Selective encryption algorithms: Overview with performance evaluation

Abstract

Digital multimedia content is becoming widely used and increasingly exchanged over computer network and public channels (satelite, wireless networks, Internet, etc.) which is unsecured transmission media for ex changing that kind of information. Mechanisms made to encrypt image and video data are becoming more and more significant. Traditional cryptographic techniques can guarantee a high level of security but at the cost of expensive implementation and important transmission delays. These shortcomings can be exceeded using selective encryption algorithms. Introduction In traditional image and video content protection schemes, called fully layered, the whole content is first compressed. Then, the compressed bitstream is entirely encrypted using a standard cipher (DES - Data Encryption Algorithm, IDEA - International Data Encryption Algorithm, AES - Advanced Encryption Algorithm etc.). The specific characteristics of this kind of data, high-transmission rate with limited bandwidth, make standard encryption algorithms inadequate. Another limitation of traditional systems consists of altering the whole bitstream syntax which may disable some codec functionalities on the delivery site coder and decoder on the receiving site. Selective encryption is a new trend in image and video content protection. As its name says, it consists of encrypting only a subset of the data. The aim of selective encryption is to reduce the amount of data to encrypt while preserving a sufficient level of security. Theoretical foundation of selective encryption The first theoretical foundation of selective encryption was given indirectly by Claude Elwood Shannon in his work about communication theory of secrecy systems. It is well known that statistics for image and video data differ much from classical text data. Indeed, image and video data are strongly correlated and have strong spatial/temporal redundancy. Evaluation criteria for selective encryption algorithm performance evaluation We need to define a set of evaluation criteria that will help evaluating and comparing selective encryption algorithms. - Tunability - Visual degradation - Cryptographic security - Encryption ratio - Compression friendliness - Format compliance - Error tolerance Classification of selective encryption algorithms One possible classification of selective encryption algorithms is relative to when encryption is performed with respect to compression. This classification is adequate since it has intrinsic consequences on selective encryption algorithms behavior. We consider three classes of algorithms as follows: - Precompression - Incompression - Postcompression Overview of selective encryption algorithms In accordance with their precedently defined classification, selective encryption algorithms were compared, briefly described with advantages and disadvantages and their quality was assessed. Applications Selective encryption mechanisms became more and more important and can be applied in many different areas. Some potential application areas of this mechanism are: - Monitoring encrypted content - PDAs (PDA - Personal Digital Assistant), mobile phones, and other mobile terminals - Multiple encryptions - Transcodability/scalability of encrypted content Conclusion As we can see through foregoing analysis, we can notice that tunability, cryptographic security and error tolerance are the main unsatisfied criteria. Selective encryption algorithms based on static encryption parameters do not allow tunability. Tunability is a desirable property especially for content protection systems targeting different applications with different requirements in terms of security or visual degradation and different devices with different capabilities in terms of memory, computational power, or display capabilities. It is therefore appreciated to design a tunable selective encryption algorithm with dynamic encryption parameters.