Ethereum Transactions – Creating and Signing Offline Raw Transactions

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I'm looking to create a raw transaction, sign it, and broadcast it using the eth.sendRawTransaction RPC method.

I would like to do this offline using code or an open-source library OR online but without the need of using an specific wallet or software.

There is neither a RPC method to create/sign the raw transaction, nor can I find any libraries to do this (offline).

Any help or guidance on how this could be achieved would be most thanked. If there is a python implementation that would be awesome.

Best Answer

ethereumjs-tx is a library with this example:

npm install ethereumjs-tx

const Tx = require('ethereumjs-tx').Transaction
var privateKey = new Buffer('e331b6d69882b4cb4ea581d88e0b604039a3de5967688d3dcffdd2270c0fd109', 'hex')

var rawTx = {
  nonce: '0x00',
  gasPrice: '0x09184e72a000', 
  gasLimit: '0x2710',
  to: '0x0000000000000000000000000000000000000000', 
  value: '0x00', 
  data: '0x7f7465737432000000000000000000000000000000000000000000000000000000600057'
}

var tx = new Tx(rawTx)
tx.sign(privateKey)

var serializedTx = tx.serialize()
console.log(serializedTx.toString('hex'))

Related Solutions

Security – How to Safely Send Money from a Cold Wallet Using an Untrusted Computer

We at MyEtherWallet.com implemented an offline transaction tool to do exactly this. While most people who are cold-storage fanatics know command line and the inner-workings of gas price and nonce, this allows you to do it via GUI.

Navigate to the "Offline Transaction" tab via your online computer. Here you will generate the current gas price and nonce.
Enter the FROM address in the field. Please note, this is the address you are sending FROM, not TO. This generates the nonce and gas price.
Move to your offline computer. Enter the TO ADDRESS and the AMOUNT you wish to send.
Enter the GAS PRICE as it was displayed to you on your online computer in step #1.
The GAS LIMIT has a default value of 21000. This will cover a standard transaction. If you are sending to a contract or are including additional data with your transaction, you will need to increase the gas limit. Any excess gas will be returned to you.
Enter the NONCE as it was displayed to you on your online computer in step #1. If you wish, enter some data. Data is optional. If you enter data, you will need to include more than the 21000 default gas limit. All data is in HEX format. Select your wallet file -or- enter/paste your private key and unlock your wallet.
Press the "GENERATE SIGNED TRANSACTION" button.
The field below this button will populate with your singed transaction. Copy this and move it back to your online computer, or you can use the QR code provided.
On your online computer, paste the signed transaction into the text field in Step 3 and click "SEND TRANSACTION". This will broadcast your transaction.

Alternatively, you could use other services in order to broadcast the signed transaction. Etherscan has a field you can use to broadcast transactions.

drcode on reddit made a tool to decode a signed transaction so you can verify it's doing what it says it is doing.

FYI, everything on MyEtherWallet is almost 100% offline. The only thing that leaves your computer while sending is the signed transaction, which is generated by your browser. We also receive the nonce and gas price, which is why you must generate those bits of information online.

Web3.js – How to Properly Create and Sign a Raw Transaction in Browser

Below is working code that calls "increment" on https://programtheblockchain.com/dapps/counter. (More info about that sample here: https://programtheblockchain.com/posts/2017/12/13/building-decentralized-apps-with-ethereum-and-javascript/.)

It uses web3.js and ethereumjs-tx:

<!-- from https://github.com/ethereumjs/browser-builds/raw/master/dist/ethereumjs-tx/ethereumjs-tx-1.3.3.min.js -->
<script src="ethereumjs-tx-1.3.3.min.js"></script>
<script>
  var address = "0xf15090c01bec877a122b567e5552504e5fd22b79";
  var abi = [{"constant":true,"inputs":[],"name":"getCount","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[],"name":"increment","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"inputs":[{"name":"_count","type":"uint256"}],"payable":false,"stateMutability":"nonpayable","type":"constructor"}];

  var account = "<REDACTED ACCOUNT ADDRESS>";
  var privateKey = "<REDACTED PRIVATE KEY WITHOUT 0x PREFIX>";

  var web3 = new Web3(new Web3.providers.HttpProvider(
    "https://ropsten.infura.io/<REDACTED API KEY"));

  web3.eth.getTransactionCount(account, function (err, nonce) {
    var data = web3.eth.contract(abi).at(address).increment.getData();

    var tx = new ethereumjs.Tx({
      nonce: nonce,
      gasPrice: web3.toHex(web3.toWei('20', 'gwei')),
      gasLimit: 100000,
      to: address,
      value: 0,
      data: data,
    });
    tx.sign(ethereumjs.Buffer.Buffer.from(privateKey, 'hex'));

    var raw = '0x' + tx.serialize().toString('hex');
    web3.eth.sendRawTransaction(raw, function (err, transactionHash) {
      console.log(transactionHash);
    });
  });
</script>

Best Answer

Related Solutions

Security – How to Safely Send Money from a Cold Wallet Using an Untrusted Computer

Web3.js – How to Properly Create and Sign a Raw Transaction in Browser

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