Double Check

Double Check is a browser extension that prevents data-entry errors by mathematically validating high-stakes numbers and requiring you to independently retype them for a direct, on-site comparison within a website’s input field.

Double Check catches simple transcription errors, such as transposed digits, while also guarding against deliberate fraud, such as fake “changed bank details” emails and hidden look-alike characters.

And the values you check never leave your computer. No transmission, no storage, no analytics — OCR and speech recognition run on your device, and the extension can’t even see a page until you invoke it there. Privacy isn’t a policy here; it’s the architecture. (This website has no JavaScript and no tracking either.)