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Beyond the metadata/EXIF/IPTC (which can be easily altered), is it possible to prove that an image is authentic? If this is not possible, how does a photojournalist prove the authenticity of an original image?

Can digital cameras sign images to prove authenticity? How does this work, and what cameras can do it?

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Inspired by @Itai's comment on photo.stackexchange.com/questions/15305/… –  dpollitt Aug 31 '11 at 13:35
    
@mattdm - I disagree with you adding the tag "ethics"! –  dpollitt Aug 31 '11 at 13:54
    
I was looking through the existing tags for ones that might apply, and it seems that issues of veracity have an ethical aspect. Given that we're limited to five tags, though, maybe something like digital-signatures would be more useful. –  mattdm Aug 31 '11 at 13:59
    
Most (if not all) Canon EOS cameras has an option to include a hash of the unaltered image in its metadata. Unfortunately the metadata can still be stripped from the file, or even faked since the algorithm was cracked in '10 (source: vizworld.com/2010/11/… ) –  Tzarium Aug 31 '11 at 14:00
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@Tzarium - Can you add that in an answer instead of a comment? The comments are not really meant for this, and I'd like to vote you up! –  dpollitt Aug 31 '11 at 14:02

6 Answers 6

up vote 9 down vote accepted

Yes, this capability exists to some extent, but not through "signing" the image in the normal sense. It's based on the sensor noise patterns. Jan Lukáš, Jessica Fridrich, and Miroslav Golja (and a few others) at SUNY Binghamton have done work relating to two fields - identification of digital cameras using sensor noise patterns and identification of digital image forgeries using sensor noise patterns.

Something like this paper probably discusses what you are looking for. By detecting interruptions and inconsistencies in the noise pattern produced by the sensors that capture an image, it is possible to detect what parts of a digital image may have been manipulated. It's not a trivial process by any means, but research has been done on using the characteristics of the hardware to perform this type of task.

The last time I did work in this field was about 5 years ago, so I'm a bit out of touch with the latest and greatest, but I do know that law enforcement and the press are both interested in this capability (or at least were 5 years ago). You might have to do some digging to see if/how this has advanced, but it seems to be the best bet at proving authenticity. I just don't see it as something an individual would have on their own.

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There IS also a signature method too. IIRC, the patent belongs to NASA. –  Itai Aug 31 '11 at 15:29
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@Itai A signature other than the sensor noise pattern? I know a few patents have come out of a few universities (SUNY Binghamton being one) sponsored by US Government funding. That's the work that I'm most familiar with, since I was sitting on the DoD side around the time it was happening. The problem with most signatures, though, is that they can be broken or manipulated. The technique relies on the characteristics of CCDs and the other electrical components of the camera to generate noise, which can't easily be modified consistently. –  Thomas Owens Aug 31 '11 at 15:42
    
@Thomas Owens - You sound like our resident expert on this topic! Thank you for sharing. I would be interested to know how this has progressed along with noise improvements and in camera removal of noise. –  dpollitt Aug 31 '11 at 15:52
    
@dpollitt I was more knowledgable a few years ago, when I was doing some work supporting the R&D in this topic area, but more from a camera identification rather than a forgery detection point of view. Unfortunately, it's been several years since I've worked on anything like this, so my knowledge is very out of date on the latest available. –  Thomas Owens Aug 31 '11 at 16:17
    
patents.com/us-5499294.html - The author's biography claims this is the one used in Canon and Nikon digital cameras. It creates a hash from a key in the camera apparently. –  Itai Aug 31 '11 at 18:19

Yes, they can sign images.

This should prove authenticity although a team claims to have cracked Canon's implementation. Another team did the same for Nikon.

So this is like most digital security issues, it will prove authenticity or monumental effort to circumvent it ;)

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Direct link to technical details on the Canon crack: elcomsoft.com/presentations/… –  mattdm Aug 31 '11 at 14:15
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And for Nikon, although with less detail: blog.crackpassword.com/2011/04/… –  mattdm Aug 31 '11 at 14:18
    
It actually appears to be the same team in both cases. –  mattdm Aug 31 '11 at 14:18
    
And in both cases the main problem is insufficient protection of the signing key on the camera. –  mattdm Aug 31 '11 at 14:19
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I don't think your conclusion is correct. The cracks are real, meaning that no, digital signing is not all that effective. From the Nikon bit: "The private signing key has been compromised, which automatically invalidates digital signatures placed by all current models manufactured by Nikon." Crypto signing in this context is extremely difficult to get right, and Nikon at least has not done so and does not appear (according to the blog post) to have much interest in doing so. I would not trust the signing and shudder to think a judge might. –  Reid Sep 2 '11 at 18:23

Based on the mix of tags applied to this question, I think there is an important distinction to be made. While there may be technologies which prove an image is unaltered at a digital level, that does not extend to the content as it might be applied with respect to the photojournalism tag. Suppose I take a lemon and paint it the appropriate shade of green. I then take a picture. I may be able to prove to you the picture is unaltered, but it is still a picture of a lemon and not a lime. Ethics aside, I'm not sure you can apply the same analysis to the journalism as to the image.

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is it possible to prove that an image is authentic?

I've been told that Japanese police are using cameras with "tamper-proof storage". As far as I can tell, they are ordinary off-the-shelf digital cameras, with special write-once read-many (WORM) storage cards.

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The Canon image authentication kit can do this. As with anything related to security, there are technical and physical aspects. Others have pointed out the technical aspects have been called into question, but they still apply assuming you have good physical security. The kit includes an sd card, and it is important to verify where the card has been, and in whose possession at all times. Only then, along with the software kit, can you be assured of authenticity.

http://www.canon.co.jp/imaging/osk/index.html

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The inherent problem here is that if you have physical access to the device that can sign/decrypt the data, you can always break the security. So it is fundamentally impossible, while you can make it harder by using various tricks.

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