App lock

A code in front of your diabetes log, so that opening the app needs more than picking up the phone. Everything stays on your device; nothing about the lock is sent anywhere.

What it does

Asks for a 4 or 6 digit code every time the app opens, and again after the time you choose in the background.
Keeps your whole log encrypted on the device (AES-256-GCM). It is decrypted in memory only after the code is entered, and the plain copy is removed from storage when you turn the lock on.
Optionally accepts your fingerprint or face through your device's own check. The code always still works.
Slows down guessing: four wrong codes are free, then waits grow from 30 seconds to an hour.
Refuses codes everyone tries first, such as 1111 or 123456.

How the code protects your data

The code is never stored. A random data key encrypts your log, and that key is itself locked by a second key derived from your code with PBKDF2-SHA-256 (600,000 rounds, a random salt). A wrong code simply fails to open it. Changing the code re-wraps the key without touching your data.

Honest limits

A forgotten code cannot be recovered by anyone, including us. You can erase the log on the device and start fresh. Keep an encrypted backup somewhere else.
A short code is guessable by someone who copies the stored files off the device and tries codes without the app. 6 digits is far stronger than 4, and each try is deliberately slow, but neither matches a long passphrase.
The lock protects a closed or locked app. It does not protect a phone that is unlocked and in someone else's hands.
Only one window can use the log at a time while the lock is on.
The optional automatic backup file you can choose is a plain copy and is not covered by the lock. Use an encrypted backup instead.
Fingerprint and face unlock depend on your browser and device, and have not been exercised on every device.

Turn it on

Open Settings and choose "Turn on app lock". You can change the code, the auto-lock time and fingerprint or face unlock there, or turn the lock off again with your code. More about what leaves the device is in the Privacy centre.