psbt_example.ts

import * as bitcoin from "bitcoinjs-lib";
import * as ECPairFactory from "ecpair";
import tinysecp from "tiny-secp256k1";

// Initialize ECPair for signing
const ECPair = ECPairFactory.ECPairFactory(tinysecp);

// Configuration (replace these with your real values or import from config)
const NETWORK = bitcoin.networks.testnet; // or bitcoin.networks.bitcoin for mainnet
const PLATFORM_KEY_WIF = "<YOUR_PLATFORM_WIF>"; // WIF of your platform's private key
const FEE_WALLET = "tb1q..."; // platform fee recipient
const FEE_BPS = 200; // 2%

interface Utxo {
  tx_hash: string;
  tx_pos: number;
  value: number; // in satoshis
  address: string;
}

/**
 * Build, sign, and finalize a PSBT for a sticker purchase:
 * - Input: buyer UTXO (paid to pool address)
 * - Output1: platform fee
 * - Output2: seller payout
 * - Output3: OP_RETURN(metadata)
 */
export function createStickerPurchasePsbt(
  utxo: Utxo,
  sellerAddress: string,
  productId: string,
  productName: string
): string {
  const platformKey = ECPair.fromWIF(PLATFORM_KEY_WIF, NETWORK);

  // 1. Calculate sats
  const grossSat = utxo.value;
  const feeSat = Math.floor((grossSat * FEE_BPS) / 10000);
  const netSat = grossSat - feeSat;

  // 2. Create PSBT
  const psbt = new bitcoin.Psbt({ network: NETWORK });

  // 2a. Add input
  psbt.addInput({
    hash: utxo.tx_hash,
    index: utxo.tx_pos,
    witnessUtxo: {
      script: bitcoin.address.toOutputScript(utxo.address, NETWORK),
      value: utxo.value,
    },
  });

  // 2b. Add fee output
  psbt.addOutput({ address: FEE_WALLET, value: feeSat });

  // 2c. Add seller payout
  psbt.addOutput({ address: sellerAddress, value: netSat });

  // 2d. Embed metadata via OP_RETURN (up to 80 bytes)
  const metadata = `${productId}|${productName}`;
  const data = Buffer.from(metadata, 'utf8');
  if (data.length > 80) throw new Error('Metadata too long');
  const embed = bitcoin.payments.embed({ data: [data] });
  if (embed.output) psbt.addOutput({ script: embed.output, value: 0 });

  // 3. Sign input with platform key
  psbt.signInput(0, platformKey);

  // 4. Finalize and extract transaction hex
  psbt.finalizeAllInputs();
  const tx = psbt.extractTransaction();
  return tx.toHex();
}

PSBT example

This utility builds and signs a PSBT for a sticker purchase, automatically splitting a buyer’s payment into:

• Platform fee (2%)
• Seller payout (98%)
• OP_RETURN metadata carrying productId|productName (up to 80 bytes)

Example Usage

import { createStickerPurchasePsbt } from './stickerPurchase';

// UTXO from buyer Bob’s payment (in sats)
const utxo = {
  tx_hash: 'abcd1234...',
  tx_pos: 0,
  value: 10000,
  address: 'tb1qbuyer...',
};

// Seller Alice’s address and product info
const sellerAddress = 'tb1qseller...';
const productId = 'sticker123';
const productName = 'CoolSticker';

// Create, sign, and finalize the PSBT
const rawTxHex = createStickerPurchasePsbt(
  utxo,
  sellerAddress,
  productId,
  productName
);

// rawTxHex can now be broadcast to the Bitcoin network

How It Works

1. Calculates fee: 2% of the input UTXO value.
2. Determines payout: input value minus fee.
3. Builds PSBT:
   • Input: buyer’s UTXO
   • Output 1: platform fee address
   • Output 2: seller’s address
   • Output 3: OP_RETURN with metadata payload
4. Signs with the platform’s private key.
5. Finalizes and returns the raw transaction hex.

Perfect for on‑chain marketplaces, tip bots, and any BTC‑priced digital store!