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How Jimby keeps your screen private

Jimby is built on standard, well-studied cryptography: TLS 1.3, ECDSA P-256 and HPKE (RFC 9180). We don't invent our own. Our code only decides who is allowed to talk to whom.

Two layers

Every connection goes through our relay, because your computer usually sits behind a home router and can't be reached directly. So the design assumes the relay could be hostile, and gives it nothing to read.

To the relay: TLS 1.3 with a pinned key

  • Jimby apps only accept the relay whose key is built into them. No certificate authority is involved, so nobody can impersonate it with a look-alike certificate.
  • Each device proves who it is by signing a random challenge with its own key. Unknown devices are dropped and repeat offenders are blocked.

Between your devices: end-to-end encryption

  • Inside that connection, your device and your computer set up their own encrypted channel with HPKE in authenticated mode, using keys exchanged when you paired them.
  • Every frame of your screen and every keystroke is sealed with AES-256-GCM. The relay forwards bytes it cannot open.
  • Messages that are replayed, reordered or tampered with break the session instead of getting through.

Keys stay on your devices

  • On Mac and iPhone, keys live in the Secure Enclave. They cannot be copied off the device, not even by us.
  • On Windows, for both Jimby Host and Jimby, keys are encrypted with DPAPI and tied to your Windows account on that PC.

Pairing

  • Jimby Host shows a one-time code as a QR code or link. It's valid for 10 minutes and works once.
  • The relay only ever sees a hash of it. Your new device sends a proof that only your computer can open, so even a compromised relay couldn't slip its own keys in.
  • Someone at the computer has to click Allow. The default button is Deny. On a Mac, software without Accessibility permission can't click it for you.

You stay in charge

  • Your computer shows a notification when a session starts and when it ends, and the Jimby icon shows while someone is connected.
  • Remove a device from Jimby Host and its session closes within seconds. It won't be let back in.
  • The computer only makes outgoing connections. Nothing on your network waits for incoming ones.

What the relay knows

Which devices are online, which ones connect to each other and when, and how much data flows. It does not know what's on your screen, what you type or what files you open.

What Jimby can't protect against

We'd rather be clear about the limits:

  • Malware already running as you on the computer can see your screen anyway, with or without Jimby.
  • On Windows, DPAPI protects your keys from other users and from a copy of the disk, not from a program running under your own account.
  • Jimby Host needs permission to control the pointer and keyboard (Accessibility on macOS). Anyone you pair gets that control, so only pair devices you own or trust.

Found a problem?

Write to [email protected]. We read every report and reply.