Some formats may keep an internal history log. You want to draw a black box, not scribble over it with a black pen that might leave gaps.

Of course any attempt to redact with solid blocks must completely cover the original contents to be safe. Unless there was some very good reason to blur rather than black out, I just wouldn't do it. No way would I rely on it without thoroughly understanding the algorithm. If the blurring does not involve any randomness, if it's a deterministic algorithm, it may be possible to undo the blur with appropriate software. You'd have to know how the software does the blur. Drawing a black box over a section of a Photoshop, Corel Draw, or Paint Shop Pro native image does not destroy the previous contents if it's on a different layer. Drawing a black box over a section of a GIF, JPG, or PNG image destroys the previous contents. I'd emphasize the distinction between layered and un-layered data formats. (This attack isn't a problem if the image never went through JPEG compression before your redaction.)ĭitto Ben N, but let me add a couple of points that are too long to fit as comments. It never hurts to overwrite more information than necessary when you're concerned about secrecy. One possible thing to be wary of is JPEG compression artifacts around the secret data, which could be used to get clues about the shape of the text. Of course, Paint doesn't have any special effects, so you should be fine. Some effects can be undone almost perfectly, so make sure the one you use involves a lot of randomness or actual data destruction (e.g. If you use a blur rather than a plain opaque rectangle/brush, a determined attacker could try lots of different possibilities in the image to see what text(s) get close to your image when blurred.

Any spaces in the blanked region provide information about the relative lengths of the data's parts/words (mentioned in David Schwartz's comment).

The height of the region could tell attackers whether the text representation of the data has ascenders or descenders (like in the letters b and p).

The width of the blanked region places an upper bound on the length of the secret data.(Paint doesn't.) There are some things to watch out for, though: As mentioned in the answers to a very similar question, scribbling over part of an image will destroy the original pixels, assuming that your editor doesn't store any layers or undo history in the saved image.

How to use layers in ms paint