On-device encryption

A lot of vault apps put a lock screen in front of your photos and call it protection. The files behind it can still be ordinary, readable files. Anyone who gets at the phone’s storage, or a copy of it, gets the photos.

Silo works the other way round. The lock screen is real, but the protection is that the files themselves are scrambled and only your keys can unscramble them.

What it does

When you add a photo, video or file, Silo encrypts it on your phone before anything is written to storage. What lands on the phone is a scrambled container. The original name, the date and the album it sits in are scrambled too, not only the picture.

The cipher is XChaCha20-Poly1305 with 256-bit keys. In plain terms, it does two jobs at once. It hides the contents, and it notices if even one byte has been changed, so a damaged or tampered file can’t pass as a good one.

How it works, step by step

1. You choose a PIN (or a passphrase on Pro). Silo runs it through Argon2id, a method designed to be slow and memory-hungry so that guessing millions of PINs on a powerful computer is expensive.
2. That produces the key that opens your vault. The key is never saved as it is. Behind it sits a long random master key, and your recovery phrase can rebuild it if you forget your PIN.
3. You open the vault. Silo unlocks the keys in memory and decrypts items only as you view them.
4. Photos and videos are shown from memory. Silo serves them to the viewer without writing a decrypted copy to storage.
5. You lock, or the vault locks itself. Silo clears the keys and caches from memory. The vault goes back to scrambled files that nobody can read.

Why it matters

If a phone is lost, repaired, copied or backed up somewhere you didn’t expect, the scrambled files are all anyone finds. Without your keys they are noise.

It also means the answer to “who can see my photos?” is short. There is no Silo account and no Silo server. Your keys exist on your phone and in the recovery phrase you wrote down, and nowhere else. That is also why a forgotten PIN and a lost phrase together can’t be fixed by anyone, including us.

Because the design is written down in a published format, you don’t have to take our word for any of this. Anyone can read how the files are built, and the independent decrypt tool opens a vault without the app.

What you’ll notice

Not much, and that is the aim. Importing takes a moment longer because each item is encrypted and then read back and checked. Unlocking takes a moment because the slow key step is deliberate.