Security, in plain language

SimTether moves sensitive messages — verification codes, carrier alerts — between two phones. Here's exactly what protects that traffic, stated plainly enough to verify.

The short version

• Everything between your phones is end-to-end encrypted — not "encrypted to our server," but encrypted from one of your phones to the other.
• Pairing happens once, by QR code you physically scan. After that the phones authenticate by cryptographic key — nothing to type, nothing to phish.
• The optional internet relay forwards ciphertext only. It can see that two sockets are talking; it cannot see what they're saying.
• There is no account, no analytics, no crash reporting. The app's network code talks to exactly one thing: your other phone.
• The bridge phone shows a persistent notification whenever the service is active — it cannot run invisibly on someone's device.

Pairing: one scan, then keys

When you scan the bridge's QR, your phone learns the bridge's public key and a one-time pairing token. From then on:

• Your phone keeps a persistent identity key. The bridge pins the first client key it sees — a photographed QR or leaked token is useless after your phone's first connect.
• The pairing token rotates on every connection. A copied token stops working as soon as your real phone reconnects.
• Re-pairing on the bridge rotates its identity and revokes everything — a lost phone loses access instantly.

The encryption itself

The relay, honestly

Remote access is off by default. If you turn it on, your phones meet at a small splice server that:

• forwards bytes between the two sockets — it never parses application data, by design;
• verifies room ownership cryptographically — claiming a room requires proving the bridge's private key, not just knowing a token;
• sees only metadata: IP addresses, timing, and byte counts.

You can also run your own relay — it's a single small jar — and point both phones at it, in which case we see nothing at all.