Natural Language Watermarking and Tamperproofing

Abstract

Two main results in the area of information hiding in natural language text are presented. A semantically-based scheme dramatically improves the information-hiding capacity of any text through two tech- niques: (i) modifying the granularity of meaning of individual sentences, whereas our own previous scheme kept the granularity fixed, and (ii) halving the number of sentences affected by the watermark. No longer a “long text”, short watermark. approach, it now makes it possible to wa- termark short texts, like wire agency reports. Using both the above- mentioned semantic marking scheme and our previous syntactically- based method hides information in a way that reveals any non-trivial tampering with the text (while re-formatting is not considered to be tampering.the problem would be solved trivially otherwise by hiding a hash of the text) with a probability 1-2 ^-β(n+1), n being its number of sentences and β a small positive integer based on the extent of co- referencing.

Portions of this work were supported by Grants EIA-9903545 and ISS-0219560 from the National Science Foundation, Contract N00014-02-1-0364 from the Office of Naval Re- search, and by sponsors of the Center for Education and Research in Information Assurance and Security. An online demo can be found at http://www.cerias.purdue.edu/homes/wmnlt/semdemo.html.