Data hiding in MPEG-2 bitstream by creating exceptions in code space

Abstract

In this work, we have developed a file-size preserving algorithm to hide data directly in MPEG-2 bitstreams without violating the syntax. The approach is based on the realization that embedding of a bit directly in the bitstream can be achieved by creating an exception in the code space. An exception is created by inserting code words not occurring in the original content into the bitstream. Because the entropy-coded segment of MPEG-2 is fully used, every valid code word appears somewhere in the video. However, exceptions can still be created by recognizing that there are certain variable length codes (VLC) that never occur together in any video block. These are unused VLC pairs and although they vary from stream to stream, they always exist. It is then possible to embed one bit of data by forcing an unused pair in a block subject to a number of constraints, including transparency, file-size preservation and quality metric. The list of unused pairs constitutes a secure key that can be identified by pointer. This pointer can be efficiently encoded for sharing through a secure download. The content-dependence of the key allows a secure exchange of information because each key is valid only for the corresponding video. Because the cover video still uses standard VLCs, the marked bitstream is transparent to external observers. Application areas for the proposed approach are metadata binding and general data hiding when the size of the key is less important than the security and value of attaching data to content. Examples are surveillance video footage and medical applications.