A Study of Arithmetic Codes for Joint Encryption and Compression

Abstract

The requirements for encrypting multimedia contents are very different from general purpose symmetric encryptions. Those requirements including low computation power, small data size, and enabling partial encryptions, make neither common block ciphers like DES/AES nor other stream ciphers applicable to protect multimedia contents. Recently, many works have been proposed to compress and encrypt multimedia contents simultaneously. Being a compression code adopted in several industrial standards, the arithmetic code is studied in some of these works. In particular, Kim extit{et al.} proposed a secure compression code called Secure Arithmetic Code (SAC) in 2007. It had been believed to be an efficient and secure algorithm. However, we find that SAC is not as secure as the authors have claimed. In the first part of the thesis, we show that SAC is prone to two attacks. The first attack completely breaks the code using an adaptive chosen plaintext attack with a polynomial number of queries. The second attack is a ciphertext-only attack, which removes a part of the output permutation. In the second part of the thesis, we present a novel and efficient scheme called Arithmetic Code Encryption (ACE) to jointly compress and encrypt multimedia contents base on the arithmetic code. Through the experience in breaking SAC, we realize that the diffusion property offered by the code is the key of defense against differential attacks. We design ACE by exploring the characteristics of the arithmetic code to provide high security levels that previous schemes fail to deliver. Experiments have been conducted to help us verify ACE's security properties and select appropriate system parameters. We also discuss how parital encryptions can be implemented with ACE that provides leverages between security and flexibility.