MPC Protocol & Node Architecture

Planbok's security is anchored by our 2-of-2 Multi-Party Computation (MPC) protocol. This architecture ensures that a single compromise of a server, employee, or database can never lead to the loss of funds, as signing never assembles the full private key in any single location.

The Distributed Node Architecture

Planbok operates two cryptographically isolated nodes that collaborate to perform sensitive operations.

• Node 1 (Coordinator): Handles request orchestration and stores the "blinded" key shares. In trustless models, this node is powerless without the client's entropy.
• Node 2 (Signer): An independent cryptographic engine that holds the second share. It only participates in protocols after validating the authenticity and authorization of the request.

Isolation & Communication

Node-to-node communication uses gRPC. Service-to-service traffic is encrypted and mutually authenticated with TLS. The nodes run in separate environments, so a breach of one does not grant lateral access to the other.

Distributed Key Generation (DKG)

Traditional wallets generate a private key and then backup its shards. Planbok reverses this: DKG and signing work from independently generated shares, so the full key is never assembled in either protocol.

Code
sequenceDiagram
    participant C as Client (SDK/API)
    participant N1 as MPC Node 1
    participant N2 as MPC Node 2
    
    C->>N1: Initiate DKG Request
    N1->>N2: Start DKG Session
    Note over N1,N2: Round 1: Exchange Commitments
    N1->>N2: Commitment 1
    N2->>N1: Commitment 2
    Note over N1,N2: Round 2: Generate Shares
    N1-->>N1: Local Secret S1
    N2-->>N2: Local Secret S2
    Note over N1,N2: Joint Public Key Derived
    N1->>C: Public Key + Address

Threshold Signing

Signing a transaction is a collaborative process that uses the partial shares to produce a standard signature (ECDSA or EdDSA) compatible with public blockchains.

The Signing Flow

Code
sequenceDiagram
    participant C as Client (PIN/Org Secret)
    participant N1 as MPC Node 1
    participant N2 as MPC Node 2
    participant BC as Blockchain
    
    C->>N1: Sign Request (with Blinding Secret)
    N1-->>N1: Unwrap Share S1
    N1->>N2: Initiate Signing Protocol
    N2-->>N2: Valdiate Authorization
    Note over N1,N2: Multi-round Cryptographic Handshake
    N1->>N2: Partial Signature P1
    N2->>N1: Partial Signature P2
    N1-->>N1: Aggregate (P1 + P2) = Final Signature
    N1->>BC: Broadcast Signed TX
    BC-->>C: Transaction Confirmed

Key Security Properties

1. No Reconstruction: The shares $S1$ and $S2$ are never combined to form the private key $k$.
2. Stateless Request Validation: Node 2 independently verifies the authorization of every signing request before participating.
3. Indistinguishability: The resulting signature is mathematically identical to one produced by a single-key wallet, ensuring compatibility with all smart contracts.

Implementation Details

• SECP256K1: Used for Bitcoin and EVM chains like Ethereum, BSC, and Polygon.
• ED25519: Used for high-performance chains like Solana, Near, and Substrate.
• Communication Layer: Protobuf-based gRPC for sub-millisecond coordination.