How do you redact private information in an iPhone screenshot?

Why blur is not redaction

A blur is a convolution: every output pixel is a weighted average of its neighbours. It discards precision, but it does not discard information the way it appears to. Given the blurred result and the ability to render candidate text the same way, an attacker can search for the input that produces a matching output.

That attack needs three things: the font, the size, and the layout. On a screenshot, all three are free. It is the system font, at one of a small set of sizes, in a layout the attacker can reproduce by opening the same app.

Pixelation is worse, not better — it is a blur with a smaller, more predictable kernel.

The failure mode is what makes this dangerous rather than merely imperfect: a blurred account number looks handled. It reads as done. Nobody re-checks it, and the file gets sent.

What actually works

Opaque fill, then flatten. A solid rectangle with no transparency, rendered into the pixels of the file you send. The information is destroyed, not covered.

Three details that catch people out:

• Extend past the text. Ascenders, descenders and antialiasing spill outside the glyph box. A rectangle that stops at the last pixel of the last character can leave enough shape to read.
• Flatten before sharing. An editor that stores your shapes as a separate layer has not redacted anything; it has drawn on top of the original. Export a flattened copy and send that.
• Check the file you actually send. Not the preview in the editor. Open the exported file, zoom to 100 percent, and try to select text.

What to look for on an app screenshot

The obvious things are the ones people remember. These are the ones they miss:

• The status bar. Carrier name, and on some devices a personal hotspot indicator.
• A notification banner that dropped in mid-capture.
• Autofill suggestions above the keyboard, which are drawn from your own data.
• The tab bar badge count, which can reveal an internal figure.
• Deep-link URLs visible in a share sheet, which often carry account identifiers.
• QR codes and barcodes, which are machine-readable at surprisingly low resolution.
• Reflected data in charts — a balance you redacted in text may still be plotted.

How Walkthru does it

Walkthru offers redaction as a drag-a-box tool, filled opaque, baked into every export. Blur and pixelation are not options in the app. That was a decision, not a gap: the product’s premise is that the recording never leaves your device, and shipping a redaction that can be reversed would undercut the one guarantee it makes.

Two things it adds on top:

It reads each screenshot on the device and offers redactions. Emails, phone numbers, postal addresses, card numbers, IBANs, US Social Security numbers, ABA routing numbers, crypto wallet addresses, and strings that look like secrets — masked password fields, known API-key prefixes, high-entropy tokens. Where a value carries a checksum, the checksum is verified rather than the shape merely matched — Luhn and IIN for cards, mod-97 for IBANs, the FedACH check for routing numbers, Base58Check and bech32 for wallet addresses — because a tool that cries wolf is one you stop reading.

It offers; it never applies. A proposed redaction appears as a dashed outline you accept with one tap or dismiss with another, and it is never baked into an export until you accept it. Walkthru does not decide what is private in your screenshot. It decides what is worth asking about.

When the same detail repeats down a flow — a header showing an account number on every screen — one tap accepts that redaction on every step at once.