Neal Krawetz, who owns a computer consulting firm called Hacker Factor, presented his own image-analysis tools last month at the Black Hat 2008 conference in Washington, DC. Among his tools was one that looks for the light direction in an image. The tool focuses on an individual pixel and finds the lightest of the surrounding pixels. It assumes that light is coming from that direction, and it processes the image according to that assumption, color-coding it based on light sources. While the results are noisy, Krawetz says, they can be used to spot disparities in lighting. He says that his tool, which has not been peer-reviewed, is meant as an aid for average people who want to consider whether an image has been manipulated--for example, people curious about content that they find online. Cynthia Baron, associate director of digital media programs at Northeastern University and author of a book on digital forensics, is familiar with both Krawetz's and Farid's work. She says that digital forensics is a new enough field of research that even the best tools are still some distance away from being helpful to a general user. In the meantime, she says, "it helps to be on the alert." Baron notes that, while sophisticated users could make fraudulent images that would evade detection by the available tools, many manipulations aren't very sophisticated. "It's amazing to me, some of the things that make their way onto the Web and that people believe are real," she says. "Many of the things that software can point out, you can see with the naked eye, but you don't notice it." Johnson says that he sees a need for tools that a news agency, for example, could use to quickly perform a dozen basic checks on an image to look for fraud. While it might not catch all tampering, he says, such a tool would be an important step, and it could work "like an initial spam filter." As part of developing that type of tool, he says, work needs to be done on creating better interfaces for existing tools that would make them accessible to a general audience. |
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Photoshop Finds its Way to the Web
04/04/2008
Comments
mdale9 on 03/17/2008 at 4:04 AM
2
nealkrawetz on 03/17/2008 at 10:36 AM
3
There is one primary light source (the Sun), but there should be lots of ambient light of varying degrees (stars and reflection of any earthlight).
As far as analysis goes, there is usually no difference between a "real" and "staged" photo -- both are "real" in that they were not digitally manipulated. However, this does not mean that the content is real. For example, Imad Mughniyeh staged the famous photo of him holding a gun to a pilot's head (http://edition.cnn.com/2008/WORLD/meast/02/13/btsc.clancy.mugniyah/index.html). The interpretation of the photo's content is deceptive but the image itself is real.
When you ask about the moon landing, the photos that I have looked at appear to be real. However, I will not to get into a debate on whether they were staged or actually taken on the moon -- too many people ignore facts in place of agendas and conspiracies.
- Dr. Neal Krawetz
smithsomian on 03/19/2008 at 11:42 AM
18
deejay on 03/17/2008 at 5:15 AM
22
Erica Naone on 03/17/2008 at 9:03 AM
Assistant Editor
25
-- Erica Naone
nealkrawetz on 03/17/2008 at 10:17 AM
3
It depends on the type of manipulation. Light direction may still be detectable, as will any poorly pasted borders. However, my tools also look for compression artifacts and format anomalies. These will not be present if a photo is manipulated before being digitized.
- Dr. Neal Krawetz
dsouza on 03/17/2008 at 12:01 PM
1
Erica Naone on 03/18/2008 at 10:04 AM
Assistant Editor
25
-- Erica Naone
gabrielg01 on 03/18/2008 at 12:17 PM
297
As a forger, you would take the fake image and run tests on it with the forensics software, and see which part of the image is flagged as fake. Then you modify the image accordingly, and test again. A few iterative steps like these, and you would be able to produce an image that passes the test.
Sure this means more work on the part of the forgers, but then they can claim to have a "genuine" picture.
We should never rely solely on pictures as proof anymore. A picture can tell a thousand lies.
Erica Naone on 03/19/2008 at 9:39 AM
Assistant Editor
25
One additional point is that, like all questions of security, if someone is intent on pulling off a beautiful and sophisticated forgery, and has the skills to back that up, they probably can. However, many forgeries simply aren't done with that level of dedication and skill, and tools can help filter out those forgeries.
nealkrawetz on 03/19/2008 at 12:30 PM
3
Having the tools is not difficult. However, having the skills to use the tool is a different story. (Just because I own a hammer does not mean I can build a house.) Most people who modify images lack the skills to overcome most image analysis techniques. In fact, many forgeries by professional artists still test positive for being modified.
My own tools are around 86% accurate. The few misses were by professional artists who have won awards from the CG Society. And two of the misses were by the same artist who spent over two years on the same model (wireframe, skin texture, hair, etc.). Fortunately for detecting forgeries, most amateurs don't have the skill or time to devote to their edits. (Even I, knowing exactly how the tools work, cannot make a good forgery because I lack the skills to use the drawing tools at a professional level.)
Given enough time, an amateur artist should be able to defeat a few of the detection methods. However, I use a suite of analysis tools and methods -- light analysis is just one approach. Very few doctored images can make it past all of the tests, and those that do were likely by professional artists who have plenty of time to devote to the forgery.
You are correct that a professional forger could use these forensic techniques to better the forgery. Fortunately, most doctored images are not by professionals, and even professionals may not have the time to overcome all of the signs of modification.
There is a saying in the computer security field: the defender has an impossible task; they must defend against all possible attacks, while the attacker only has to find one vulnerability. The same can be said about photo forensics: the forger must hide all possible signs of modification or forgery, while the forensic examiner only needs to find one clue.
jmaximus9 on 04/04/2008 at 9:22 PM
34