I'm wondering if it's safe to black out sensitive information from a picture just by using Microsoft Paint?

Let's take in this scenario that EXIF data are stripped and there is no thumbnail picture, so that no data can be leaked in such a way.

But I'm interested in whether there is any other attack, that can be used in order to retrieve hidden information from the picture?

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. (Paint doesn't.) There are some things to watch out for, though:


The width of the blanked region places an upper bound on the length of the secret data
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)
Any spaces in the blanked region provide information about the relative lengths of the data's parts/words (mentioned in David Schwartz's comment)


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. 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. a blocky pixellization). Of course, Paint doesn't have any special effects, so you should be fine.

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. It never hurts to overwrite more information than necessary when you're concerned about secrecy. (This attack isn't a problem if the image never went through JPEG compression before your redaction.)
