Research On Digital Watermark For Text

The rapid development of computer and Internet technology has led to digital products being distributed deeply and widely to a previously unpredicted level. Meanwhile their copyright protection is an urgent problem. Fortunately Digital Watermarking has become an effective method to protect copyright for digital products, which has solved the problem of traditional cryptography that encrypted contents are not secure after decryption, therefore it has been studied and used widely as an effective method of knowledge copyright protection and preventing from sham for digital multimedia.The information hided should be related with the text content. And it is better if the document added into watermark is much similar with the original one. The best way to hide information is that there is no change after watermarking add to document.There are two kinds of text digital watermark, one based on its format and the other based on its content. The former is mainly used in early days, the classic algorithm of watermark are usually embeded in the space field such as line sihft coding,word shift coding and character coding. For example , It is marked"1"if current line is shifted up, or it is marked"0"if the current line shift down.The greatest weakness of these wartermark methods is the hided information is related with the formats of the documents, but not related with the contents of the documents. So it is easy to erase the watermark by formatting the documents. On the contrary, the latter is mainly used in present days, watermark embedded in the space field or the transform domain by algorithm such as synonyms replacement according to the content ,sentence pattern transformation according to the natural language characteristc.The advantage of this mothod is hiding information is closed related with contents of documents, which is better able to copy with attack.The natural language is the set of sentences, and the sentence is made up by words, the phrases etc. On the premise of unchanging text content, natural language sentences are transformed into grammar sentence pattern. This paper proposes a new information hiding algorithm based on Huffman code. The algorithm constructs code of patterns according to their distribution and transforms the secret message into a pattern by Huffman decoding and then computes the secret key of position which pattern appears in the cover text.In the experiment, digital watermarking based on the text format and on the part of speech tagging are tested.Compared the two results ,the latter performs better than the former no matter in the rubustness of watermark or in the watermaking capacity.Furthermore,the latter has little strict reqirement of text.and is more convient to carry out.