How to redact an image so the hidden data cannot be recovered
Why some blur and pixelation can be reversed, how coarse mosaics discard detail for good, and a safe checklist for censoring photos.
Redaction that only looks safe
A gentle blur or a fine mosaic often hides text to the human eye while leaving enough structure for software to reconstruct it. Researchers have recovered pixelated faces and even unmasked blurred numbers by testing which original patterns would produce the same smeared result. The lesson is that a weak effect is a preview, not a redaction. If the censored area still shows a faint outline of letters or features, an attacker with the same tool can probe it.
Why coarse block averaging is stronger
This tool replaces every block with the single average colour of the pixels inside it, then writes that flat colour back over the whole block. The larger the block, the more distinct pixels collapse into one value, so a 40 or 80 px block over a small face leaves only a few featureless squares. Because the original values are overwritten rather than blended, there is no residual gradient to reverse-engineer. The trade is visual: a coarse mosaic is obvious, which is usually fine for redaction.
Flatten the result before you share it
Redaction fails most often through the file, not the pixels. Export to a flat raster format such as PNG or JPEG rather than a layered format that could carry a hidden original underneath. Re-encoding on the canvas also drops EXIF metadata, so location and camera tags do not leak. Never send the source file alongside the censored one, and delete any intermediate copies your workflow created.
A quick redaction checklist
First, switch to Region mode and box the exact area, checking that the dashed outline fully covers the sensitive part with a small margin. Second, raise the block size until you cannot read or recognise anything inside the box. Third, download and reopen the saved file to confirm the mosaic looks the same as the preview. Finally, keep only the redacted copy and remove the original from shared folders, chats, and your trash.