Image encryption methods aim to protect content privacy. Typically, they encompass scrambling and diffusion. Every pixel of the image is permuted (scrambling) and its value is transformed according to a key (diffusion). Although several methods have been proposed in the literature, some of them have been cryptanalyzed. In this paper, we present a novel method that deviates the traditional schemes. We use variable length codes based on Collatz conjecture for transforming the content of the image into non-intelligible audio; therefore, scrambling and diffusion processes are performed simultaneously in a non-linear way. With our method, different ciphered audio is obtained every time, and it depends exclusively on the selected key (the size of the key space equal to 8 . 57 × 10 506 ). Several tests were performed in order to analyze randomness of the ciphered audio signals and the sensitivity of the key. Firstly, it was found that entropy and the level of disorder of ciphered audio signals are very close to the maximum value of randomness. Secondly, fractal behavior was detected into scatter plots of adjacent samples, altering completely the behavior of natural images. Finally, if the key was slightly modified, the image could not be recovered. With the above results, it was concluded that our method is very useful in image privacy protection applications.

The purpose of image encryption is to provide data privacy and security. The former ensures that only authorized personnel can access the original content, while the latter implies that there is no evident relationship between the encrypted and the original content, and that the key space is equally likely and large enough. In the current state of the field, there are several proposals of image encryption techniques with very high privacy (in terms of entropy) but weak in terms of security (i.e., small key space). Recently, a new encoding-based method that provides a long key space (namely 8,57 × 10506) with a middle value of entropy (87%) was proposed. Our proposal preserves the strength of the image encryption methods based on encoding, but with a higher value placed on security than the preliminary works. Every pixel of an image is mapped into an orthogonal code based on 256 bits. The 8-OVSF codes are selected to encode the image, given that the entropy of the inter-symbol is near the possible maximum. Numerous test results verify that our ciphered data have a very high value of entropy (98,5%) with an equally likely and long key space (8,57 × 10506), thus providing an adequate balance between privacy and security.