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Forensics v. Photoshop
On 2012.09.20 04:26 , Martin Brown wrote:
There are a handful of independent JPEG implementations - most follow the original spec closely enough that there is little or no distinction between them (apart from in PSPro 8 which contained gross errors). It has to be like that or you would see much worse artefacts if some JPEG encoders made significant mistakes (as in fact happened with PsP 8). These folks seem to have statistical evidence to the contrary which trumps your knee jerk assumptions. It will be judged in the marketplace of those who are concerned with such. If there is value it will be quickly found. Or not. The only real variation is the exact choice of quantisation table and Photoshop is distinctive there, but most of the rest use a scaled version of the canonical JPEG standard Qtables from the original spec. Assumption. Not really. Tools to do this have been around more or less since the JPEGLIB codec was completed. CJPEG can compress an image with any arbitrary quantisation if you ask it nicely. Splicing in the fake Exif data for a given camera signature is a pretty trivial binary edit. It probably is - but that's not what this sw is detecting. Above. That is all that they have claimed. I have already pointed a link at freeware that does exactly the same job (and much the same way). The file signature is pretty much determined by the miscellaneous dross that each vendor adds to the header for the JPEG stream and how many implementation ambiguities/mistakes they make in their encoding of Exif data. Luckily most decoders can cope with quite badly malformed Exif so there is scope for recognising certain brands there. Again, as if you didn't read the article or see the video, the signature aspect has to do with the content, not the additional data. -- "C'mon boys, you're not laying pipe!". -John Keating. |
#2
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Forensics v. Photoshop
On 20/09/2012 21:49, Alan Browne wrote:
On 2012.09.20 04:26 , Martin Brown wrote: There are a handful of independent JPEG implementations - most follow the original spec closely enough that there is little or no distinction between them (apart from in PSPro 8 which contained gross errors). It has to be like that or you would see much worse artefacts if some JPEG encoders made significant mistakes (as in fact happened with PsP 8). These folks seem to have statistical evidence to the contrary which trumps your knee jerk assumptions. It will be judged in the marketplace of those who are concerned with such. If there is value it will be quickly found. Or not. I agree. Snake oil will be fairly quickly smoked out. The only real variation is the exact choice of quantisation table and Photoshop is distinctive there, but most of the rest use a scaled version of the canonical JPEG standard Qtables from the original spec. Assumption. Not an assumption at all - I know that for a fact. I have written software that analyses damaged JPEGs. There are only a handful of cameras and applications that use custom non-standard Qtables that are unrelated to the "examples" given in the original spec. Virtually everything apart from Photoshop uses scaled copies of the JPEG example. One such was PsPro 8 which included a typo in the Y matrix and various faulty chroma downsampling algorithms which distorted the results. Such errors are rare and are seldom detected by end users. Not really. Tools to do this have been around more or less since the JPEGLIB codec was completed. CJPEG can compress an image with any arbitrary quantisation if you ask it nicely. Splicing in the fake Exif data for a given camera signature is a pretty trivial binary edit. It probably is - but that's not what this sw is detecting. Above. That is all that they have claimed. I have already pointed a link at freeware that does exactly the same job (and much the same way). The file signature is pretty much determined by the miscellaneous dross that each vendor adds to the header for the JPEG stream and how many implementation ambiguities/mistakes they make in their encoding of Exif data. Luckily most decoders can cope with quite badly malformed Exif so there is scope for recognising certain brands there. Again, as if you didn't read the article or see the video, the signature aspect has to do with the content, not the additional data. I did read the site and their description matches my interpretation of what they are offering. The camera "signature" is in the fluff around the JPEG stream and not in the coefficient stream itself. http://www.fourandsix.com/fourmatch Such "signatures" can be easily forged with the right tools. The JPEG coefficient stream has a limited number of encoding possibilities and only a few of them are actually seen in practice. -- Regards, Martin Brown |
#3
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Forensics v. Photoshop
On 2012.09.20 17:07 , Martin Brown wrote:
On 20/09/2012 21:49, Alan Browne wrote: On 2012.09.20 04:26 , Martin Brown wrote: There are a handful of independent JPEG implementations - most follow the original spec closely enough that there is little or no distinction between them (apart from in PSPro 8 which contained gross errors). It has to be like that or you would see much worse artefacts if some JPEG encoders made significant mistakes (as in fact happened with PsP 8). These folks seem to have statistical evidence to the contrary which trumps your knee jerk assumptions. It will be judged in the marketplace of those who are concerned with such. If there is value it will be quickly found. Or not. I agree. Snake oil will be fairly quickly smoked out. The only real variation is the exact choice of quantisation table and Photoshop is distinctive there, but most of the rest use a scaled version of the canonical JPEG standard Qtables from the original spec. Assumption. Not an assumption at all - I know that for a fact. I have written software that analyses damaged JPEGs. There are only a handful of cameras and applications that use custom non-standard Qtables that are unrelated to the "examples" given in the original spec. Virtually everything apart from Photoshop uses scaled copies of the JPEG example. You've completely missed the point. I'll leave to you to go figure out what it was. One such was PsPro 8 which included a typo in the Y matrix and various faulty chroma downsampling algorithms which distorted the results. Such errors are rare and are seldom detected by end users. Not really. Tools to do this have been around more or less since the JPEGLIB codec was completed. CJPEG can compress an image with any arbitrary quantisation if you ask it nicely. Splicing in the fake Exif data for a given camera signature is a pretty trivial binary edit. It probably is - but that's not what this sw is detecting. Above. That is all that they have claimed. I have already pointed a link at freeware that does exactly the same job (and much the same way). The file signature is pretty much determined by the miscellaneous dross that each vendor adds to the header for the JPEG stream and how many implementation ambiguities/mistakes they make in their encoding of Exif data. Luckily most decoders can cope with quite badly malformed Exif so there is scope for recognising certain brands there. Again, as if you didn't read the article or see the video, the signature aspect has to do with the content, not the additional data. I did read the site and their description matches my interpretation of what they are offering. The camera "signature" is in the fluff around the JPEG stream and not in the coefficient stream itself. http://www.fourandsix.com/fourmatch Such "signatures" can be easily forged with the right tools. The JPEG coefficient stream has a limited number of encoding possibilities and only a few of them are actually seen in practice. As above. -- "There were, unfortunately, no great principles on which parties were divided – politics became a mere struggle for office." -Sir John A. Macdonald |
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