Cipher: Secret Notes with Lock

Secret notes with a lock. What you write is encrypted on this device behind a passcode and is never sent anywhere: no account, no phone number, no upload. If you forget the passcode, nothing can recover it.

Start with what this app does not do.

What you write is never sent anywhere. There is no account, no phone number, and nothing is uploaded. There is no export or share button for what you write: 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.) Network permissions appear in the listing because Google's billing library adds them; no line of this app's code sends what you wrote.

A passcode is the only way in.
You set a passcode of four digits or more 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 unlock. There is no reset. No security question, no recovery email, no way for us to open it. Forget the passcode and the contents are gone.

How sealing works.
What you write is 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. You do not get to read something that was tampered with and never notice.

Entries are grouped into threads.
The app calls a thread a note and the button that files an entry says SEAL. Pressing it seals that entry into the vault on this device and leaves it there; nothing is transmitted. Each entry carries a byte figure that is rounded into fixed buckets, so one word and a long paragraph read the same size.

Keys and cards.
The app issues you a card — a QR code and a cipher:// link that carry a public key rather than your name. A shelf addressed to yourself is ready the first time you open the app, so you can start writing without anyone's card. Only a thread addressed to someone else needs their card link entered, and the signature on that card is checked before anything is stored. Your own card link and ID can be copied, so how you hand them over is your choice. There is no directory to search, nobody can look you up, and the app can display a QR code but cannot read one — it requests no camera permission.

Things that can be counted.
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 59 unit tests hold that behaviour in place.

There is not one line of code in this app that sends your text, your keys or your contacts anywhere.

— 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 3 contacts; Pro (monthly, yearly or one-time) removes that limit. The number of contacts is the only thing Pro changes.

— Please know before installing
This is 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. A shelf for yourself needs nothing; only a thread addressed to someone else needs their card link. Seal, hold, open — those three things, and nothing beyond them.