Smart Contracts

RAIL20 runs on Base (chain 8453), Robinhood Chain (chain 4663, an Arbitrum Orbit L2), Arbitrum (chain 42161), and BNB Smart Chain (chain 56). It runs one independent zero-knowledge privacy pool contract per token on each chain - there is no single shared vault. Each chain has a native-asset pool plus stablecoin pool(s): USDC on Base, USDG on Robinhood, USDC on Arbitrum, and both USDC + USDT on BSC (18-decimal Binance-Peg tokens).

Contract Addresses

Base (chain 8453)

Robinhood Chain (chain 4663)

Arbitrum (chain 42161)

Arbitrum and Robinhood share the same pool addresses because each pool was deployed from the same address at the same account nonce on both chains - the resulting contract address is deterministic and identical across chains. The deployments are independent contracts on separate chains; there is no shared state.

BNB Smart Chain (chain 56)

BSC's BNB and USDC pool proxies share the Robinhood/Arbitrum addresses (same deployer + nonce, deterministic CREATE); the USDT pool is unique to BSC. Binance-Peg USDC and USDT are both 18 decimals, unlike the 6-decimal USDC on Base/Arbitrum - the client and API pin TOKEN_DECIMALS=18 for these pools. Same-chain BNB ↔ USDC/USDT swaps route via PancakeSwap V3 (router 0x13f4EA83D0bd40E75C8222255bc855a974568Dd4).

Fee recipient: 0xd962c7a67b74EEe8D418189f7917A18577A4775d. Relayer fees for every pool are paid to this address.

Staking & Governance

The RAIL20 staking and governance system lets stakers lock $RAIL20 to earn a share of protocol fees paid in WETH and to gain voting power over on-chain governance. Live on Robinhood Chain (4663).

Governance parameters (identical to upstream): sponsorship threshold 500,000 $RAIL20 within a 30-day window, voting quorum 2,000,000, 30-day unlock cooldown, snapshot interval 1 day, reward distribution interval 14 days, distribution rate 2% of the treasury balance per interval (intervalBP = 200).

Ownership chain: Treasury, ProxyAdmin, and GovernorRewards are all owned or admin-controlled by Delegator; Delegator is owned by Voting. All privileged actions execute only through passed governance proposals.

Privacy Pool

Each pool is a self-contained ZK privacy contract for a single token. It holds shielded balances as UTXO notes, maintains a Poseidon commitment Merkle tree, verifies zk-SNARK proofs, and tracks spent nullifiers. On-chain, a spend reveals only commitments and nullifiers - never amounts or recipients. The proving circuit produces at most 2 outputs per transaction: outputs[0] is the recipient note and outputs[1] is the change note.

// Shield (deposit) into the pool: create up to 2 output commitments.
// USDC pool: caller approves the pool first. ETH pool: send value.
function shield(uint256 amount, uint256[2] calldata outputCommitments) external payable;

// Spend a note by proving ownership. A zk-SNARK proof reveals only the
// Merkle root, nullifiers, and new output commitments - never amounts.
// outputs[0] = recipient note, outputs[1] = change note (max 2 outputs).
function transact(
    uint256 root,
    uint256[] calldata nullifiers,
    uint256[2] calldata outputCommitments,
    address recipient,        // 0 = fully private (no on-chain withdraw)
    uint256 withdrawAmount,   // 0 = no public withdraw
    bytes calldata proof
) external;

UTXO Notes & Nullifiers

Shielding produces private UTXO notes. Each note commits to its value and owner as a Poseidon hash inserted into the pool's Merkle tree. To spend, the owner proves in zero knowledge that the note exists in the tree and publishes its nullifier; the nullifier marks the note spent without revealing which leaf it was. Double-spends are impossible because a nullifier can be published only once.

commitment  = Poseidon(amount, pubkey, blinding, mintAddress)
nullifier   = Poseidon(commitment, merklePath, signature)  // signature = Poseidon(privKey, commitment, merklePath)
merkleRoot  = Poseidon-based tree over all commitments (BN254 field)

Relayer Fee

Users submit proofs to a relayer, which broadcasts the transaction so the user pays no gas and their wallet never touches the chain. The relayer takes a flat fee plus a percentage; there is no on-chain fee-splitting contract. All fees go to the fee recipient address above.

Integration Guide

import { createPublicClient, http } from "viem";
import { base } from "viem/chains";

const client = createPublicClient({ chain: base, transport: http() });

// Base (chain 8453)
const BASE_POOLS = {
  ETH:  "0x99205B045Fcf5ff2689ad6B038abF3dd35b79ddE",
  USDC: "0x764FF96EabEeF2b197f845dD8D39441Ac68FDE11",
};
// Robinhood Chain (chain 4663)
const ROBINHOOD_POOLS = {
  ETH:  "0xBf6a26CEF8f6251B46Cc09D03A0f12Df5972d163",
  USDG: "0x04F8d8E4C3f23fE2BfB80c6C46b7d7dfE6f853F0",
};
// Arbitrum (chain 42161) - same deterministic addresses as Robinhood
const ARBITRUM_POOLS = {
  ETH:  "0xBf6a26CEF8f6251B46Cc09D03A0f12Df5972d163",
  USDC: "0x04F8d8E4C3f23fE2BfB80c6C46b7d7dfE6f853F0",
};
// BNB Smart Chain (chain 56) - BNB + USDC share Robinhood/Arbitrum addresses; USDT is unique.
const BSC_POOLS = {
  BNB:  "0xBf6a26CEF8f6251B46Cc09D03A0f12Df5972d163",
  USDC: "0x04F8d8E4C3f23fE2BfB80c6C46b7d7dfE6f853F0",
  USDT: "0x8d484006882fDe2604968D5BE24D092DbAE2C04b",
};
const POOLS = BASE_POOLS;

// Read the current Merkle root of the ETH pool
const root = await client.readContract({
  address: POOLS.ETH,
  abi: poolAbi,
  functionName: "getLastRoot",
});

Backend API

The RAIL20 backend at https://api.rail20.org serves proof, balance, and settlement data used by the app: