack. For a 512 512 256 grayscale image file, the entropy of ciphertext is 7.999413 which approximate to 8 while the plaintext one is 7.475746, and the average encryption / decryption time is about 0.9 second. It’s 2.5 second faster than Wang’s cryptosystem. The correlation coefficient of the encrypted image is approximated to zero. For key sensitive test, the value of NCPR between two images encrypted / decrypted using slightly different keys is over 99%. The size of ciphertext is the same as the plaintext size. Simulation results show that the performance of the proposed cryptographic scheme is suitable for practical use in the secure transmission.
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