SEEING SHOULDN8217;T ALWAYS BE BELIEVING

As Hany Farid sat in his office at Dartmouth College in New Hampshire on a recent morning, he fiddled with his laptop and cracked disconcerting little jokes. The 41-year-old engineer is the founder of a subdiscipline within computer science: digital forensics. Intelligence agencies, news organisations and scientific journals employ Farid8217;s consulting services when they need to authenticate the validity of images. A conversation with Claudia Dreifus of The New York Times.

Let8217;s start with some definitions. What exactly is digital forensics?
It8217;s a new field. It didn8217;t exist five years ago. We look at digital media8212;images, audio and video8212;and we try to ascertain whether or not they8217;ve been manipulated. We use mathematical and computational techniques to detect alterations in them. In society today, we8217;re now seeing doctored images regularly. If tabloids can8217;t obtain a photo of Brad Pitt and Angelina Jolie walking together on a beach, they8217;ll make up a composite from two pictures. And it8217;s happening in the courts, politics and scientific journals, too. As a result, we now live in an age when the once-held belief that photographs were the definitive record of events is gone.

Why do scientists need to know about this?
Because not long ago, researchers from South Korea had to retract papers published in Science because the photographs used to prove that human stem cells had been cloned were effectively Photoshop-cloned, and not laboratory-cloned. There have been other recent cases, too. And today, in science, more and more, photographs are the data. The Federal Office of Research Integrity has said that in 1990, less than 3 percent of allegations of fraud they investigated involved contested images. By 2001, that number was 26 percent. And last year, it was 44.1 percent.

Are there policy changes that you think scientists should be considering?
I think it8217;s very hard to define inappropriate manipulation. Sometimes you can change 30 percent of the pixels in an image and it won8217;t fundamentally change anything. At other times, you can change 5 percent of the pixels and it radically changes meaning. I8217;m not a purist. I think there8217;s room for cropping, adjusting, contrast enhancement, but I want to know what was done. I think journal editors need to see the unadulterated, unretouched original images.
No.2, the scientific community as a whole needs to come out with a well-thought-out policy on what is and isn8217;t acceptable when it comes to altering photographs. And this is something that must be refined, updated and changed as the technology changes.

You make software to detect forgeries. How do you design your programs?
I think like a forger. I spend a lot of time in Photoshop making digital forgeries to learn the tools and techniques a forger uses. By working backwards, we learn the forger8217;s techniques and how to detect them. For instance, when looking at composites of two people, we8217;ve discovered that one of the hardest things for a forger to match is the lighting. So we8217;ve developed a way of measuring whether the lighting is consistent within various parts of the image. Lately, I8217;ve become obsessed with eyes. In a person8217;s eyes, one sees a slight reflection of the light in the room. So I8217;ve developed a technique that can take that little image of the reflection of light and tell us where the light was while you were being photographed. Does that match what we see in the image? We also look at numbers. The pixels of a digital image are represented on a computer by numbers. Once you8217;ve altered an image, the numbers change.

You consult regularly in legal cases. How is your work used in the courts?
I8217;ve consulted for the FBI, which sometimes uses images in prosecutions. They make surveillance tapes. At a trial, the defence might argue that the FBI doctored the images. So how do you prove they weren8217;t doctored? That8217;s my job. I8217;ve also been an expert witness in several child pornography cases. The Supreme Court in 2002 ruled that computer-generated child porn is protected under the First Amendment. So now in these cases, defence lawyers will sometimes argue that the images aren8217;t real. So far, I have only testified on the side of the prosecution. But I8217;ve been approached by defendants several times and I8217;ve told them, 8220;I8217;ll work on your case, but I8217;m going to testify to whatever I find.8221; And in every situation, the defense lawyers said, 8220;No, thank you.8221; In my opinion, that8217;s because they knew the photographs were not computer generated.