Device-to-Device MPC

In Device-to-Device MPC, two or more smartphones connect directly to each other (peer-to-peer) to perform threshold signing. No central server is required. The phones discover each other, establish a secure session, and run the MPC protocol locally. Each phone holds one key share and uses its own cryptographic processor to sign.

Transport options: Bluetooth Low Energy (BLE), MultipeerConnectivity (iOS), peer-discovery over local WiFi, or relay servers for out-of-range pairing.

Both phones advertise themselves on the local network using Bonjour (mDNS) or BLE scanning. Each device shows a friendly name and status indicator.

Both users verify that the codes match on both screens (out-of-band). This prevents man-in-the-middle attacks. Once verified, both phones establish a TLS session authenticated by the shared nonces.

Both phones now have a secure, encrypted, and authenticated channel. They can send messages without eavesdropping or tampering.

Round 1 (Commitment): Each phone generates a random key share and a commitment hash. The commitments are exchanged and verified against received messages.

Round 2 (Key Derivation): Both phones derive the same public key Q = (x_0 + x_1)*G and verify it matches.

Round 3 (Storage): Each phone stores its share in the Secure Enclave (iOS) or StrongBox (Android) — a hardware-backed keystore that prevents extraction.

Round 1 (Nonce Generation): Each phone generates a random nonce k_i and broadcasts a commitment to it.

Round 2 (Partial Signatures): Each phone computes a partial signature component and sends it to the other. The partial components are combined.

Round 3 (Final Signature): One phone (designated by access policy) receives the final signature (r, s) and stores it.