So much for hiding your face

| Comments (2) | COMSEC
Redacting digital information turns out to be a tricky proposition, at least if you go by how often people screw it up. The usual situation is some declassified document where the government has just put easily removed black boxes over the relevant text, but in this case it's an individual who made the mistake, a certain alleged pedophile who posted incriminating pictures. of himself with his face obscured by the Photoshop twirl filter. Unfortunately for him, it turns out that this effect is reversible:
Apparently, the suspect, or whoever handled the pictures, did not think it was possible to reverse the twirling, a capability that at least one Interpol official was intent on keeping confidential.

Now the cat is out of the bag. Officials are declining to say just how they did it, leaving Interpol in the strange position of urging the public to help find one pedophile suspect while refusing to divulge a tool that might identify others before they hear today\u2019s news and rush to delete potentially incriminating twirled images of themselves.

By publishing the untwirled photos of their suspect today, the international police organization also decided to risk the possibility that the man -- or men who happen to look like him -- may face violence from vigilantes.

Apparently, this effect is really trivial to reverse. According to this BoingBoing post you can just set the twirl filter to negative and you get the original picture back. Obviously, there are transformations which would be more complicated to reverse; for instance you could encrypt the relevant pixels or randomly permute them, though any fixed transformation which is one-to-one and onto should be reversible with enough effort. In addition, there are transformations which destroy information and are partly or wholly irreversible. The obvious case is replacing the relevant pixels with pixels all of the same color. This is of course simple, but apparently not as obvious as you might think.

Now where things go wrong with a lot of redaction operations—especially with formats like PDF—is that the basic formats are more complicated than bitmaps. The redactors just create a new black object that is in front of the the text to be redacted. The underlying information is still there, so it's just a matter of removing/ignoring the black object and you have the original text. It's much safer to work with a bitmap format where you know that you're changing the relevant pixels rather than just masking them. Of course, you also need to use a transformaton that actually can't be easily reversed.

2 Comments

Here's something that's confused me about the coverage of this case: whenever referring to the man in the pictures, the media has taken care to describe him as an "alleged pedophile."

Alleged.

Based on the descriptions of the portions of the photographs that haven't been published by the mainstream media, these are photographs of the man having sex with clearly under-aged boys.

I can see how you would need to be careful if you were attaching a name or specific identity to the statement -- any identified person would merely be an alleged pedophile until the case goes to trial and a conviction is obtained.

But the man in the photos? The man in the pictures that depict him engaging in pedophilia is a pedophile.

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