I ask because what was there would have affected the original compression algorithm. So there might be some signal left.
So, if you want to redact a rectangle between pixels [ 12, 4 ] – [ 34, 18 ], redact [ 8, 0 ] – [ 40, 24 ] instead, this way the black rectangle will cover complete blocks.
The only issue I see is meta tags and embedded thumbnails, which some cameras do. Stripping EXIF is always required.
JPEG compression causes ringing artifacts that can extend out past the pixels you're trying to redact, up to the next block boundary. This is a problem if what you're trying to redact is already a JPEG, rather than a pristine uncompressed image.
This.
I made a response to the parent question that goes into the practical side of this in a bit more detail, but this is a much better one-sentence summary of /how/ the information leaks.
Whether that's enough to decode the redacted information will depend on how many pixels were redacted and other intricacies of JPEG.
This is not too dissimilar to compression oracle attacks, such as CRIME (https://en.wikipedia.org/wiki/CRIME), an infamous vulnerability in HTTPS.
https://en.wikipedia.org/wiki/Rescue_of_Giuliana_Sgrena
https://en.wikipedia.org/wiki/Rescue_of_Giuliana_Sgrena#Rele...
Programs that appear to black out a section of image, but actually leak information.
(Of course, you don't actually have to know how to pronounce something in order to write it! But if you decide what to write by saying it to yourself, you might find yourself instinctively reaching for some alternative turn of phrase.)
Consider the following case: You have limited previously knowledge of the unredacted document; for example, you know that it was a pure black-and-white document that was compressed as a standard color JPEG (not too unusual). Suppose further that in the block extending from (16, 16) to (24, 24) (exclusive), you wish to redact the single pixel at 23, 23, presumedly as the upper left corner of a larger black bar. Finally, suppose that the post-redacted image is losslessly compressed; this simplifies the argument, but is not strictly necessary.
In the way we've phrased this problem, there are only two values for the original color of the redacted pixel; it was black, or white. The rest of the pixels in the block are known in their encoded form (unredacted), but also known in their pre-encoded form (pure black or pure white, while JPEG compression will put them somewhere into dark gray or light gray, respectively). With all known pixels set to their pre-encoded form, each possible value of the redacted pixel can be considered, using (known, deduced, or guessed) JPEG compression parameters to regenerate the compressed block. Whichever value of the redacted pixel leads to the observed encoded values of the unredacted pixels is the correct value of the redacted pixel.
(In practice, the problem might be less constrained; and you might have multiple but less than a block of pixels redacted; these make it harder to make use of the leaked information, but do not change that some information has indeed leaked.)
Let’s say you’re covering a red pixel with a black pixel. There’s no way that red pixel will in any way “survive JPEG compression” because the compression happens on the raw pixel data; the raw pixel data no longer has a red pixel.
They're saying that before encoding the red dot could have been X pixels wide, and to the naked eye it would seem that after encoding it's still only X pixels wide... but due to compression artifacts it may now actually be affecting an area X+Y pixels wide. So only covering X pixels could leak Y pixels of information.
It honestly sounds a bit paranoid to me, but it's definitely possible.
This is trivially untrue for JPEG; JPEG often stores chroma info as 4:2:0, that is at a quarter the resolution of luminance info. In this common case, covering a single red pixel in the decoded form only removes a quarter of the chroma information from its presence (and this is easy to not notice if, for example, the other three pixels that share its chrominance are nearly black).
I have also seen people try to redact by drawing a black block on a new layer in Photoshop, for example, and then exporting as PDF. They're unaware that the layers are being exported in the PDF and the block can be removed. Always flatten your document completely before export.