Pairing

1. Host generates a pairing code (9 digits, 2-minute TTL, single use)
2. User displays the QR, or reads the code aloud
3. Client scans the QR, and dials the routes in it in order
4. Client claims the code on the first route that answers
5. Client verifies the host fingerprint pinned in the QR
6. Both sides exchange signed public keys
7. Host sends a challenge; the client signs it
8. Both derive the SAS digits
9. User confirms the SAS digits match
10. Client persists the paired-app record and the routes it learned

9 digits: XXX-XXX-XXX
Example: 123-456-789

const code = await server.getPairingCode();
// code = {
//   code: "123456789",
//   uri: "crosslink://pair?v=2&e=lan~ws://192.168.1.83:54676&c=123456789&a=…",
//   endpoints: [{ kind: "lan", url: "ws://192.168.1.83:54676" }],
//   expiresAt: 1699999999999
// }

crosslink://pair?v=2&e=<endpoints>&c=<code>&a=<appId>&n=<appName>&f=<fingerprint16>

crosslink://pair?v=2
  &e=lan~ws://192.168.1.83:54676,wan~ws://203.0.113.9:54676
  &c=754524531&a=com.crosslink.chat&n=Crosslink+Chat&f=110ed6212454f46f

http://203.0.113.9:54676/mobile.html#pair=crosslink%3A%2F%2Fpair%3Fv%3D2…

sas = HKDF-SHA256(
  IKM: X25519(hostEphemeral, clientEphemeral),
  salt: hostIdentity || clientIdentity,
  info: "crosslink-sas"
)[:6]

fingerprint16 = SHA-256(hostPublicKey)[:16]

pairing: {
  approve: async (req) => {
    // req.deviceName, req.requestedCaps, req.sas
    return true; // approve
  },
  autoApprove: false, // require explicit approval
  codeLength: 9,      // digits (default: 9)
  codeTtlMs: 120_000  // TTL (default: 2 minutes)
}

await client.pairFromQr(uri, requestedCaps, {
  onConfirmPairing: async ({ sas, grantedCaps, hostName }) => {
    return window.confirm(`Pair with ${hostName}? SAS: ${sas}`);
  }
});