Custodial account transactions are unique as these users cannot sign transactions using their web3 wallets. To ensure security and align with the characteristics of web3 transactions, we have developed a special application called the Custodial Signer. This application securely handles the transaction signing process by communicating with a server specifically designed to manage custodial account transactions.

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Integration Guide

To use custodial accounts to complete transactions, requires users to implement a popup window and communicate with the custodial signer via post messages to obtain the signature. Then, the obtained signature is used to send the transaction to the blockchain.

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Code Example: Barebones Solution

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Step 1: Get Signature by Communicating with Custodial Signer

import { ethers } from 'ethers'

const rawTransactionWithoutSignature = {
  to: 'the destination wallet address',
  value: ethers.parseEther('0.01'),
  chainId: 'the chain id',
  gasLimit: 210000,
  gasPrice: ethers.parseUnits('10.0', 'gwei'),
}
let nonce = 0

const custodialSignerUrl = 'the custodial signer service url'

async function signTransaction() {
  if (typeof window === 'undefined') {
    return
  }

  const fromAccount = 'your own wallet address'

  const transactionCount = await provider.getTransactionCount(fromAccount)
  nonce = transactionCount + 1

  const serializedUnsignedTransaction = ethers.Transaction.from({
    ...rawTransactionWithoutSignature,
    nonce,
  }).unsignedSerialized

  const signTransactionPayload = {
    account: fromAccount,
    transaction: serializedUnsignedTransaction,
  }

  // If using native clients (games) payload can include callbackUrl to which signature will be sent
  // const signTransactionPayload = {
  //   account: fromAccount,
  //   transaction: serializedUnsignedTransaction,
  //   callbackUrl: 'http://localhost:3000/signature-callback' // <- your game callback URL here
  // };

  const id = 'client:2' // must be formatted as `client:${ an identifier number }`
  const tag = 'fv/sign-tx' // do not change this

  const encodedPayload = {
    id,
    tag,
    payload: signTransactionPayload,
  }

  window.open(
    `${custodialSignerUrl}?request=${base64UrlEncode(
      JSON.stringify(encodedPayload),
    )}`,
    'futureverse_wallet', // don't change this
    'popup,right=0,width=290,height=286,menubar=no,toolbar=no,location=no,status=0',
  )

  window.addEventListener('message', (ev) => {
    if (ev.origin === custodialSignerUrl) {
      const dataR = signMessageType.decode(ev.data)

      if (E.isRight(dataR)) {
        transactionSignature = dataR.right.payload.response.signature
      }
    }
  })
}

Refer: getSignatureFromCustodialSigner to get signature on web and native(server) clients.

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Step 2: Send Transaction to Blockchain

sync function sendTransaction() {
  if (transactionSignature == null || fromAccount == null) {
    return;
  }

  const rawTransactionWithSignature = {
    ...rawTransactionWithoutSignature,
    signature: transactionSignature,
    from: fromAccount,
    nonce,
  };

  const serializedSignedTransaction = ethers.Transaction.from(
    rawTransactionWithSignature
  ).serialized;

  const transactionResponse = await provider.broadcastTransaction(
    serializedSignedTransaction
  );
}

Refer: transactions for signEthTransaction, signRootTransaction and sendTransaction.