A Hierarchical Selective Encryption Technique in a Scalable Image Codec

Modern still image codecs furnish more than just good distortion-rate performances. They must also provide some services. Scalability in resolution and quality, error resilience and embedded bitstreams were among the first one to be available. There is still room for enhancement, especially when it comes to security-oriented features. Image encryption is one of the aspect of image security. This paper presents the embedding of an encryption service in a multiresolution lossless codec. Partial encryption is performed using ciphering only the keystone part of the codec algorithm, a quadtree decomposition. This results in a hierarchical encryption scheme, showing to be a good tradeoff between encryption speed, selective access and robustness. 1 Selective image encryption Nowadays, huge amount of digital visual data are stored on different media and are exchanged over various networks . Often, these visual data contain private, confidential or proprietary informations or are associated with economical interests. As a consequence, techniques especially designed for these data are required so that to provide security functionalities such as privacy, integrity, or authentication. Multimedia security[3] is aimed towards these technologies and applications. Besides watermarking, steganography, and techniques for assessing data integrity and authenticity, providing confidentiality and privacy for visual data is among the most important topics in the area of multimedia security. Applications range from digital rights management to secured personal communications. Contrary to classical encryption[2], security may not be the most important aim for an encryption system for images. Depending on the type of applications, other properties (such as speed or bitstream compliance after encryption) might be equally important as well. In that context, naive or hard encryption consists in putting in the whole image data bitstream into a standard encryption system, without taking care of its nature. However, considering the typical size of a digital image compared to that of a text message, the naive algorithm usually cannot meet the speed requirements for real-time digital image processing or transmission applications. In contrast, soft or selective encryption trades off security for computational complexity. They are designed to protect multimedia content and fulfil the security requirements for a particular multimedia application. Research is focused on fast encryption procedures specifically tailored to the target environment. There are two levels of security for digital image encryption: low level and high-level security encryption. In low-level security encryption, the encrypted image has degraded visual quality compared to that of the original one, but the content of the image is still visible and understandable to the viewers. In the high-level security case, the content is completely scrambled and the image just looks like random noise. In this case, the image is not understandable to the viewers at all. Selective encryption [4]aims at avoiding the enthe image decrypted without the decryption key 1 ha l-0 03 36 40 3, v er si on 1 3 N ov 2 00 8 Author manuscript, published in "N/P"