How Units encryption works
This is the full design in plain words, including what it does not protect against. If any of it changes, this page changes first.
From pattern to key
- You draw a pattern on a 5 by 5 grid. It must use at least four dots and change direction at least once.
- The pattern is turned into a fixed string of dot positions and stretched with PBKDF2-HMAC-SHA512, 600,000 rounds, using a random salt stored in this iPhone's Keychain (this device only). The result is a 256-bit vault key.
- The vault key unwraps a random 256-bit master key. The master key encrypts every file, thumbnail and the vault index with AES-256-GCM through Apple's CryptoKit, with a fresh nonce for every item.
- The vault's storage folder name is derived from the vault key, so a wrong pattern finds nothing to try and the folder names do not reveal which pattern they belong to.
No recovery
Units keeps no second copy of the master key. There is no recovery phrase, no reset email and no developer override. The pattern is the only thing that can derive the vault key, so if you forget it, the vault cannot be opened by anyone, including us. That is the point: any recovery path would also be a way in for someone else. You can change a vault's pattern at any time while it is open.
Hidden vaults and the emergency pattern
The unlock screen has no vault list. A pattern either opens its vault or offers to create one. With Pro you can set an emergency pattern for when someone forces you to open the app: drawing it opens only the vault you set it in, and erases every other vault on this iPhone.
Honest limits
- Your pattern is the ceiling. PBKDF2 makes every guess slow, but a short, simple pattern is still easier to guess than a long one. Units marks 4 to 5 dots as Weak, 6 as Safe and 7 or more as Perfect. Use seven or more dots with several turns.
- Someone with full access to the app's files can count vault folders. They cannot read them or link them to patterns, but they can see that several exist. Plausible deniability is about the screen, not forensic analysis.
- While a vault is open, its keys are in the app's memory. Locking discards them, but iOS does not guarantee memory is wiped immediately.
- Units does not block screenshots or screen recording. It shows the ruler in the app switcher, but anything shown on screen can be captured.
- Playing a video or previewing a document makes a temporary decrypted copy. It lives in the app's private temporary folder and is deleted when you close it, lock the app or open it again.
- Imported originals stay where they were. Photos and notes you import remain in Photos and Notes until you delete them there.
- Version 1 has no backup. Deleting the app deletes your vaults.
What we receive
Nothing from inside your vaults. The app has no account and talks to no server of ours. Purchases go through Apple; we see only the anonymous sales totals Apple reports to every developer.