P2P Cryptocurrency Exchange Mobile App Development

P2P Crypto Exchange Mobile App Development

P2P exchange is not just a bulletin board with prices. It's a system where money and crypto change hands with minimal trust between parties. Key mechanism — escrow: seller's crypto locked on smart contract or custodial wallet until fiat receipt confirmation. Implement this carelessly — either seller loses crypto without payment or buyer pays and gets nothing.

Escrow: Smart Contract vs Custodial Service

Two approaches to choose:

Smart contract escrow (non-custodial). Seller deposits crypto in contract via approve() + deposit(). Buyer transfers fiat any way → clicks "confirm receipt" → contract calls release() and sends crypto to buyer. If dispute — arbitrator can call resolveDispute(winner).

// Simplified escrow logic on EVM-compatible chain
contract P2PEscrow {
    enum Status { ACTIVE, RELEASED, REFUNDED, DISPUTED }

    struct Trade {
        address seller;
        address buyer;
        address token;
        uint256 amount;
        Status status;
    }

    mapping(uint256 => Trade) public trades;
    address public arbiter;

    function deposit(uint256 tradeId, address buyer, address token, uint256 amount) external {
        IERC20(token).transferFrom(msg.sender, address(this), amount);
        trades[tradeId] = Trade(msg.sender, buyer, token, amount, Status.ACTIVE);
    }

    function release(uint256 tradeId) external {
        Trade storage t = trades[tradeId];
        require(msg.sender == t.seller, "Only seller");
        require(t.status == Status.ACTIVE);
        t.status = Status.RELEASED;
        IERC20(t.token).transfer(t.buyer, t.amount);
    }

    function dispute(uint256 tradeId) external {
        Trade storage t = trades[tradeId];
        require(msg.sender == t.buyer || msg.sender == t.seller);
        t.status = Status.DISPUTED;
    }

    function resolveDispute(uint256 tradeId, address winner) external {
        require(msg.sender == arbiter);
        Trade storage t = trades[tradeId];
        require(t.status == Status.DISPUTED);
        IERC20(t.token).transfer(winner, t.amount);
    }
}

Advantage — decentralized, users control wallets. Disadvantage — gas fees per operation, complexity for newcomers.

Custodial escrow. Server holds crypto on hot wallet. Faster, no gas, simpler UX. Requires high security standards: HSM for private keys, multi-sig wallets (Gnosis Safe), cold/hot wallet separation.

Mobile Client: Architecture and Web3

On iOS — web3.swift or custom integration via URLSession to JSON-RPC. On Android — web3j. For user wallet — WalletConnect v2 (Sign SDK): allows connecting MetaMask, Trust Wallet, Rainbow and sending transactions via deep link without storing keys in app.

// WalletConnect v2 connection on iOS
import WalletConnectSign

class WalletService {
    func connect() async throws {
        let methods: Set<String> = ["eth_sendTransaction", "personal_sign"]
        let chains = [Blockchain("eip155:1")!] // Ethereum mainnet
        let namespaces: [String: ProposalNamespace] = [
            "eip155": ProposalNamespace(
                chains: chains,
                methods: methods,
                events: ["chainChanged", "accountsChanged"]
            )
        ]
        let uri = try await Sign.instance.connect(requiredNamespaces: namespaces)
        // Open Deep Link to wallet or show QR
        await UIApplication.shared.open(uri.deepLink)
    }
}

Order Feed and Real-time Updates

Buy/sell order list must update in real time — WebSocket from server. New order, price change, trade close — all via WebSocket. On client URLSessionWebSocketTask (iOS) or OkHttp WebSocket (Android).

Order filtering: currency, payment method (bank transfer, cash), amount range, seller rating. Rating system — mandatory element for P2P. Without it platform doesn't gain trust. Store transaction history and reviews, calculate rating on server.

User Verification (KYC)

Regulatory requirements for P2P: at least basic KYC (passport photo + selfie). On iOS — VisionKit document scanner, on Android — ML Kit Document Scanner. Offload document checks to Sum Sub, Veriff or Jumio via their mobile SDK — no point building liveness detection yourself.

Default limits for unverified users, raised after KYC — standard scheme.

Trader Chat

Built-in chat between buyer and seller within trade — critical function. Without it impossible to resolve disputes. Implement via Firebase Realtime Database or Stream Chat SDK — latter provides ready UI for iOS/Android, saves 2–3 days development. Encrypt messages end-to-end via libsignal if extra privacy needed.

Fiat payment confirmation — bank screenshot, uploaded to chat. Stored in Firebase Storage or S3 with presigned URL.

Security

Rate limiting on order publishing endpoints — without it competitors flood market with junk orders. Anti-scam checks: new account can't immediately publish huge orders. 2FA via TOTP (Google Authenticator) — mandatory for withdrawals. Biometric auth via LocalAuthentication (iOS) / BiometricPrompt (Android) for trade confirmation.

Work Timeline

Total: 8–12 weeks depending on escrow type and feature set. Cost calculated individually after requirements analysis.