hidden in a system is key because it's important to know that you can't hide any more and if you are attacking (trying to disable the message) that you can't block anymore than this," O'Sullivan says. "It's also important because knowing this theory you can derive what are the properties of the optimal strategy to hide information and what are the properties of the optimal attack." O'Sullivan is associate direc- tor of Washington University's Center for Security Technologies, a research center devoted to developing technologies that safeguard the United States against terrorist attack. Although the intellectual pursuit of data hiding is relatively new, with the first international conference on the topic held in 1996, the practice goes back to the ancient Greeks. Herodotus was known to have sent a slave with a message tattooed on his scalp to Mellitus; the slave grew his hair out to hide the message, which was an encouragement to revolt against the Persian king. In World War II, the Germans used microdots as periods at the ends of sen- tences. Magnified, the microdots carried lots of information. The German usage is a classic instance of steganography. There will be much work ahead before O'Sullivan's theory will be fully implemented. "This is an example of one kind of work we do at the center that has a big impact in the theory community, but it's a couple of layers away from implementation," O'Sullivan says. "But the theory answers the questions, what is the optimal attack and what's the optimal strategy for information hiding." 7.7.3 Data Hiding's Positive Side Although terrorists and other evil doers often use data hiding to disguise their nefarious plans, the technique has also a positive side. Copyright holders, for example, use data hiding to safeguard intellectual property, particularly images, sent over the Internet and various types of telecom devices. Scientists from the University of Rochester and from Xerox have invented a new way to hide information within an ordinary digital image and to extract it