Encryption on the device

This complements How the end-to-end encryption works, which covers what leaves the device.

The device data key

Data on the device is encrypted at rest, account or not. Each profile has a device data key that seals every database row and snapshot with XChaCha20-Poly1305. The operating system’s keychain holds it: Electron’s safeStorage on the desktop (the Windows data protection API, the macOS keychain or the Linux Secret Service), and the Android Keystore through Expo’s SecureStore on the phone. A Linux desktop without a keychain protects it with a passphrase you choose, through Argon2id.

The app lock

The app lock (PIN or passphrase) is a screen in front of the open app, not another layer of encryption. While it’s locked, the keys already unlocked stay in memory.

Backup files

A .sternwaybackup file is encrypted with a key derived by Argon2id from its backup password, independent of any account, server or device key. That’s why it opens on any device that knows the password.

What this doesn’t protect

Malware running as your user can read the keychain, the app’s files and its memory, as you can. See Threat model: teams and devices.