5 ms·

There is nothing scientific about any of these tests. What's a 'high' ELA or 'low'? The thing that triggers a 'positive' seems to be only the authors intuition.

There is nothing scientific about any of these tests. What's a 'high' ELA or 'low'? The thing that triggers a 'positive' seems to be only the authors intuition. While some of the tests might show something, I'm pretty sure ELA is absolute garbage - you can't separate the number of needed resaves because of the content frequencies from number of times it has already been resaved.

sliverstorm 15y ago
What, may I ask, makes a test scientific?
- greendestiny 15y ago
  You don't need to get technical about it, but I think most people would expect any test described as scientific to be objective.
  - tantalor 15y ago
    Do you mean free of bias or reproducible? I think the author was clearly free from bias, and his results are easily reproduced by applying the same algorithms. The subjectivity which you complain about may be the conclusions drawn from the results of the tests, which I think are distinct. Specifically, you cite his "intuition" as the origin of the conclusion. I think the author's intuition is reliable because, like "real" scientists, he's an expert and speaks publicly about his work.[1] Or at least he appears to be. Are you prepared to challenge him as an expert? [1] http://www.hackerfactor.com/papers/bh-usa-07-krawetz-wp.pdf http://www.hackerfactor.com/papers/bh-usa-07-krawetz-wp.pdf
    - greendestiny 15y ago
      I could, but shouldn't we demand he show some proof for his claims? It seems to me that if you pull a bit of maths and technical magic out the normal skepticism the tech community melts away into a compliant bundle of gullibility.
- pazimzadeh 15y ago
  For one, there should be a control. In this case, analyzing another ticket that has not been faked, but preferable he would not know if the control was real or not. http://en.wikipedia.org/wiki/Blind_experiment#Double-blind_trials http://en.wikipedia.org/wiki/Blind_experiment#Double-blind_t...
drx 15y ago
He used PCA as well as ELA, making your first statement unfounded.
- greendestiny 15y ago
  While the result of the PCA is quite well defined, there is nothing well defined about the inferences he draws from that. In the end it shows a marked difference in the suspicious area, but there is no proof that the technique will only show those results on tampered areas. If you can simply run filters until the suspicious area 'looks a bit different' and thats your success criteria, you haven't proven anything.
  - Natsu 15y ago
    Neal specializes in this sort of thing. This blog post wasn't written to convince you that PCA (or ELA) work. If you've read his other posts on forensics, you'd see that he always starts with the common tests, then works through the most likely explanations, trying to see what you can rule out and where you should focus. A lot of the time, you can rule things out with common sense (the out-of-order numbers jumped out at me), though he always goes on to ferret out what manipulation was performed, because that's what the whole blog is about. Anyhow, people have done lots of tests with lots of different tools to see what they report in different circumstances. So this isn't the first time that someone has used ELA or something. You will notice if you read past explanations that he always tries to find the exact manipulations done, rather than just running a tool and declaring something to have been Photoshopped. I'm no expert, but I've read his blog for long enough to know that he knows an awful lot about the various quirks of many different image editing programs (Photoshop being by far the quirkiest). But if you still have questions, you can always email him. He was kind enough to reply with a lot of useful information when I asked him something a long time ago. Actually, I think he even blogged about it.
    - greendestiny 15y ago
      It doesn't matter how good your intentions are if you place human intuition into the loop you will fool yourself.
maurits 15y ago
One basically looks for roundings caused by quantization and places in the image where the compression (again quantization) is either inconsistent or non-optimal. A nice tutorial (and a lot of articles) can be found on the Dartmouth site of Hany Farid. http://www.cs.dartmouth.edu/farid/downloads/tutorials/digitalimageforensics.pdf http://www.cs.dartmouth.edu/farid/downloads/tutorials/digita... Personally, I have had very very mixed results with this method, and never managed to model it correctly. Interpreting results always was a very human job.
- greendestiny 15y ago
  I understand the theory and I certainly agree resaving jpegs will reduce the error level. If you have an undoctored image to compare it to, you could probably use that to determine which parts have been changed. But given only an image that may or may not have been doctored the error level will vary so much with the image content that it won't be meaningful.

sliverstorm 15y ago

What, may I ask, makes a test scientific?

greendestiny 15y ago
You don't need to get technical about it, but I think most people would expect any test described as scientific to be objective.
- tantalor 15y ago
  Do you mean free of bias or reproducible? I think the author was clearly free from bias, and his results are easily reproduced by applying the same algorithms. The subjectivity which you complain about may be the conclusions drawn from the results of the tests, which I think are distinct. Specifically, you cite his "intuition" as the origin of the conclusion. I think the author's intuition is reliable because, like "real" scientists, he's an expert and speaks publicly about his work.[1] Or at least he appears to be. Are you prepared to challenge him as an expert? [1] http://www.hackerfactor.com/papers/bh-usa-07-krawetz-wp.pdf http://www.hackerfactor.com/papers/bh-usa-07-krawetz-wp.pdf
  - greendestiny 15y ago
    I could, but shouldn't we demand he show some proof for his claims? It seems to me that if you pull a bit of maths and technical magic out the normal skepticism the tech community melts away into a compliant bundle of gullibility.
pazimzadeh 15y ago
For one, there should be a control. In this case, analyzing another ticket that has not been faked, but preferable he would not know if the control was real or not. http://en.wikipedia.org/wiki/Blind_experiment#Double-blind_trials http://en.wikipedia.org/wiki/Blind_experiment#Double-blind_t...

drx 15y ago

He used PCA as well as ELA, making your first statement unfounded.

greendestiny 15y ago
While the result of the PCA is quite well defined, there is nothing well defined about the inferences he draws from that. In the end it shows a marked difference in the suspicious area, but there is no proof that the technique will only show those results on tampered areas. If you can simply run filters until the suspicious area 'looks a bit different' and thats your success criteria, you haven't proven anything.
- Natsu 15y ago
  Neal specializes in this sort of thing. This blog post wasn't written to convince you that PCA (or ELA) work. If you've read his other posts on forensics, you'd see that he always starts with the common tests, then works through the most likely explanations, trying to see what you can rule out and where you should focus. A lot of the time, you can rule things out with common sense (the out-of-order numbers jumped out at me), though he always goes on to ferret out what manipulation was performed, because that's what the whole blog is about. Anyhow, people have done lots of tests with lots of different tools to see what they report in different circumstances. So this isn't the first time that someone has used ELA or something. You will notice if you read past explanations that he always tries to find the exact manipulations done, rather than just running a tool and declaring something to have been Photoshopped. I'm no expert, but I've read his blog for long enough to know that he knows an awful lot about the various quirks of many different image editing programs (Photoshop being by far the quirkiest). But if you still have questions, you can always email him. He was kind enough to reply with a lot of useful information when I asked him something a long time ago. Actually, I think he even blogged about it.
  - greendestiny 15y ago
    It doesn't matter how good your intentions are if you place human intuition into the loop you will fool yourself.

maurits 15y ago

One basically looks for roundings caused by quantization and places in the image where the compression (again quantization) is either inconsistent or non-optimal. A nice tutorial (and a lot of articles) can be found on the Dartmouth site of Hany Farid. http://www.cs.dartmouth.edu/farid/downloads/tutorials/digitalimageforensics.pdf http://www.cs.dartmouth.edu/farid/downloads/tutorials/digita... Personally, I have had very very mixed results with this method, and never managed to model it correctly. Interpreting results always was a very human job.

greendestiny 15y ago
I understand the theory and I certainly agree resaving jpegs will reduce the error level. If you have an undoctored image to compare it to, you could probably use that to determine which parts have been changed. But given only an image that may or may not have been doctored the error level will vary so much with the image content that it won't be meaningful.