Veiled Messages of Terrorists May Lurk in Cyberspace
By Gina Kolata

The investigation of the terrorist attacks on the United States is drawing new attention to a stealthy method of sending messages through the Internet. The method, called steganography, can hide messages in digital photographs or in music files but leave no outward trace that the files were altered.

Intelligence officials have not revealed many details about whether, or how often, terrorists are using steganography. But a former French defense ministry official said that it was used by recently apprehended terrorists who were planning to blow up the United States embassy in Paris.

The terrorists were instructed that all their communications were to be made through pictures posted on the Internet, the defense official said.

The leader of that terrorist plot, Jamal Beghal, told French intelligence officals that he trained in Afganistan and that before leaving that country for France, he met with an associate of Osama bin Laden. The plan was for a suicide bomber to drive a minivan full of explosives through the embassy gates.

The idea of steganography is to take advantage of the fact that digital files, like photographs or music files, can be slightly altered and still look the same to the human eye or sound the same to the human ear.

The only way to spot such an alteration is with computer programs that can notice statistical deviations from the expected patterns of data in the image or music. Those who are starting to look for such deviations say that their programs are as yet imperfect but that, nonetheless, some are finding widespread use of steganography on the Internet. For national security reasons some of these experts do not want to reveal exactly what they find, and where.

"Quite an alarming number of images appear to have steganography in them," said one expert who has looked for them, Chet Hosmer, the president and chief executive of WetStone Technologies in Cortland, N.Y.

Mr. Hosmer says his company has not decided whether to reveal all the sites where he is finding steganography. He has found it on the auction site eBay, where people can post pictures anonymously, inserting hidden messages if they choose to, and just as anonymously download them, retrieving the messages. WetStone works under a contract to the Air Force.

At George Mason University, Dr. Neil F. Johnson, a steganography expert, said he became so worried by steganography's potential to be used by terrorists and criminals that he stopped publishing his research on how to detect it, reasoning that if people knew how he detected it, and where, they could devise methods to thwart him and move their messages to sites he has not checked.

"I have no reason to think that Al Qaeda is not using steganography," Dr. Johnson said, but he, like others, pointed to no proof. His research, he said, is financed by "law enforcement."

"I think it's foolish to disclose what I'm scanning for, whether I'm scanning and whether I'm detecting anything," Dr. Johnson said. "To give that away tips one's hands."

Steganography, Greek for "hidden writing," is one of the most ancient ways of passing secret messages, but until very recently few computer scientists paid it much attention - it seemed more a relic of ancient times, sort of a Paul Revere-type "one if by land two if by sea" way of sending information.

The ancient Greeks used it, writing a message on a wooden tablet and covering the wood with wax. Sentries would think the tablets were blank, but when they were delivered, their recipients would simply scrape off the wax and read the message.

In World War II, Dr. Johnson said, the Allies became so suspicious about hidden messages that the United States Office of Censorship "took extreme actions, such as banning flower deliveries which contained delivery dates, crossword puzzles and even report cards."

But in recent years, steganog raphy has arrived on the Internet in a big way, experts said, with free and easy-to-use programs to insert messages into music or picture files. Many programs also allow users to choose an encryption scheme to further hide the message, so even if the recipients know it is there, they have to decode it to read it.

"In the past two years, the number of steganography tools available over the Internet has doubled - it's 140 and growing," Dr. Johnson said. Some of the newer ones, he said, prompt users at each step on how to proceed.

Bruce Schneier, a founder of Counterpane, an Internet security company, likened steganography to what is known as a dead drop - a message, money or papers left in a hiding place to be picked up by someone.

"The effect is that the sender can transmit a message without ever communicating directly with the receiver," Mr. Schneier wrote in a recent newsletter. "There is no e-mail between them, no remote log-ins, no instant messages. All that exists is a picture posted in a public forum, and then downloaded by anyone sufficiently enticed by the subject (both third parties and the intended receiver of the secret message.)"

Mr. Hosmer said he became interested in steganography three years ago when he conducted a study for the Air Force looking at potential areas for cybercrime and cyberterrorism.

"We wanted to see what kinds of tools and weapons were being used by terrorist organizations," he said. To his surprise, he said, steganog raphy, an area he had paid little attention to, stood out because it could be so effective

in hiding the very fact that people were communicating - thwarting attempts to detect terrorist activities by looking for flurries of communications between members.

Mr. Hosmer found more than 100 free steganography programs on the Internet and said he was shocked when the providers of the programs said there had been over a million downloads of the technology.

"It really struck us: why were there so many downloads?" Mr. Hosmer said. Some, he said, may be hackers or people who are using it for fun. But, he said, he doubts that those are the only users.

"We said, `This is really startling, that there are so many people who are communicating without people knowing that they are communicating.' And because these programs were coming from around the world, we were very concerned."

Mr. Hosmer's company began looking at millions of digital pictures that were posted on the Internet. They scanned auction sites and pornographic sites, where people can post and download digital images anonymously.

"We started getting hits," Mr. Hosmer said, adding that about 0.6 percent of millions of pictures on auction and pornography sites had hidden messages. The messages they found on eBay were encrypted and unreadable, he said. The company also noticed that some of the same photos seemed to be used over and over again, with different messages each time. "If you're very sophisticated at this, you would never use an image again," Mr. Hosmer said.

One limitation in published steganography detection programs is that often they miss images hidden in the most frequently used format, JPEG, said Dr. Jessica Fridrich, a research professor at the Center for Intelligent Systems at the State University of New York at Binghamton.

It is hard to see evidence of steganography in such files because the detection methods look for statistical evidence that an image's data have been distorted. But JPEG files are distorted by their very nature - the digital data are altered when the files are compressed to send them electronically.

Dr. Fridrich said that a steganography detection program she developed also had that limitation but that she had greatly improved the program so that, even though it still did not work well for JPEG images, it was much better at finding images in other formats. She said she was providing it to the Air Force, which was paying for her group's work. "I believe that the Air Force made this program available to other government agencies," she said.

The best published method for finding steganography in JPEG files, Dr. Fridrich said, is one developed by Niels Provos, a graduate student at the University of Michigan. Mr. Provos said he had seen no steganography in the two million images from eBay he had examined.

On the other hand, Mr. Provos can miss steganography - he said he had trouble finding small messages and was unable to detect a short message in a photograph that was sent to him. He was told beforehand that an unencrypted message had been inserted.

Mr. Provos publishes his research, enabling others to know how he detects steganography and, as a consequence, how to avoid his detection system. "When I started my research, which was a couple of years ago, it was, of course, in a completely different political situation," he said.

Now, he says, he asked himself again if publication was advisable. He concluded it was, arguing that research thrived when people could freely exchange ideas.

Of course, those whose business it is to intercept terrorist communications would never reveal anything they have learned about steganog raphy.

Asked what the National Security Agency - the nation's codemaking and codebreaking agency - knows, Dr. Robert Morris, a retired cryptographer who was chief scientist there, said, "We wouldn't talk about it."

30 October 2001

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