Marking Text Documents

Electronic documents are more easily copied and redistributed than paper documents. This is a major impediment to electronic publishing. Illegal redistribution can be discouraged by placing unique marks in each copy and registering the copy with the original recipient. If an illegal copy is discovered, the original recipient can be identified. In this work we describe several invisible techniques for encoding information in text documents. We also describe a marking system, for electronic publishing, that is scalable to large numbers of users.