Designing a vault whose front door is a working calculator
A lot of people hand their phone to someone else several times a day: to show a photo, pass along directions, or let a friend make a quick call. Nothing wrong with that. The awkward part is the one app on the home screen that says "Vault" or "Private" in big letters. It practically asks to be opened.
That small problem is where Lock It: Calculator Vault, our Android app, started. These are the design notes behind it, including a few things we changed our minds about.
The disguise only works if it isn't a disguise
Our first rule was that the calculator could not be a prop. It does real arithmetic, and someone who borrows the phone to split a dinner bill gets a normal calculator and a normal answer.
The vault opens only when you type your own equation and press =. There is no separate unlock button and no hint on screen. A wrong answer is just math.
Your equation is the key
During setup you create an unlock equation on the calculator and enter it a second time to confirm it. The setup screen says the app stores a secure hash of the equation, not the equation itself, and offers an optional recovery email. We put that text on the screen on purpose: people should know what is kept and what is not before they trust an app with anything.
What lives inside
Once you are in, the vault holds the things people actually worry about:
- Photos and notes
- Passwords and 2FA keys, so backup codes are not sitting in the camera roll
- Private messages and live calls with only the people you invite
Security you can see
We did not want encryption to be a line buried in a FAQ. The app has a screen that shows vault contents are protected with AES-256 on the phone.
What we learned
- The private path should feel ordinary. The less an app draws attention to itself, the better it does its job.
- Recovery matters as much as locking. A vault you can lock yourself out of is a support nightmare, so pick an equation you will actually remember.
- The free version has to be useful on its own. Lock It is free with ads. Premium Pro is $4.99 a 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.
Questions and criticism from Android developers and privacy-minded readers are welcome in the replies.
Disclosure: this post comes from the team behind Lock It: Calculator Vault. It was written with AI help and reviewed before publishing.
