由Logistic映射产生的混沌序列常被用来置乱加密数字图像，但迄今为止，在国内外有关文献中，均未提到由Logistic映射产生的混沌序列中存在“平凡密钥”和“拟平凡密钥”的现象。如果用“平凡密钥”和“拟平凡密钥”作为Logistic映射的初始值，则将无法产生可用于图像置乱的混沌序列，并且在Logistic映射中存在无穷多个“平凡密钥”和“拟平凡密钥”，这可能会导致对图像置乱加密无效，这是值得注意的问题．针对该问题，在对由Logistic映射产生的混沌序列中存在的“平凡密钥”和“拟平凡密钥”进行研究的基础上，提出了一种新的基于混合混沌序列的图像置乱加密算法，从而彻底解决了“平凡密钥”和“拟平凡密钥”对图像置乱加密无效的问题。

The Logistic-Map chaotic sequence is often used to scramble and encrypt the digital images.In the former papers,the invalid-keys and the quasi invalid-keys existing in the Logistic-Map have not yet been discussed.If using the invalid-keys or the quasi invalid-keys as the initial value of the Logistic-Map,we can't get the chaotic sequence to scramble the digital images.Furthermore,there are infinite invalid-keys and quasi invalid-keys in the Logistic-Map.So we should take the problems seriously.In this paper,the invalid-keys and the quasi invalid-keys existing in the Logistic-Map chaotic sequence are deeply discussed.Then,an image-scrambling algorithm based on mixed chaotic sequences is proposed.Using this algorithm,the invalidity of the invalidkeys and the quasi invalid-keys is avoided thoroughly.