混沌加密系統在這近二十年一直是一個重要的研究領域。由於混沌理論與加密系統密切的關係，使混沌理論廣泛的應用在安全性傳輸與加密系統當中。基於混沌訊號中雜訊般的訊號以及混沌系統對初始值的高敏感性，許多混沌加密系統被提出並應用在安全性傳輸上。

本論文提出一基於遞迴多混沌系統與使用外部秘密金鑰的複合式區塊加密系統。此加密系統使用一個128位元的外部秘密金鑰及四個一維混沌系統，此方法利用 J. Wei 2007及X. Wang 2009兩個加密系統建構而成。所提系統由Wang的演算法組成核心部份，再加上Wei的演算法來提升速度。它保持了上述兩加密系統原有的優點且避開其缺點。再者，我們亦修改部份的運算法以更提升安全性與加快系統速度。

所提加密系統的金鑰空間高達 ，到達可以抵擋暴力破解攻擊的標準。對於一大小為512 512的灰階影像檔案，加密後的影像將原來熵值7.475746提升到7.999413，此數值非常接近理想值8，而加 / 解密的平均時間為0.9秒，比起Wang的加密系統快了2.5秒。而加密後影像的相關係數幾乎為零，代表兩相鄰的點幾乎不相關。在key的敏感度測試上，我們對原始 / 加密後的影像，使用非常微小變化的key來加 / 解密，得到的NCPR值均高達99%以上。而使用本加密系統得到的密文大小與明文相同。實驗結果顯示本加密系統非常適合實際應用於安全傳輸。

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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