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#11
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Forensics v. Photoshop
On 2012.09.22 04:48 , Eric Stevens wrote:
Having looked at Wikipedia on the subject I've come away more than a little gob-smacked. There are so many ways to construct a JPG file it would be surprising if any two cameras produced identical finger prints from their files. But any given camera co. probably re-uses the same algorithms (generally) over time so they would be similar or exactly the same over dozens and dozens of models. That isn't what (or solely what) this software purports to do. Rather it looks at fine grained differences in the actual image data to fingerprint the effects of its sensor/d-a/n-r circuits/algorithms. That will not be common from a given manufacturer except for a few closely related variations of a model. -- "There were, unfortunately, no great principles on which parties were divided – politics became a mere struggle for office." -Sir John A. Macdonald |
#12
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Forensics v. Photoshop
On Sat, 22 Sep 2012 10:56:08 -0400, Alan Browne
wrote: On 2012.09.22 04:48 , Eric Stevens wrote: Having looked at Wikipedia on the subject I've come away more than a little gob-smacked. There are so many ways to construct a JPG file it would be surprising if any two cameras produced identical finger prints from their files. But any given camera co. probably re-uses the same algorithms (generally) over time so they would be similar or exactly the same over dozens and dozens of models. That isn't what (or solely what) this software purports to do. Rather it looks at fine grained differences in the actual image data to fingerprint the effects of its sensor/d-a/n-r circuits/algorithms. That will not be common from a given manufacturer except for a few closely related variations of a model. It's quite possible that a model X and model Y may share the same sensor and image generation software. But there will be differences in the EXIF data which should show up in the file. But that doesn't matter. The question is, can anyone alter the image in such a way that the fact that it has been altered is indetectable? I'm sure that it will be possible to stuff another image inside the file but to do so indetectably would require that the new image be constructed in exactly the same way as the camera would have done it. I expect that finding out how that should be would be very difficult. -- Regards, Eric Stevens |
#13
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Forensics v. Photoshop
On 2012.09.22 18:33 , Eric Stevens wrote:
On Sat, 22 Sep 2012 10:56:08 -0400, Alan Browne That isn't what (or solely what) this software purports to do. Rather it looks at fine grained differences in the actual image data to fingerprint the effects of its sensor/d-a/n-r circuits/algorithms. That will not be common from a given manufacturer except for a few closely related variations of a model. It's quite possible that a model X and model Y may share the same sensor and image generation software. But there will be differences in the EXIF data which should show up in the file. But that doesn't Though it analyzes that as well, it appears to be the duller edge of its sword. matter. The question is, can anyone alter the image in such a way that the fact that it has been altered is indetectable? I'm sure that it will be possible to stuff another image inside the file but to do so indetectably would require that the new image be constructed in exactly the same way as the camera would have done it. I expect that finding out how that should be would be very difficult. Unless you can emulate the fine grained noise of the camera being spoofed the s/w at discussion claims to be able to sniff out a fake. Such emulation is not out of the question but you would have to do as much work collecting the statistics (as they did) to be able to carry it out. Or get the data from the company at discussion perhaps. ;-) -- "There were, unfortunately, no great principles on which parties were divided – politics became a mere struggle for office." -Sir John A. Macdonald |
#14
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Forensics v. Photoshop
Eric Stevens wrote:
matter. The question is, can anyone alter the image in such a way that the fact that it has been altered is indetectable? Of course someone can. Change a single pixel in such a way that it's still within it's typical statistic value range for the true image. Don't do anything except exchanging the single JPEG block that contains the pixel (write your own program to do it or do it by hand). Presto: even having a second photo (with pixel-exact registration) of the same scene and access to the same scene you cannot detect the manipulation. (Of course, the manipulation will not be relevant.) -Wolfgang |
#15
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Forensics v. Photoshop
John A wrote:
On Sun, 23 Sep 2012 22:37:01 +0200, Wolfgang Weisselberg Eric Stevens wrote: matter. The question is, can anyone alter the image in such a way that the fact that it has been altered is indetectable? Of course someone can. Change a single pixel in such a way that it's still within it's typical statistic value range for the true image. Don't do anything except exchanging the single JPEG block that contains the pixel (write your own program to do it or do it by hand). Presto: even having a second photo (with pixel-exact registration) of the same scene and access to the same scene you cannot detect the manipulation. (Of course, the manipulation will not be relevant.) You can embed a message that way, using the least-significant bits to encode it. http://en.wikipedia.org/wiki/Steganography Yep --- and if there's then any sort of pattern in the LSB, your hiding there has failed. Note: Often an encryption or compression output contains something like a header ... -Wolfgang |
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