Every once in a while, you’ll come across an image or video where sensitive information has been redacted using a blur or mosaic filter. Just take a look at license plates on COPS or do a search for “google check” and you’ll see a few common examples.

Putting an image through a blur filter is the visual analogue of a one way hash. While it’s difficult to read, it’s still subject to a dictionary attack, and when the subject consists of a 10 digit routing number or a 6 digit license plate, well, the dictionary is very small. Simply identify the filter mechanism used, filter/hash all of the possibile values, and find the resulting hash that measures closest to the source image.

The solution is simple: Don’t blur your images! Instead, just color over them. Remember, you want to leave your visitors with NO information, not blurred information.

Why blurring sensitive information is a bad idea – [via] Link.