Overview
NEXUS is a cross-chain metaprotocol that provides smart contract programmability across Bitcoin, Zcash, and Dogecoin. V1 supports independent chain operations, with unified liquidity via intent exchange coming in V2.
Core Components
┌─────────────────────────────────────────────────────────────────┐
│ NEXUS Metaprotocol │
├─────────────────────────────────────────────────────────────────┤
│ │
│ ┌──────────────────────────────────────────────────────────┐ │
│ │ Execution Layer │ │
│ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │
│ │ │ WASM VM │ │ State │ │ Mempool │ │ │
│ │ │ Executor │ │ Processor │ │ Manager │ │ │
│ │ └─────────────┘ └─────────────┘ └─────────────┘ │ │
│ └──────────────────────────────────────────────────────────┘ │
│ │
│ ┌──────────────────────────────────────────────────────────┐ │
│ │ Bridge Layer │ │
│ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │
│ │ │ Bitcoin │ │ Zcash │ │ Dogecoin │ │ │
│ │ │ Bridge │ │ Bridge │ │ Bridge │ │ │
│ │ └─────────────┘ └─────────────┘ └─────────────┘ │ │
│ └──────────────────────────────────────────────────────────┘ │
│ │
│ ┌──────────────────────────────────────────────────────────┐ │
│ │ Security Layer │ │
│ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │
│ │ │ ZK Proofs │ │ Fraud │ │ Economic │ │ │
│ │ │ STARK/Groth │ │ Proofs │ │ Bonds │ │ │
│ │ └─────────────┘ └─────────────┘ └─────────────┘ │ │
│ └──────────────────────────────────────────────────────────┘ │
│ │
│ ┌──────────────────────────────────────────────────────────┐ │
│ │ V2: Intent Exchange Layer │ │
│ │ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │ │
│ │ │ Intent │ │ Solver │ │ Unified │ │ │
│ │ │ Matching │ │ Network │ │ Liquidity │ │ │
│ │ └─────────────┘ └─────────────┘ └─────────────┘ │ │
│ └──────────────────────────────────────────────────────────┘ │
│ │
└─────────────────────────────────────────────────────────────────┘
V1 vs V2
| Feature | V1 (Current) | V2 (Planned) |
|---|
| Smart Contracts | ✅ | ✅ |
| Cross-Contract Calls | ✅ | ✅ |
| Bitcoin Deposits | ✅ | ✅ |
| Zcash Deposits | ✅ (t-addr) | ✅ + z-addr |
| Dogecoin Deposits | ✅ | ✅ |
| Cross-Chain Swaps | ❌ | ✅ Intent Exchange |
| Unified Liquidity | ❌ | ✅ |
Execution Layer
WASM Virtual Machine
- Executes WebAssembly smart contracts
- Deterministic execution for verifiability
- Gas metering for resource limits
- Host functions for storage, crypto, events
State Processor
- Processes transactions in order
- Maintains global state tree
- Generates state commitments
- Handles cross-contract calls
Mempool Manager
- Transaction ordering and prioritization
- EIP-1559 style fee market
- Backpressure handling
- Parallel transaction validation
Bridge Layer
Each supported chain has a dedicated bridge component:
Bitcoin Bridge
- Vault Type: Taproot (P2TR) with escape hatch
- Verification: ZK-STARK proofs + BitVM2 fraud proofs
- State Commits: OP_RETURN outputs every N blocks
- Confirmations: 6 blocks (~60 min)
Zcash Bridge
- Vault Type: Transparent (t-addr) with escape hatch
- Verification: Groth16 proofs (native Zcash verification)
- State Commits: OP_RETURN outputs
- Confirmations: 10 blocks (~25 min)
- V2: Shielded (z-addr) support
Dogecoin Bridge
- Vault Type: P2PKH with CLTV escape hatch
- Verification: ZK-STARK proofs + fraud proofs
- State Commits: OP_RETURN outputs
- Confirmations: 6 blocks (~6 min)
Security Layer
ZK Proofs
NEXUS uses different ZK proof systems optimized for each chain:
| Chain | Proof System | Trusted Setup | Verification |
|---|
| Bitcoin | ZK-STARK | ❌ No | BitVM2 bisection |
| Zcash | Groth16 | ✅ Zcash’s | Native on-chain |
| Dogecoin | ZK-STARK | ❌ No | Fraud proofs |
Fraud Proofs
When a prover posts an invalid state:
1. Challenger detects invalid state commitment
2. Challenger posts bond and initiates challenge
3. Bisection game narrows to single disputed step
4. Disputed step verified (on-chain or via ZK)
5. Loser's bond transferred to winner
Economic Bonds
- Provers must post bonds on L1 before committing state
- Bond amount covers potential fraud proof costs
- Slashed bonds go to successful challengers
- Creates economic incentive for honest behavior
Vault Architecture
All vaults follow a two-path spending structure:
Vault Address
│
├── Protocol Path
│ └── L1 balance always matches L2 balance (1:1 backing)
│
└── Escape Path
└── User can spend after timelock (emergency recovery)
Escape Hatch Timelocks
| Chain | Timelock | Approximate Time |
|---|
| Bitcoin | 2016 blocks | ~2 weeks |
| Zcash | 576 blocks | ~1 day |
| Dogecoin | 1440 blocks | ~1 day |
The escape hatch ensures users can always recover their funds, even if the protocol becomes unresponsive.
Transaction Flow
Deposit Flow
┌─────────────────────────────────────────────────────────────────┐
│ Deposit Flow │
├─────────────────────────────────────────────────────────────────┤
│ │
│ 1. User requests vault address │
│ │ │
│ ▼ │
│ 2. Protocol derives deterministic vault address │
│ │ │
│ ▼ │
│ 3. User sends L1 funds to vault address │
│ │ │
│ ▼ │
│ 4. Scanner detects deposit after N confirmations │
│ │ │
│ ▼ │
│ 5. L2 balance credited to user │
│ │
└─────────────────────────────────────────────────────────────────┘
Withdrawal Flow
┌─────────────────────────────────────────────────────────────────┐
│ Withdrawal Flow │
├─────────────────────────────────────────────────────────────────┤
│ │
│ 1. User requests withdrawal on L2 │
│ │ │
│ ▼ │
│ 2. L2 balance debited │
│ │ │
│ ▼ │
│ 3. Withdrawal added to batch queue │
│ │ │
│ ▼ │
│ 4. Batch processed and L1 transaction broadcast │
│ │ │
│ ▼ │
│ 5. User receives L1 funds │
│ │
└─────────────────────────────────────────────────────────────────┘
State Commitments
NEXUS posts state commitments to each L1 chain periodically:
Block N: State Root 0xabc123...
Block N+100: State Root 0xdef456...
Block N+200: State Root 0x789abc...
- Downloading full state from NEXUS nodes
- Computing Merkle root of state
- Comparing with on-chain commitment
Node Architecture
┌─────────────────────────────────────────────────────────────────┐
│ NEXUS Node │
├─────────────────────────────────────────────────────────────────┤
│ │
│ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │
│ │ RPC │ │ Gateway │ │ Sync │ │
│ │ Server │ │ (REST) │ │ Service │ │
│ └─────────────┘ └─────────────┘ └─────────────┘ │
│ │
│ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │
│ │ WASM │ │ Vault │ │ Watchtower │ │
│ │ VM │ │ Manager │ │ │ │
│ └─────────────┘ └─────────────┘ └─────────────┘ │
│ │
│ ┌─────────────┐ ┌─────────────┐ ┌─────────────┐ │
│ │ Bitcoin │ │ Zcash │ │ Dogecoin │ │
│ │ Scanner │ │ Scanner │ │ Scanner │ │
│ └─────────────┘ └─────────────┘ └─────────────┘ │
│ │
│ ┌──────────────────────────────────────────────────────────┐ │
│ │ Storage (RocksDB) │ │
│ └──────────────────────────────────────────────────────────┘ │
│ │
└─────────────────────────────────────────────────────────────────┘
Node Modes
| Mode | Description | Use Case |
|---|
| Primary | Produces blocks, commits state | Main sequencer |
| Replica | Syncs from primary, read-only | Scaling reads |
| Archive | Full history, no pruning | Historical queries |
V2: Intent Exchange (Planned)
V2 will introduce cross-chain unified liquidity via intent exchange:
┌─────────────────────────────────────────────────────────────────┐
│ Intent Exchange Flow │
├─────────────────────────────────────────────────────────────────┤
│ │
│ User Solver Network Chains │
│ ┌─────┐ ┌─────────────┐ ┌─────────┐ │
│ │ │── Create ──────►│ │ │ Bitcoin │ │
│ │ │ Intent │ Solvers │────────►│ │ │
│ │ │ │ compete │ └─────────┘ │
│ │ │◄── Best ────────│ to fill │ ┌─────────┐ │
│ │ │ Execution │ │────────►│ Zcash │ │
│ └─────┘ └─────────────┘ └─────────┘ │
│ ┌─────────┐ │
│ ────────►│Dogecoin │ │
│ └─────────┘ │
│ │
└─────────────────────────────────────────────────────────────────┘
Intent Exchange Benefits
- Best execution: Solvers compete for best price
- No slippage: Price locked at intent creation
- MEV protection: Intents prevent front-running
- Cross-chain atomic: All-or-nothing execution
