Watermaking For The Authentication Of Compressed Document

Document Watermarking by exploiting the multiplicity of encoding of a file using LZ-77 compressed method and its variations (we called"LZS-77"), has been proposed recently as a new method of document authentication. It allows one to hide enough information within the compressed document to warrant its authenticity. However, the watermark embedding will degrade the compression performance of gzip (a universal compression standard for lossless data). In this paper, some variations have been made to the LZS-77 algorithm to reduce the compression degradation. In order to achieve this purpose, the main technique used is to select only from the nearer longest matches instead of the all longest matches. The methods used can be concluded as follows:①Limit the search range of the longest match to a certain distance relative to some benchmark (for example, the current compression position or the nearest longest match).②Given the relationship between the positions of the longest match and the binary codes of the complete tree's leaf nodes, and then add an extra bit or more as an index to the next bits of the secret message.③Use the above two methods together.The experimental results show that our algorithm, compared to the LZS-77 algorithm, can improve compression rates on twenty files of the Cargary corpus and Canterbury corpus. So these changes are feasible.The means used for the security of the document authentication are also given in this paper.