Electronic and printed text documents are still the most common means of information communication among humans. One of the major problems that our society faces is the ease with which dishonest people can forge official documents. Although modern cryptography can deal with this security issue in the case of electronic documents, it cannot be applied to the case of printed documents. Classical ways to secure a printed document are expensive and usually do not protect the document's content itself. In this thesis, we address the above problems by developing new low-cost methods that can be applied to both electronic and printed text documents. The main contributions include the development of a new state-of-art for two-dimensional bar codes, a novel information theoretic framework for document authentication as well as practical text data hiding and robust text hashing algorithms that can be used as building blocks for text document authentication systems.