We propose an algorithm to embed data directly in the bitstream of JPEG imagery. The motivation for this approach is that images are seldom available in uncompressed form. Algorithms that operate in spatial domain, or even in coefficient domain, require full (or at best) partial decompression. Our approach exploits the fact that only a fraction of JPEG code space is actually used by available encoders. Data embedding is performed by mapping a used variable length code (VLC) to an unused VLC. However, standard viewers unaware of the change will not properly display the image. We address this problem by a novel error concealment technique. Concealment works by remapping run/size values of marked VLCs so that standard viewers do not lose synchronization and displays the image with minimum loss of quality. It is possible for the embedded image to be visually identical to the original even though the two files are bitwise different. The algorithm is fast and transparent and embedding is reversible and file-size preserving. Under certain circumstances, file size may actually decrease despite carrying a payload.

In this letter, we propose a fragile digital watermarking algorithm that is directly applicable to compressed bit streams and is mathematically lossless with practically no increase in file size. The algorithm exploits the inefficiency of the code space of popular compression standards by recognizing that not all code words appear in the bit stream. By mapping the code words to the outside of the used code space, we can embed the watermark into the stream as forced bit errors. The decoder recognizes such code words, recovers the watermark from them, and restores the media to its original state. As proof of the concept, the algorithm is then applied to the JPEG bit stream.