Photo DNA

March 22nd, 2012 at 11:56 am by David Farrar

Fairfax reports:

Internal Affairs has teamed up with to develop software to detect child sex abuse images.

The PhotoDNA software will be used during the forensic analysis of a seized computer, to identify known abuse images.

Internal Affairs minister Amy Adams said it was common for the ministry’s censorship compliance unit to review more than 100,000 images files on seized computers.

“This technology will make the process much faster. It will also allow a greater level of information sharing with our international partners as more systems come online that use this technology.

“New Zealand is one of the first countries in the world to have access to this technology, which gives investigators another valuable tool to help us in the fight against this problem.”

Generally I’m skeptical of technological “solutions” to child abuse images due to the problem with false positives. Filtering software for example sometimes block legitimate sites.

But this initiative is not like this. How PhotoDNA works is it uses a “fingerprint” of the images, which means only known objectionable images are flagged by automated PhotoDNA scans. More precisely, the basis for PhotoDNA is a technology called “robust hashing,” which calculates the particular characteristics of a given digital image — its digital fingerprint or “hash value” — to match it to other copies (and variations) of that same image.

So basically a human views the image once, and classifies it as a child abuse image, and then creates a hash value of it, so it can be easily detected in future. As I understand it, will not use in its filtering but for when the seize a computer under warrant.

If the computer has 100,000 images on it, they will no longer has to review each one. They can run this software on it, and if it says it find 5,000 matches, then they’ll review those 5,000 only. It is possible there will be some other objectionable images in the other 95,000 – but for prosecution purposes they don’t need an exact count.

Some general info on PhotoDNA is here. As I said, while I am usually skeptical, a technological solution which doesn’t create false positives is a very good tool to be using.

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12 Responses to “Photo DNA”

  1. Danyl Mclauchlan (1,066 comments) says:

    hashMaps. Is there anything they can’t do?

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  2. peterwn (3,211 comments) says:

    Internal Affairs has successfully used a similar technology to cross check all passport photos on file. It seems the only ‘false positives’ were through clerical errors or identical twins. They rooted out 60-70 or so fake passports through this and other checks. If there was a large number of false positives the checking process would have failed. So the prognosis for the proposal to check for ‘child abuse’ images would seem good, especially since the chances of a false positive for a particular computer would be very remote if say 50 or more images are matched.

    A good use of this technology could be by NZTA and police. Where drivers’ faces are visible in speed camera photos, the appropriate ‘demerits’ can be slapped on the driver in question, or if the speed is grossly excessive, the driver can be disqualified. This would be preferable to slapping ‘demerits’ on to vehicle owners as is done in Australia.

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  3. jams (48 comments) says:

    More importantly, they can easily cross reference the collections of kiddy porn these people have against each other and hopefully learn more about how they are traded.

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  4. PaulL (5,983 comments) says:

    Yes, seems very useful. Also, it presumably means that someone doesn’t have to wade through those 100,000 photos and decide which are kiddy porn – no doubt there’s some scary stuff in there. So long as at the end of it, they have the 5,000 ones that are potentially bad, and then someone still reviews those and demonstrates that they’re illegal under NZ law, then this seems mostly like a really good labour saving technique.

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  5. chrisw76 (85 comments) says:

    Great that they have this!

    Previously some poor bugger has had to look at each image individually and then try and determine if it is infringing. This isn’t as easy as you think, as you have to be able to conclusively prove that in the event of child pornography for instance that the subject is definitely under the age of consent. This quite often means that prosecutions are only done on the most blatant images.

    Apparently anyone doing this job is rotated out after three months. I can’t imagine what it does to you to have to look at images like that for eight hours a day.

    Cheers, Chris W.

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  6. Chthoniid (2,029 comments) says:

    I’m not sure that it’s that unbeatable. The thing about passport photos is they restrict the format and position of the subject, making comparisons easy. I’m not sure objectionable images are all done using the same pose.

    As I understand it, the DNA is based on a matrix of histogram data and perhaps edge sharpness on each sampled area of the photo. Both these can be manipulated, albeit it may be beyond the skills of most. It may of course. mean that instead of trawling through 100,000 pics the IA people can work on folders of a smaller number of likely candidates.

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  7. Kimble (4,408 comments) says:

    Not sure if I follow correctly, but does this simply mean that each image is able to be recognised after the initial flagging even if it is cropped or otherwise manipulated?

    The PhotoDNA software will be used during the forensic analysis of a seized computer, to identify known abuse images.

    The economist in me is pessimistic (surprise!) and reckons this would increase the value of new images. The higher demand for new images will lead to efforts to increase the supply, which is a bad thing.

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  8. cows4me (248 comments) says:

    Would this still work if images were digitally altered before they were circulated on the web ? No I don’t run a kiddy porn site.

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  9. Chthoniid (2,029 comments) says:

    @Kimble

    There’s a bit more to it on sciblogs
    http://sciblogs.co.nz/griffins-gadgets/2012/03/20/tracking-a-photos-digital-dna/

    It basically breaks the image down into a grid and each cell of the resulting matrix gets a value assigned to it.

    The DNA isn’t based on a grab of the whole picture. I think, just like a partial fingerprint can identify a suspect, a cropped photo can too.

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  10. Kimble (4,408 comments) says:

    But it still increases the value of new images. Not just different images, but new images.

    New images carry some risk, but this risk is effectively being reduced by massively increasing the risk of existing images.

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  11. Chthoniid (2,029 comments) says:

    @cows4me

    They claim no, but I’m a little skeptical.

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  12. scrubone (3,082 comments) says:

    I read somewhere a while back that most child sex images are from the ’70s when they were briefly legal in Europe.

    If that were true, then this would be very useful.

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