Initiating a Withdrawal

In order to create a withdrawal and convert our sBTC back to BTC, we need to create and broadcast a Bitcoin transaction with the necessary data in addition to signing a Stacks message to prove we own the sBTC we are trying to withdraw.
Below is an example code snippet for doing that using the Leather wallet in a Next.js app.
If you want to see how to do this in the context of a complete full-stack app, check out the tutorial, which this example was pulled from. We have it here for easy reference.
This example heavily relies on the sbtc package. Documentation is in progress, but you can see the repo here and take a look at the test files to see examples of how to use it to construct transactions.
"use client";
import { useState, useContext } from "react";
import {
DevEnvHelper,
sbtcWithdrawHelper,
sbtcWithdrawMessage,
TESTNET,
TestnetHelper,
} from "sbtc";
import { bytesToHex, hexToBytes } from "@noble/hashes/utils";
import * as btc from "@scure/btc-signer";
import { openSignatureRequestPopup } from "@stacks/connect";
import { UserContext } from "../UserContext";
import { StacksTestnet } from "@stacks/network";
export default function WithdrawForm() {
const { userData, userSession } = useContext(UserContext);
const [satoshis, setSatoshis] = useState("");
const [signature, setSignature] = useState("");
const handleInputChange = (event) => {
setSatoshis(event.target.value);
};
const signMessage = async (e) => {
e.preventDefault();
// First we need to sign a Stacks message to prove we own the sBTC
// The sbtc paclage can help us format this
const message =
sbtcWithdrawMessage({
network: TESTNET,
amountSats: satoshis,
bitcoinAddress: userData.profile.btcAddress.p2wpkh.testnet,
});
// Now we can use Leather to sign that message
openSignatureRequestPopup({
message,
userSession,
network: new StacksTestnet(),
onFinish: (data) => {
// Here we set the signature
setSignature(data.signature);
},
});
};
const buildTransaction = async (e) => {
// Once the signature has been set, we can build and broadcast the transaction
e.preventDefault();
// Helper for working with various API and RPC endpoints and getting and processing data
// Change this depending on what network you are working with
const testnet = new TestnetHelper();
// const testnet = new DevEnvHelper();
// setting BTC address for devnet
// Because of some quirks with Leather, we need to pull our BTC wallet using the helper if we are on devnet
// const bitcoinAccountA = await testnet.getBitcoinAccount(WALLET_00);
// const btcAddress = bitcoinAccountA.wpkh.address;
// const btcPublicKey = bitcoinAccountA.publicKey.buffer.toString();
// setting BTC address for testnet
// here we are pulling directly from our authenticated wallet
const btcAddress = userData.profile.btcAddress.p2wpkh.testnet;
const btcPublicKey = userData.profile.btcPublicKey.p2wpkh;
let utxos = await testnet.fetchUtxos(btcAddress);
// If we are working via testnet
// get sBTC deposit address from bridge API
const response = await fetch(
"https://bridge.sbtc.tech/bridge-api/testnet/v1/sbtc/init-ui"
);
const data = await response.json();
const sbtcWalletAddress = data.sbtcContractData.sbtcWalletAddress;
// if we are working via devnet we can use the helper to get the sbtc wallet address, which is associated with the first wallet
// const sbtcWalletAccount = await testnet.getBitcoinAccount(WALLET_00);
// const sbtcWalletAddress = sbtcWalletAccount.tr.address;
const tx = await sbtcWithdrawHelper({
// comment this line out if working via devnet
network: TESTNET,
sbtcWalletAddress,
bitcoinAddress: btcAddress,
amountSats: satoshis,
signature,
feeRate: await testnet.estimateFeeRate("low"),
fulfillmentFeeSats: 2000,
utxos,
bitcoinChangeAddress: btcAddress,
});
const psbt = tx.toPSBT();
const requestParams = {
publicKey: btcPublicKey,
hex: bytesToHex(psbt),
};
const txResponse = await window.btc.request("signPsbt", requestParams);
const formattedTx = btc.Transaction.fromPSBT(
hexToBytes(txResponse.result.hex)
);
formattedTx.finalize();
const finalTx = await testnet.broadcastTx(formattedTx);
console.log(finalTx);
};
return (
<form className="flex items-center justify-center space-x-4">
<input
type="number"
placeholder="Amount of BTC to deposit"
className="w-1/3 px-4 py-2 text-gray-300 bg-gray-700 rounded focus:outline-none focus:border-orange-500"
value={satoshis}
onChange={handleInputChange}
/>
<button
className="px-6 py-2 bg-orange-500 rounded hover:bg-orange-600 focus:outline-none"
onClick={(e) => {
signature ? buildTransaction(e) : signMessage(e);
}}
>
{signature ? "Broadcast Withdraw Tx" : "Sign Withdraw Tx"}
</button>
</form>
);
}