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Comment by Const-me | Hacker News Reader

JPEG format normally compresses pictures in 8x8 blocks, and these blocks are aligned so the first one starts at top-left corner of the image. I think these leaks you’re talking about are limited to the inside of the 8x8 blocks.

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.

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(Monochrome pictures in 8x8 blocks, 4:2:0-downsampled colour images in 16x16 blocks; 4:2:2 and non-downsampled 4:4:4 colour images are technically possible but I’ve never seen one.)

Is another low tech way simply print screening the redacted image + cropping? Naturally you will lose resolution if the OS/app is downsampling the image to render but could be good enough and a good hedge against the image format somehow encoding history. AFAIK, some law firms still redact, print, scan for this reason.

I don’t think what you’re saying makes sense. If you covered all information, no information is left. The blocks are irrelevant since 100% of the information is gone. There’s no “history leak” in JPEG, which is basically what’s being asked.

The only issue I see is meta tags and embedded thumbnails, which some cameras do. Stripping EXIF is always required.

> If you covered all information, no information is left.

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.

> JPEG compression causes ringing artifacts that can extend out past the pixels you're trying to redact, up to the next block boundary.

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.

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