Transform encryption coding (TEC) is a universal technique for the performance improvement of conventional transform coding (TC) techniques. It not only increases the compression ratio, quality, and security level of coded image but also decreases the coded image sensitivity to channel noise. In TEC, TC technique is applied to the encrypted image instead of the natural scene image. The sample of encrypted image is the weighted sum of the original image samples. Apparently, TEC is compatible with all the TC techniques.
JPEG system is applicable to continuous-tone gray-scale or color digital still image data. Since TC technique is employed in JPEG baseline system, TEC technique can then be used to improve its performance. In this paper, the quantization tables in JPEG system are redesigned to match the statistical characteristics of encrypted images. In addition, some parameters required by encryption process is also chosen. Therefore, the performance of JPEG baseline system on the encrypted images can then be increased. According to the simulation results, about 0.9 dB luminance and 0.7 dB chrominance SNR increase in JPEG demonstration images is obtained by the combined JPEG-TEC technique.