Pattern Encryption: A Sign That Becomes Your Key

In sigil., the sign you draw on a 5×5 dot grid is the secret that opens your photos. It is not a lock screen sitting in front of ordinary files. Each vault has its own random 256-bit key; your sign, run through Argon2id and bound to the phone's secure chip, is the only thing that unlocks it, and the files are protected with authenticated 256-bit encryption. The sign itself is never stored, so there is nothing on the phone to compare against or read out.

The simple version

Think of a safe whose combination is not written anywhere — not even inside the safe. When you draw your sign, sigil. uses it to build the key on the spot, and opens the vault. The sign itself is not kept. If someone draws a different sign, they get a different key, and that key does not open anything.

This is the main difference from most "locked" photo features. Apple's Hidden album, for example, is protected by your phone's Face ID or passcode, which you also use to unlock everything else (see whether the Hidden album is secure). Many calculator-style vault apps check a PIN and then show files that are not separately encrypted; our guide on whether calculator vault apps are safe explains how to tell.

Why a sign on a 5×5 grid

A sign is fast to draw, easy to remember as a shape, and does not need a keyboard. The 5×5 grid has 25 dots, which gives far more possible paths than the classic 3×3 Android-style grid with its 9 dots. More dots means more room for a sign that is both memorable and hard to guess.

Signs also make multiple hidden vaults natural: each sign derives a different key, so each sign opens a different vault.

The technical version

1. Input. Your sign is recorded as the ordered sequence of dots you pass through.
2. Key derivation. That sequence is fed into Argon2id together with a salt. Argon2id is deliberately slow and memory-hungry: a fraction of a second when you draw once, but a real cost for every guess made in bulk.
3. Bound to the phone. The result is combined with a key that lives inside the phone's secure chip (the Secure Enclave on iPhone) and never leaves it. Only that combination unlocks the vault key. Without this phone, the sign alone unlocks nothing, so guessing cannot be moved to a fast computer.
4. Encryption. The vault key protects your photos with authenticated 256-bit encryption: along with the ciphertext it produces a tag that detects any modification. A wrong key does not produce a slightly wrong photo; it fails authentication cleanly.
5. Nothing to read out. No copy of the sign is saved. The vault key exists on the phone only in wrapped form, and it can be unwrapped only with your sign, on this phone. Data at rest is ciphertext plus non-secret parameters such as salts and nonces.

If you want the background on the cipher itself, read AES-256 encryption explained.

Honest limits

Encryption is only as strong as the secret behind it. AES-256 is not the weak link; your sign is. A few points worth knowing:

• Short signs are weak. A sign that touches four or five dots in a straight line has relatively few possibilities, and people tend to choose similar shapes.
• Shapes are predictable. Letters (L, Z, N, M), squares, and initials are the first things anyone guesses. So are signs that start in a corner.
• Shoulder surfing beats math. If someone watches you draw, the key derivation does not matter. Draw out of sight.
• A copied file is not the shortcut it used to be. Because the key is bound to the phone's secure chip, someone who copies the encrypted data can't guess signs against it on another machine. Guessing has to happen on the phone, where Argon2id, growing waits and the optional self-destruct apply. A long, irregular sign is still what makes it impractical.

The pattern strength meter lets you test a sign in your browser and see how it compares. For a broader comparison with PINs and passwords, see pattern lock vs PIN vs password.

Tips for a strong sign

• Touch more dots — eight or more is a good starting point.
• Change direction several times, including diagonals.
• Avoid letters, digits, initials, and symmetric shapes.
• Do not start in a corner by habit.
• Use different signs for different vaults, and never reuse your phone passcode pattern.

If you forget it

Because the sign is never stored, forgetting it is serious. sigil. gives you a 12-word recovery phrase as a backup way in. Write it down and keep it offline. Without the sign or the phrase, nobody can help, not even us: there is no server, no account, no reset and no copy of your key anywhere.