UniSpaCh: A text-based data hiding method using Unicode space characters

► A visual attack named DASH is proposed to reveal existence of information embedded by the conventional methods, and UniSpaCh is proposed to counter DASH. ► UniSpaCh utilizes the combination of ordinary space character and Unicode space characters to embed information. UniSpaCh offers higher carrier efficiency than the conventional methods. ► UniSpaCh produces output document of size almost 9 times smaller than that of the existing method in the best case scenario. ► UniSpaCh is also reversible where the embedded information can be removed to completely reconstruct the original Microsoft Word document. This paper proposes a text-based data hiding method to insert external information into Microsoft Word document. First, the drawback of low embedding efficiency in the existing text-based data hiding methods is addressed, and a simple attack, DASH, is proposed to reveal the information inserted by the existing text-based data hiding methods. Then, a new data hiding method, UniSpaCh, is proposed to counter DASH. The characteristics of Unicode space characters with respect to embedding efficiency and DASH are analyzed, and the selected Unicode space characters are inserted into inter-sentence, inter-word, end-of-line and inter-paragraph spacings to encode external information while improving embedding efficiency and imperceptivity of the embedded information. UniSpaCh is also reversible where the embedded information can be removed to completely reconstruct the original Microsoft Word document. Experiments were carried out to verify the performance of UniSpaCh as well as comparing it to the existing space-manipulating data hiding methods. Results suggest that UniSpaCh offers higher embedding efficiency while exhibiting higher imperceptivity of white space manipulation when compared to the existing methods considered. In the best case scenario, UniSpaCh produces output document of size almost 9 times smaller than that of the existing method.