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Forensics v. Photoshop



 
 
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  #11  
Old September 22nd 12, 03:56 PM posted to rec.photo.digital.slr-systems,alt.photography,rec.photo.digital
Alan Browne
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Posts: 12,640
Default 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  
Old September 22nd 12, 11:33 PM posted to rec.photo.digital.slr-systems,alt.photography,rec.photo.digital
Eric Stevens
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Posts: 13,611
Default 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  
Old September 22nd 12, 11:53 PM posted to rec.photo.digital.slr-systems,alt.photography,rec.photo.digital
Alan Browne
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Posts: 12,640
Default 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  
Old September 23rd 12, 09:37 PM posted to rec.photo.digital.slr-systems,alt.photography,rec.photo.digital
Wolfgang Weisselberg
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Posts: 5,285
Default 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  
Old September 25th 12, 02:32 PM posted to rec.photo.digital.slr-systems,alt.photography,rec.photo.digital
Wolfgang Weisselberg
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Posts: 5,285
Default 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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