NullKey 
Why the choice matters
A passphrase is harder to guess than a short PIN, and that matters most when someone holds a copy of your encrypted files. They can try guesses without the lockout rules a phone applies to its own screen. A longer passphrase adds possible combinations, and a slow key derivation step adds cost to every guess. A backup file stored in the cloud or on a computer is exactly that kind of copy.
A PIN can be a reasonable choice for a phone's own lock, where the device's lockout rules help. It is a weaker fit for protecting a set of files that could be copied away from the device.
How the strength is built
A well-designed vault never uses the passphrase directly as the encryption key. It runs it through a deliberately slow, memory-heavy function with a random salt, so every guess has to pay that cost, and the result unlocks a random master key rather than your files directly.
This is why the passphrase deserves more care than the convenience unlocks around it. Those unlocks are routes into the same vault, not replacements for the secret that protects it. A PIN of a few digits has so few combinations that even a slow function only delays a determined guesser, while several unrelated words multiply the work many times over.
Choose a passphrase you can keep
A passphrase you cannot type reliably is a weak choice, because you will be tempted to write it down somewhere unsafe. Pick a phrase that makes sense to you but is not obvious to someone who knows you only casually.
- Use several ordinary words that you can type without looking at the keys.
- Avoid anything printed on a card, stored in a wallet or reused on other accounts.
- Lock and unlock the vault with it a few times so typing it becomes automatic.
- Write the recovery key down and store it apart from the phone.
- Test unlocking after any change, using the keyboard you will use later.
How NullKey handles this
NullKey asks you to choose a passphrase when you create the vault, and it stays the root secret alongside your recovery key. Argon2id, set to use 64 MiB of memory with a fresh random salt, turns it into a wrapping key for the vault key, so every guess against a copied backup pays that cost. The app's wait after repeated wrong passphrases is only a courtesy: someone working on a copied backup is not slowed by it.
Limit: if you turn on phone screen-lock unlock, your phone's PIN, pattern or password also opens the vault, so a short phone PIN becomes a second way in for anyone holding the phone.
Pick the trade-off for each case
Use a passphrase for the vault itself, and treat convenience unlocks as a second route you can switch on or off. You may use them for everyday access if you wish, but never choose a weaker secret just because the passphrase is slow to type. If you do enable a quicker unlock, still type the passphrase now and then, because a secret you never use is one you can forget. Review the choice when your habits change, such as when you start using a new phone.
