Designing a vault whose front door is a working calculator

Lock It: Calculator Vault — Mon, 12 Oct 2026 06:27:19 GMT

Phones get passed around all day: to show a photo, share directions, or let someone make a quick call. That's normal. But most of us also keep a few things on our phones that are nobody else's business, and a home-screen icon labeled "Vault" invites exactly the question you'd rather not answer.

That was the starting point for Lock It: Calculator Vault, our Android app. Here are the design choices behind it and what we learned along the way.

The front door has to be a real calculator

The first rule was that the calculator can't be a prop. It does real math and behaves like any other calculator app. Someone borrowing the phone can use it and never see anything unusual.

The vault opens only when you type the equation you picked during setup and press =. There's no separate "unlock" button and no hint on screen.

What lives inside

Once you're in, the vault holds the things people actually worry about:

Photos and notes
Passwords and 2FA keys, so backup codes don't sit in your camera roll
Private messages and live calls with only the people you invite

Security people can see

We wanted the security story to be visible, not buried in a FAQ. The app's security screen explains the AES-256 encryption used for vault contents.

UX lessons

Make the private path feel ordinary. The less the app draws attention to itself, the better it works.
Make setup and recovery obvious. A vault you can lock yourself out of is a support nightmare, so pick an unlock equation you will remember.
Keep the free tier genuinely useful. Lock It is free with ads. Premium Pro is \(4.99/month, and Offline Vault is a \)9.99 one-time purchase for people who want it.

Try it

Search "Lock It: Calculator Vault" on Google Play, or use this link: Lock It: Calculator Vault on Google Play

Feedback from Android developers and privacy folks is welcome in the comments.