Offline AES-256 Encryption

Every document, photo, and note you store in UnderKey is encrypted with AES-256-GCM before it touches your device's storage. Your unlock pattern derives the encryption key, which exists only in memory and is never written to disk. Because there are no servers and no cloud sync, there's no remote copy for anyone to hack or subpoena.

How it works

When you create a vault by drawing an unlock pattern:

Your pattern is hashed using PBKDF2 (a key derivation function) with a random salt unique to that vault. This produces a 256-bit encryption key.
The key is used to encrypt and decrypt your documents with AES-256 in GCM mode (Galois/Counter Mode), which provides both confidentiality and authenticity.
The key exists only in memory. When you lock the app or leave it, the key is erased. It's never stored on disk, never uploaded, and never cached.
The pattern itself is never stored. UnderKey stores only the salt and an encrypted test value. When you unlock, your pattern is hashed again, and if the hash matches, the vault unlocks. If it doesn't, the vault stays locked.

This means the only way to decrypt your documents is to know the unlock pattern. If you forget it, the documents are permanently inaccessible — including to us, because we don't have the key and we never saw your documents.

What AES-256-GCM means

AES-256 is the encryption standard used by governments, banks, and militaries worldwide. It's been extensively analysed and is considered unbreakable with current technology.
GCM (Galois/Counter Mode) adds authentication, meaning UnderKey can detect if an encrypted file has been tampered with. If someone tries to modify an encrypted document, UnderKey refuses to decrypt it.
256-bit keys mean there are 2^256 possible keys — more combinations than atoms in the observable universe. Brute-forcing the key is computationally infeasible.

What stays on your device

Everything:

Documents, photos, and scans (encrypted).
Filenames and notes (encrypted).
Expiry dates and reminders (stored locally, not sent to a server).
Vault metadata (encrypted, including the number of items and last access time).

Nothing is ever uploaded. UnderKey doesn't have servers to upload to.

What this means for privacy

No account to hack. There's no username, password, email, or server. If someone breaks into your email or cloud account, they don't get your documents because they're not stored there.
No server to subpoena. Because your documents never leave your device, there's no remote copy for governments, law enforcement, or anyone else to request or seize.
No cloud breach risk. Cloud breaches happen. With UnderKey, your documents aren't in the cloud, so a breach somewhere else doesn't affect you.
No backdoor, no master key. We don't have a way to decrypt your documents if you lose your pattern, because we don't have the key. That's by design.

Limits and trade-offs

If you lose your phone, your documents are gone. There's no cloud backup to recover from. Keep physical backups of truly critical documents somewhere safe.
If you forget your unlock pattern, your vault is permanently locked. There's no password reset, no recovery email, and no way for us to unlock it for you.
You can't access your documents from multiple devices. They live only on the device where you created the vault.

For most people, these trade-offs are worth it: the privacy and security you gain far outweigh the inconvenience. For documents you'd panic about if they leaked — passports, IDs, seed phrases — offline encryption is the only option that truly keeps them under your control.