This paper describes the basic idea of a novel Digital Rights Management(DRM) method which is composed of an incomplete cryptography using invariant huffman code length feature and the user identification mechanism to control the quality of the digital contents. We adopt the huffman code length feature of the DCT coefficient in the JPEG codec to the implement incomplete cryptography. The encoding process and the decoding process are presented by randomly selected the coefficients that belong to same category in the huffman table. In the our scheme, the copyright information is embedded into the decoded content while decoding process, and the size of digital contents are invariance during process. Experimental results with simulation confirmed that the modified codes keep compatibility with standard JPEG format, and revealed the proposed method is suitable for DRM in the network distribution system.