Cipher: Secret Notes with Lock

Cipher keeps secret notes behind a PIN lock. What you write is encrypted on this device and never sent anywhere: no account and no phone number. It works offline, and if you forget the PIN, nothing can recover it.

Secret notes with lock
Use it as a locked notepad: private notes you want out of your main notes app, locked notes for a private diary or a list nobody else should read. Enter the PIN, write under Personal and press SEAL; the entry is sealed into the vault on this device and nothing is transmitted. Entries are added one after another. They cannot be edited or deleted one at a time, and the only way to clear the vault is "Erase this vault" on the CONF tab, which removes everything.

What it does not do
What you write is never sent anywhere. There is no export or share button for it: it stays inside this app, on this device, and is read back here. (The one thing you can copy out is your own card link and ID, which are made of a public key, not of anything you wrote.) The app declares network permissions, which the Google Play billing component uses; no line of this app's code sends what you wrote, your keys or your contacts. The app can display a QR code but cannot read one, and it requests no camera permission.

A PIN is the only way in.
You set a PIN of four to twelve digits on first launch, and the vault key is derived from it with PBKDF2-HMAC-SHA512 over 600,000 iterations. That number exists to slow down a machine trying every combination, and you will feel it as a short pause when you open the vault. There is no reset: no security question, no recovery email, no way for us to open it.

Encrypted offline notes
Your encrypted notes are held in files sealed with ChaCha20-Poly1305, each under its own key pulled through HKDF-SHA512 from the vault key. Because it is authenticated encryption, a single altered byte causes the file to be refused rather than opened. Each entry shows a byte figure rounded into fixed steps (192, 512, 1,536 bytes and so on), so short entries of different lengths read the same size.

Notes, keys and cards
The app calls a thread a note. A shelf addressed to yourself is ready the first time you open the app. A thread addressed to someone else needs their card link, a cipher:// link that carries a public key rather than a name. Your own card is also shown as a QR code, but the app cannot read one. The signature on a card is checked before anything is stored. There is no directory to search, and nobody can look you up.

Cryptography used
X25519, Ed25519, HKDF-SHA512 and ChaCha20-Poly1305 are bundled as implementations rather than borrowed from the platform, because what Android ships varies by device generation and older devices have no X25519 at all. Bundling them keeps the arithmetic identical everywhere, and unit tests hold that behaviour in place.

Price
There are no ads. Everything to do with encryption, keys and storage is free, and none of it is held behind a purchase. The free version keeps up to 2 contacts; Pro (weekly, monthly, yearly or one-time) removes that limit. The number of contacts is the only thing Pro changes.

Please know before installing
Cipher is a private notes vault, not a messenger. Nothing in it delivers anything to anybody, and there is no way to move what you have written out to another app.

Recent changes:
The Back button now closes the conversation, Pro, licenses, add-contact and safety-number screens instead of leaving the app. On Android 14 and earlier, the message field is no longer hidden by the keyboard.