Research On Image Joint Compression-Encryption Algorithm Based On SPIHT Coding

With the increasingly development of Multimedia Technology and digital image technology, digital image has gradually become an important source of access to information. In order to improve its security and guarantee transmission efficiency of image information, the data need to be compressed and encrypted efficiently.To meet the high speed on the Internet image security transmission, traditionally, the process of compression and encryption of image data are implemented independently. However, it lacks flexibility in design and the calculation is complicated. Therefore, the research of the image joint realization of compression and encryption has the theoretical and practical significance.In order to reduce the cost in developing, testing and using brought by the traditional independent structure method, the arithmetic coder can be used for the joint compression and encryption of data. Yet, there’re two defects using the conventional arithmetic encryption:one being the limitation of accuracy in probability representation; the other being the assumed probability does not match with the actual probability. Using it will reduce the security and compression efficiency, which fails to meet the engineering requirements.Relying on the related research of image compression and image encryption currently, a new method is proposed in this paper, which is based on SPIHT (The Set Partitioning in Hierarchical Trees) coding algorithm. The algorithm firstly redefines the zerotree structure, then conducts the set partitioning of trees, which leads to encoding based on the correlation of the coefficients and the sets. The data output(i.e., the original decision and context) of the arithmetic coding based on SPIHT are modified by keys to realize the image joint compression and encryption.Using the relations between sets and coefficients, different sub bands can be selected and encrypted by user’s need, so the flexibility of image joint compression and encryption is improved.The algorithm simulation results show that our scheme has the same compression efficiency compared with the original image compression algorithm by testing two standard gray-scale images(Goldhill and Lena),which can satisfy the requirement of the human eye vision, namely it also can restore the basic image information when the rate is limited. The influence on the PSNR of reconstruction image is less than0.5dB after modification on the coefficients, which means modifying the selected coefficients has little effect in quality. The PSNR of reconstruction image reaches minimum of8.82dB. When the keys are wrong, and it is completely unable to extract image information. Thus the proposed algorithm can realize the important information privacy and confidentiality protection.The structure of proposed joint image compression-encryption algorithm is simple and it has lower complexity, more flexibility and less cost, and it can meet the requirements of compression efficiency and safety.