Automatic detection of a watermarked document using a private key

Abstract

Many algorithms which mark data in order to enforce copyright protection have recently been proposed. Among these, the family of spread-spectrum based schemes is predominant. This family has an inherent weakness when used to mark several documents: either it changes the key for each document and has to maintain a complex database of keys or it uses the same key for every document and will face collusion attacks. In this paper, we propose a new blind scheme, which embeds different marks in different documents, but allows detection with the help of a single private key. Our scheme can be used on top of most existing spread spectrum based schemes, and is much less prone to collusion attacks than the latters. We also prove that the false positive and false negative detection rates of our protocol are exponentially small. Finally, we believe the mathematical tools used in this article to prove concentration of random variables around their means will be useful for analyzing other watermarking schemes, and that they will be of further use for other problems in cryptology.