Demonstration of manually signing a legacy Ethereum transaction in TypeScript using RLP, Keccak256 and secp256k1, comparing against the viem library.

ethTxLegacy.ts

import { secp256k1 } from "@noble/curves/secp256k1";
import keccak256 from "keccak256";
import { assert } from "node:console";
import RLP, { Input } from "rlp";
import * as viem from "viem";
import { privateKeyToAccount } from "viem/accounts";

const messageToSign = {
  nonce: 0,
  gasPrice: viem.parseGwei("20"),
  value: viem.parseEther("1"),
  gas: 21000n,
  to: "0x3535353535353535353535353535353535353535",
  data: "0x",
  
};

const privateKey =
  "0xd76e578c0264d4bf7cb6f3e23a8c39feea8d16e39059b4fe69b7ac8e86ed8f5e";
const account = privateKeyToAccount(privateKey);

const signedPrivate = await account.signTransaction({type: "legacy", ...messageToSign} as any);

console.log(`Signed Transaction:`, signedPrivate);

const [rlpBytes,] = inputToRlp([
  messageToSign.nonce,
  messageToSign.gasPrice,
  messageToSign.gas,
  messageToSign.to,
  messageToSign.value,
  messageToSign.data,
]);
const hash = keccak256(Buffer.from(rlpBytes));
const { r, s, recovery} = secp256k1.sign(hash, viem.hexToBytes(privateKey));
const v = 27 + recovery;

const finalListToSign = {
  ...messageToSign,
  r,
  s,
    v,
};

const [, finalRlpSigned] = inputToRlp([
  finalListToSign.nonce,
  finalListToSign.gasPrice,
  finalListToSign.gas,
  finalListToSign.to,
  finalListToSign.value,
  finalListToSign.data,
  finalListToSign.v,
  finalListToSign.r,
  finalListToSign.s,
]);

function inputToRlp(value: Input) {
  const rlp = RLP.encode(value);
  const rlpHex = viem.toHex(rlp);

  return [rlp, rlpHex];
}

assert(signedPrivate === finalRlpSigned, "Signed transaction does not match");