Shared key encryption of JPEG color images

Several methods have been proposed for encrypting images by shared key encryption mechanisms since the work of Naor and Shamir. All the existing techniques are applicable to primarily non-compressed images in either monochrome or color domains. However, most imaging applications including digital photography, archiving, and Internet communications nowadays use images in the JPEG compressed format. Application of the existing shared key cryptographic schemes for these images requires conversion back into spatial domain. In this paper we propose a shared key algorithm that works directly in the JPEG domain, thus enabling shared key image encryption for a variety of applications. The scheme directly works on the quantized DCT coefficients and the resulting noise-like shares are also stored in the JPEG format. The decryption process is lossless preserving the original JPEG data. The experiments indicate that each share image is approximately the same size as the original JPEG image retaining the storage advantage provided by JPEG compression standard. Three extensions, one to improve the random appearance of the generated shares, another to obtain shares with asymmetric file sizes, and the third to generalize the scheme for n>2 share cases, are described as well.