Blur, pixelate or black box: hiding information in images properly

For anything that must never be revealed — passwords, card numbers, API keys, account numbers — cover it with a solid box. For faces, number plates and addresses, strong pixelation or heavy blur is fine. Avoid light blur on short, predictable text: it can sometimes be guessed back.

Why a solid box is the safe choice

A filled rectangle replaces the covered pixels with one colour. Nothing about the original survives underneath, so there is nothing to reconstruct — provided the box covers the whole area, including the tops and bottoms of the letters.

Why blur and pixelation can leak

Blur and pixelation don't delete information; they average it. Each output pixel is still calculated from the original pixels underneath. When the hidden content comes from a small set of possibilities — digits, a known font, a known layout — someone can render every candidate, apply the same effect and see which one matches. The open-source tool Depix does exactly this for passwords in pixelated screenshots, when the pixelation was a simple block average and the font and size can be recreated.

Faces and scenes are far harder to reverse because there is no small set of candidates to test. For them, the risk is mostly a weak effect that simply leaves the face recognisable.

Mistakes that leave data readable

• Redacting a layer instead of the image. A box drawn as an annotation in some editors or PDF tools sits on top of the text, which is still in the file. The redaction has to be written into the pixels on export.
• Missing a copy. The same email address in a tab title, a URL bar, a notification preview, a filename or a second window.
• Trusting a crop. In 2023 a bug known as "aCropalypse" in Google Pixel's Markup editor and the Windows Snipping Tool left much of the cropped-out image recoverable from the saved file. Export a fresh copy rather than overwriting the original in place.
• Forgetting hidden data. A redacted photo can still carry GPS coordinates in its metadata.
• Too little effect. A thin blur that leaves letter shapes visible, or pixel blocks smaller than the text height.

What to use for what

Redacting with ImageDoctor

1. 1Open Blur & PixelateBlur & Pixelate works on photos; the Screenshot tool has the same effects plus arrows and text for documentation.
2. 2Cover the areaDrag a rectangle for a blur or pixelation region, or paint over an irregular shape with the brush. For a solid box, use the Rectangle tool with Fill switched on.
3. 3Turn the strength upUse a high intensity, or “Strong blur”. The lowest settings are designed to make text unreadable, but more is safer.
4. 4Download a new fileThe effect is written into the exported pixels, not added as a layer, and the file's metadata, including GPS, is removed.

The image is processed in your browser and isn't uploaded to ImageDoctor — which matters when the whole point is that nobody else should see what is in it.