No, blurring images is not secure

Hands up, we've all been there. You make a quick screenshot using your tool of choice, only to realize something sensitive lurks in the background. No problem, just blur it out, right? Easy fix. If I can't decipher it, then surely nobody else can, right? Wrong.

Now, it's about more than just blurring, which might not be destructive. Pixelating your images or even slapping a censor box on them might also not be secure.

Deblurring an image is possible. However, you'll never fully restore the original quality because blurring removes information. The Mathematical Image Analysis Group at the University of Saarland has published several papers on this topic, complete with some examples. Different types of blur result in different losses, but it's possible to reverse all of them, although it gets progressively more complex, and the end result might bear little resemblance to the original picture. The outcome of deblurring heavily depends on the number of pixels removed and the number of passes applied. As a general rule, blurring text is much less secure than blurring something complex, like a human face, because it's relatively easy for a computer to guess what character would result in the censored pixel. Hackday has a great animation demonstrating how a computer can guess what's behind pixelated or blurred text. Even though I mentioned that deblurring a face is considerably more challenging and might not give you a great result, it could still be sufficient to identify the blurred person. A case in point: a rapist in the UK attempted to obscure his identity by distorting images of himself using Photoshop before posting them online. However, he was ultimately identified when the police utilized specialized software to deblur his face.

In the beginning, I also mentioned that censoring with a box might not be foolproof. This largely depends on the tool you use to censor your image. Some screenshot tools don't replace the pixel with the box you overlay; instead, they save the box separately. Fortunately, this is mostly a thing of the past and isn't commonly done anymore.

Oh, and for those who think they're clever by "censoring" their images with a marker on their phone, just stop it. More often than not, you can still decipher what you tried to hide by adjusting the brightness/contrast. Marker tools often mimic real markers and don't have 100% opacity, so the pixels can still be identified as they're not entirely removed.

Now, with all that said, how do you censor something correctly? The safest option will always be the most basic one: delete the pixels you want to conceal in a program like Photoshop. Something that doesn't exist anymore can't be restored. Additionally, most screenshot tools these days use 100% opacity when using the shape option, so censoring something with them is usually safe. Just ensure that your screenshot tool or file format doesn't support layers to prevent the shape from being saved on a separate layer.

Title Image by Darius Bashar from Unsplash