Yes, information leaks this way, if you redact a previously-JPEG-compressed image with a non-block-aligned black rectangle. Consider the following case: You ha

Yes, information leaks this way, if you redact a previously-JPEG-compressed image with a non-block-aligned black rectangle. 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.)