Word order may stump hackers

May 4, 2001

Scholars have devised a way to protect the integrity of sensitive documents from hackers by adding a hidden "watermark" in the syntax and grammatical structure of the text.

The computerised approach, which combines computer science and English language research, could be used to detect tampering - electronic or otherwise - in a document.

Its creators, Mikhail Atallah, professor of computer science, and Victor Raskin, professor of English, both at Purdue University, Indiana, United States, believe it will prove an attractive tool for governments and industry.

Although a watermark can be hidden easily in a photographic image - such as by switching the colour of a few specific pixels - text is far harder to manipulate because there is no redundant information.

"Every word means something - if you change it, you change the meaning of the sentence. That's the difficulty," Raskin said.

The new technique gets around this by introducing slight changes in the grammatical make-up of selected sentences in the document while keeping intact the intended meaning.

For example, the sentence "ships in the vicinity may provide some additional assistance" might be altered to "some additional assistance may be provided by the ships in the vicinity".

Raskin said: "What we embed is not something you can see - it's in the invisible syntactic structure."

The encryption algorithm that selects the sentences to be altered is based on a very large prime number, which is the key required to reveal whether the watermark is present or not.

The technique, which is subject to a patent application, is also resilient to simple changes to the text, such as changing one word for another throughout the whole document or deleting whole sentences.

The two scholars outlined their approach last week at a conference at Purdue's Center for Education and Research in Information Assurance Security.

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