Calculator: Photo/Messaging

SilentSum is a calculator. It adds, subtracts, multiplies and divides, and that is what anyone who picks up your phone sees.

Behind it, if you want one, is an encrypted vault for your photos and videos.

OPENING THE VAULT
You choose a PIN when you set the vault up. From then on you open it by making the calculator show that number: type it, press =, and the vault opens instead of a result.

You can also reach it in pieces. Any result that is a run of digits inside your PIN counts towards it, so the number never has to be typed in the open. A sum that has nothing to do with your PIN is only ever a sum.

There is no hidden gesture and no secret long-press. If you never set a vault up, it stays a calculator for good.

HOW YOUR FILES ARE PROTECTED
• Your key is derived from your PIN with Argon2id, which is slow and memory-hungry by design, so guessing it offline is expensive rather than instant.
• Photos and videos are encrypted with XChaCha20-Poly1305, which detects tampering instead of quietly decrypting altered data.
• Keys live in the Android Keystore and are excluded from system backups.
• Vault screens are marked secure, so they are left out of screenshots, screen recordings and the recent-apps preview.
• Importing a picture moves it in. It stops being an ordinary file in your gallery.

WORKS OFFLINE
No account, no sign-up, no email address. The free vault holds up to 50 items and never needs a network.

CLOUD BACKUP (OPTIONAL)
Off by default. Turn it on and your vault is encrypted on the device first, so the servers hold ciphertext and nothing else. There is no password-reset path into your files, because there is nothing on the server to reset them with. Keep your recovery passphrase.

PRIVATE MESSAGING (OPTIONAL)
SilentSum Pro adds end-to-end encrypted messages between SilentSum users, with photos and video. Messages are encrypted with keys derived from your vault, so they are readable on your device and nowhere else. There is no user directory and no search: you reach someone by typing the exact address they gave you.

WHAT IT COSTS
Free: the calculator, and a vault holding up to 50 items.
SilentSum Cloud, PHP 99 per 30 days: an unlimited vault and encrypted cloud backup. Each payment buys 30 days and stops there. Nothing renews on its own and no card is kept on file.
SilentSum Pro, PHP 299 once: private messaging. Paid once, kept for good.

WHAT IT CANNOT DO
Encryption protects your files at rest and in transit. It cannot protect you from someone who knows your PIN, from malware running on your phone with elevated privileges, or from being compelled to unlock the app. No app can.

Privacy policy: https://krueger100.github.io/silentsum-privacy/
More about the app: https://krueger100.github.io/silentsum/