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?

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    Have you considered just experimenting yourself and black-out a small area of the image and compare before/after hexdumps ? Maybe add extra gaussian blur or other features ? – Little Code Jun 13 '16 at 22:36
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    Not yet, I have that in my mind... – Mirsad Jun 13 '16 at 22:39
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    Are you meaning with "hidden information" actually "hidden" informations? Or do you mean by "hidden", informations that are removed? 2 very diferent things in respect to your question – Zaibis Jun 14 '16 at 7:11
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    @TheGreatDuck surely the reason we even have an Information Security stackexchange is that computers very often expose data in unintended ways, no? – nekomatic Jun 15 '16 at 7:47
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    For small images, I blacken out the sensitive info and just take a screenshot of that. May be time consuming to do this with a large number of pictures. – rohithpr Jun 15 '16 at 16:15

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.)

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    Don't forget that if your are blocking out information from a sorted list etc its position can give away what it said. I heard a story of the USA releasing a list of cities containing a particular type of secret installations from the cold war. With ones that were still in use blacked out. The list was alphabetized, so a list of possible cities for each blackout could be generated. Then by cross referencing this list against other facts, on how reasonable it was for a site to be in them, very good guesses could be made at which cities still had active sites. – Lyndon White Jun 14 '16 at 2:42
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    Reversal of blurring type effects isn't a hypothetical risk. At least one notorious scumbag, Mr. Swirl was busted after someone worked out a way to unscramble his face in CP pictures he shared. – Dan Is Fiddling By Firelight Jun 14 '16 at 5:04
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    A tall blacked-out region doesn't tell you whether the text representation contains descenders or not. It only tells you that it might. – Robert Harvey Jun 14 '16 at 5:07
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    Even "a blocky pixelization" may leave enough data to recover obscured text—much better to just completely cover it. – Miles Jun 14 '16 at 5:36
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    @RobertHarvey: Right, but a not-tall blacked-out region tells you that it doesn't. – Lightness Races in Orbit Jun 14 '16 at 10:18

Ditto Ben N, but let me add a couple of points that are too long to fit as comments.

I'd emphasize the distinction between layered and un-layered data formats. Drawing a black box over a section of a GIF, JPG, or PNG image destroys the previous contents. 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.

I'd be very cautious about blurring. You'd have to know how the software does the blur. 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. No way would I rely on it without thoroughly understanding the algorithm. Unless there was some very good reason to blur rather than black out, I just wouldn't do it.

Of course any attempt to redact with solid blocks must completely cover the original contents to be safe. You want to draw a black box, not scribble over it with a black pen that might leave gaps.

Some formats may keep an internal history log. Not quite the same thing, but I once had a case where my organization produced documents in PDF, another company edited those documents and then sent then back to us. We found that errors had been introduced in the documents and, to put it bluntly, blamed them. They claimed that the documents must have been like this to begin with because they didn't do it. Apparently they were unaware that PDF has an internal log of all changes, and I was able to identify exactly what text was changed and the exact time and date of every change.

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    In 2005 there was a case where a US soldier killed an Italian secret agent in Iraq. The US published a report which contained classified information, including the name of the soldier who shot. It was a PDF, and the secret information had been covered with a black layer. It was quickly discovered that the text beneath was still present, and a simple copy/paste would reveal everything. So this is a real risk! – Fabio says Reinstate Monica Jun 14 '16 at 10:02
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    In 2007, Christopher Paul Neil was arrested for child pornography, because he used the photoshop "twirl" tool to obscure his face. They were able to undo the effect, and reveal his face: schneier.com/blog/archives/2007/10/untwirling_a_ph.html – Jonathon Reinhart Jun 14 '16 at 11:07
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    It might be worth updating your answer as at least a PNG created in Adobe Fireworks does use layers so it doesn't destroy the content underneath. However I'm unsure about cross compatibility with other image editors (especially Photoshop) – Crazy Dino Jun 14 '16 at 12:07
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    It's not actually randomness that's required, just non-reversibility. (e.g. a hash function isn't random. Neither is drawing a black box. Neither is setting every pixel to the average of all pixels in a region. The latter drastically reduces the information content of the region, but still contains some of the source information.) – Peter Cordes Jun 14 '16 at 12:09
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    @CrazyDino More accurately, that's the APNG format (whether or not it is actually animated). Like animated GIFs, animated PNGs use layers as frames, and if one layer contains the sensitive information, it isn't truly destroyed. – Gallant Jun 14 '16 at 14:49

When blacking out sensitive information in Paint the original pixels are destroyed. But using Inkscape to black out part of a vector image does not destroy the pixels, but instead covers them. If someone removes the black cover they can see the pixels. The same applies to things like Foxit Reader (I almost sent a document with sensitive information which had been covered with a black square).

So using MS Paint to black out sensitive information is safe. JPEG artifacts might show some of the text like @BenN says.

Just don't blur it if you don't blur enough and MS Paint doesn't support blur anyway.

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    Good point - also more complicated editors like Photoshop have transparency settings in several places, and if one gets set to 98% instead of 100%, the color can look like black, but the original data is really just mixed with an almost black color and can be retrieved. – JPhi1618 Jun 14 '16 at 14:09

As a raster image program that does not use layers nor contain an undo history after saving, overwriting sensitive pixels in Paint irrevocably changes them in the saved image.

More reasoning:

Microsoft Paint is a proven simple piece of software with a long history and great popularity that works natively in simple raster image file formats. Serious flaws in Paint's algorithms would have likely been uncovered by now.

When redacting information in a raster image file it's best to use a simple format such as .bmp, .jpg. Simple formats are much easier to inspect and historically have resisted such forensic attacks as data recovery.

Of course, any security method can only show that there isn't any known vulnerability. But I couldn't find proof of any successful recovery of blacked out or blanked out information in a raster image in the .bmp or .jpg file formats that were edited using Paint.

Blurred or pixelated image sanitization has shown vulnerability to data recovery techniques. But that is outside the scope of the question.

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    Sometimes a very short answer is a very good one. – Joshua Jun 16 '16 at 15:14
  • @Joshua Nope. Answers must be longer in order to be considered very good. It's the size that matters; not only the quality. Also, citations and reasoning are needed. – EKons Jun 18 '16 at 8:43
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    @Έρικ Κωνσταντόπουλος "Answers must be longer in order to be considered very good." Odd thing to say. An answer should be as long as it needs to be. That's a meta question anyway. – geoO Jun 19 '16 at 14:19
  • @geoO Meta? R U SRS? This is the main site, not META! – EKons Jun 19 '16 at 14:24
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    My point is you asked a meta question. About question length. Longer != better. I greatly expanded my answer anyway. Problem is you can't prove a negative. One successful recovery of blacked out data means it isn't a secure method. I can't find one successful such attack. My hex inspections of the bitmaps edited in Paint show no data recovery is possible.. – geoO Jun 19 '16 at 14:53

Already a few good answers here, saying Paint is safe. (I have no reason to believe otherwise.)

Just want to add that while blacking out a rectangle that fully covers the area and any surrounding areas (lists that information is part of etc) using a basic well studied image editing program should be fairly safe, just using any image editor might not be safe as shown by http://www.underhanded-c.org/_page_id_17.html

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Some comments on previous answers (all good - Stack Exchange is like watching really good crossword puzzle players.) An interesting topic which occasionally might be life-and-death important. (My overactive imagination at work, but battered women at a shelter whose location is critical to keep secret are an example that come to mind).

Points that I hadn't considered that struck me as particularly important:

  1. Redact spaces, and here's why: Always redact more rather than less. If I were trying to guess, I'd assume a short (i.e. one or two word) redaction to be a name or a date (as a first approximation). So redact longer if possible.

  2. Try very hard to avoid redactions (particularly short ones) of the same length. Those would be likely to contain the same information.

  3. All of the answers provided are true with the current version of Paint (or even a Photoshop image flattened and exported as bmp, png or jpg), but any update of Paint may suddenly introduce undo-through-save, or layers, or auto-backup. And Microsoft have actually introduced some new features in Paint in Windows 10.

  4. Making sure that black is black is, as another poster pointed out, very important - an example that occurs immediately is scanned text (most often grayscale), but that's easy enough with Paint. Just make sure you're using the rectangle tool and both color selectors are set to actual black. (Although some artistry with the Paintbrush tool may give false information about ascenders and descenders. Whether this is ethical or legal I have no idea).

  5. As a developer, it strikes me there might be a use for a redaction tool that takes all of this into account, or a search-and-redact macro in Word.

  6. Obviously, redact even subtle contextual clues -- "his" or "her" eliminates 50% of the search pool (roughly). But that's drifting outside the scope of the question.

  7. I'm not sure about the method redaction for legal purposes, but replacing the redacted text with [REDACTED] would leave almost no clues, if you have access to the original text. You could use this technique in Paint, as well, but disguising the length of the original text would involve a lot of cutting-and-pasting.

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