The multimedia bitstream is comprised of variable length codes (VLC) to a large extent. If data hiding is carried out in multimedia compressed domain, VLC are usually chosen as the hidden object. The VLCcoded efficiency is a key factor which can directly affect the method of data hiding. MPEG-I Layer III (MP3) audio is chosen as analytical object in this paper. The VLC-coded rule is obtained by analyzing the VLC-coded efficiency of the MP3 audios. The process of variable length coding in MP3 audio can be understood further passing through the VLC-coded rule. At the same time, it also provides a favorable theoretical base for seeking for effective data hiding methods in MP3 audio compressed domain.

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In this paper, we present an information hiding method for MP3 audio files that operates directly in the compressed bitstream. The sign bits of huffman codes are chosen as the embedding object based on the characteristic of the Huffman coding in Count1 region. The audio file size can not be changed in the embedding process. And it doesn't need original audio file when secret information is extracted. The experimental results show that the proposed algorithm can obtain much higher data hiding capacity than MP3Stego with lower computational complexity and take on excellent imperceptibility.