Never Use Pixelation to Hide Sensitive Text (2014)

Summary

Undoubtedly you have all seen photographs of people on TV and online who have been blurred to hide faces. For example, here's one of Bill Gates:Adapted from the Wikimedia CommonsFor the most part this is all fine with peoples' faces as there isn't a convenient way to reverse the blur back into a photo so detailed that you can recognise the photo. So that's good if that is what you intended. However, many people also resort to blurring sensitivenumbers andtext. I'll illustrate why that is a BAD idea.Suppose someone posted a photo of their check or credit card online for whatever awful reason (proving to Digg that I earned a million dollars, showing something funny about a check, comparing the size of something to a credit card, etc.), blurring out the image with the far-too-common mosaic effect to hide the numbers:Seem secure because nobody can read the numbers anymore? WRONG. Here's a way to attack this scheme:Step 1. Get a blank check image.There are two ways of doing this. You can either Photoshop out the numbers in your existing image, or in the case of credit cards, you can get an account with the same organization and take a photo of your own card from the same angle, and match the white balancing and contrast levels. Then, use your own high resolution photo to photoshop out your numbers.This is easy in these example images, of course:Step 2. Iterate.Use a script to iterate through all the possible account numbers and generate a check for each, blocking out the various sections of digits as sections. For example, for a VISA card, the digits are grouped by 4, so you can do each section individually, thus requiring only 4*10000 = 40000 images to generate, which is easy with a script.Step 3. Blur each image in an identical manner to the original image.Identify the exact size and offset, in pixels, of the mosaic tiles used to blur the original image (easy), and then do the same to each of your blurred images. In this case, we see that the blurred image we have 8x8 ...