中文摘要：

摘要：针对Baptista提出的混沌加密算法及其改进算法中的密文过长及0，l比例不均匀的问题，该文通过计算密文的信息熵，给出了密文明文比例期望的下界：然后运用文中所提出的Ⅳ一截断等长编码和最优Ⅳ一截断等长编码等概念描述了编码方式与密文明文比例之间的关系，给出了0，1比例的近似计算公式。算例分析表明最优Ⅳ一截断等长编码可有效缩短密文长度，提高0，l比例的均匀度。

英文摘要：

In order to solve the two major drawbacks of Baptista＇s chaotic cryptosystem and its modified versions——excessive length of ciphertext and unbalance ratio of bit 0 to bit 1, this paper gives the lower bound of the expectation of the cipher-to-plaintext ratio by calculating the entropy of the ciphertext. Then exploiting N- truncated equal length coding, the optimal N-truncated equal length coding and so on, this paper describes the relationship between coding and cipher-to-plaintext ratio, and proposes the approximate formula to work out the ratio of bit 0 to bit 1. Numerical paradigms show that the optimal N-truncated equal length coding can efficiently shorten the length of ciphertext and obtain more uniform ratio of bit 0 to bit 1.