Do you think the photos you take with your digital camera are special? Do you believe that other people’s pictures are fine, but yours are unique in some way? The science of digital forensics would tell us you’re not entirely wrong. According to the research of Dr. Jessica Fridrich and her team at the State University of New York at Binghamton, every digital camera’s image sensor has a distinct and unique “noise signature.” That signature, a digital fingerprint of sorts, leaves its unique mark on every picture taken with that camera. As you might imagine, the implications and applications of this discovery are substantial, particularly in the field of law enforcement.
In addition, Dr. Fridrich and her team have become leading experts in the field of steganalysis – the science of digital image excavation. Through this process, Dr. Fridrich is able to uncover hidden content buried within a digital image – words, pictures or even videos which may have been embedded beneath the surface to avoid detection by the naked eye. At first glance, a seemingly harmless shot of Aunt Edith’s chihuahua could in fact be a coded message in disguise. As Dr. Fridrich could surely tell you, a picture could really be worth a thousand words.
A Digital Fingerprint
Unlike traditional film-based cameras, digital cameras use an image sensor comprised of millions of photosensitive diodes to capture light and record a moment in time. Each diode on the sensor represents one pixel in the resulting digital image. Due to slight pixel differences and imperfections resulting from the manufacturing process, it has been found that each and every digital image sensor is born with a unique pattern of these imperfections. This pattern is what Dr. Fridrich’s team refers to as a camera’s “noise signature.”
Based on an idea by former PhD student Jan Lukas and post-doctoral assistant Miroslav Goljan, and with help from former post-doctoral assistant Mo Chen, Dr. Fridrich and her team have come up with a method to accurately determine and capture the unique noise signature of any digital camera. Of critical importance is the fact that every single picture taken with a given camera is essentially fingerprinted with that same noise signature.
Much like the process used by police to determine whether or not a bullet was fired from a particular gun, Dr. Fridrich’s software has strong potential for use in law enforcement – particularly for tracking and prosecuting child pornographers. If the police have a suspect’s camera and an illicit photograph, Dr. Fridrich and her team can conclusively say whether or not that image was taken with that camera. If authorities have two images but no camera, Dr. Fridrich’s software can determine whether or not the two photos were taken with the same camera. During a police investigation of this type, such information can be crucial.
Depending on the type of images being taken, Dr. Fridrich says it takes between 10 and 50 images before a camera’s unique noise signature can be fully mapped. For example, whereas close-up pictures of a tree’s leaves featuring little color variation would be difficult to map, everyday photos of people, places and events make the signature identification process much easier.
In addition to child pornographers, video pirates have reason to fear this new software. According to Dr. Fridrich, digital video cameras and their sensors are subject to the same type of noise signatures; this means that video footage can be tied back to a specific camcorder in much the same fashion that a photograph can be tied to a still camera. If the police arrest a video pirate caught with his camera, that camera’s signature can be compared with other pirated videos being sold in the area. Although a pirate may have been arrested on suspicion of filming one movie illegally, Dr. Fridrich’s analysis software could link the pirate to other bootlegs he or she has made in the past, provided the same camera was used.
With the proliferation of digital cameras making film-based cameras all but extinct, it seems clear that the importance and need for Dr. Fridrich’s noise signature software will only increase in the foreseeable future.
What Lies Beneath
In addition to a camera’s noise signature, there may be other information lurking beneath the surface of a digital photograph. Similar to cryptography, steganography is the art of concealing hidden messages within a seemingly innocent medium, such as a digital photo. Unlike cryptography, which typically involves the scrambling of a covert message where the recipient must use a key to decrypt its contents, a steganographer does not scramble the message – instead, he simply hides it within a seemingly innocent container. On the other side of the fence lies steganalysis – the art and science of uncovering that which has been hidden. As Dr. Fridrich says of these two disciplines, “You cannot have one without the other. These are two sides of the same coin.”
Much like her research on the noise signatures of digital camera sensors, Dr. Fridrich’s work as a steganalyst presents tremendous potential for law enforcement and the military. With Middle Eastern terrorists continuing their intermittent stream of digital broadcasts via television and Internet, certainly the still images and videos received by the United States government are being scrutinized for this type of secretive communication.
According to Dr. Fridrich, steganography is rather simple and any “good graduate student could have a working program in less than one day.” As disconcerting as that may be to hear, there is hope. As Dr. Fridrich says, “Although it’s easy to do, it’s hard to do it right.” Hiding a sizeable amount of information in such a way to completely escape detection is rather difficult and thanks to Fridrich’s constantly-evolving software, it’s getting harder all the time.
Ones and Zeroes
There’s no denying the fact that we live in a digital world. When you look at the ever-increasing use of email, the Internet, cell phones and digital cameras, ones and zeroes have clearly supplanted the envelope and stamp as our de facto means of communication. Sadly, every technological breakthrough brings with it those who would use it for exploitative or evil purposes. Thankfully, there are people out there such as Dr. Jessica Fridrich working to thwart these abuses of technology. yy
