Signing a request with a signature gives the wrong from address

Question

I am using ethers to get a signed request. I have a signature for the address 0x518171C334AD3adc1cF990A884D67963201B675F.
It is, c4b71f516c36468022d61b0e22016c6bc69bc2c99666fb7e63e3d04841c82ff53d5c687ebe43b290d03caa6766e8e957192faa6b360f4102b536c54c3318304f1b.
When I use ethers.js, to serialize my transaction with my signature I get a transaction which is something like
0xf86a808477359400825208943e245df5a4de41e65cecd1f98b96ca06c3d319f087470de4df8200008029a0c4b71f516c36468022d61b0e22016c6bc69bc2c99666fb7e63e3d04841c82ff5a03d5c687ebe43b290d03caa6766e8e957192faa6b360f4102b536c54c3318304f
As it is an ECDSA signature, it keeps on changing every time I generate it. And every time I try to decode the serialized transaction I get a different from address.
This means that there must be something wrong in how I am using the function.
This is the code I am working with, with one of the signatures that I have generated (mentioned above)
const { utils } = require('ethers');
const bn = require('bignumber.js');
const { serializeTransaction, parseEther } = utils;
const tx2 = require('ethereumjs-tx');
async function serialize1() {
        const wei = parseEther('0.02');
        console.log(wei);
        const gasPrice = '0x' + (2000000000).toString(16);
        console.log(gasPrice);
        const tx = {
                gasPrice: gasPrice,
                gasLimit: 21000,
                to: '0x3e245df5a4de41e65cecd1f98b96ca06c3d319f0',
                value: wei,
                nonce: 0,
                data: '',
                chainId: 3,
        };
        const serializedTransaction = await serializeTransaction(tx);
        console.log(serializedTransaction);
        const signature = 'c4b71f516c36468022d61b0e22016c6bc69bc2c99666fb7e63e3d04841c82ff53d5c687ebe43b290d03caa6766e8e957192faa6b360f4102b536c54c3318304f1b';
        console.log('---------------------------------------------');
        const signedSerialized = await serializeTransaction(tx, Buffer.from(signature, 'hex'));
        console.log(signedSerialized);
}

serialize1();

When I decode this transaction, I get
{
  "nonce": 0,
  "gasPrice": 2000000000,
  "gasLimit": 21000,
  "to": "0x3e245df5a4de41e65cecd1f98b96ca06c3d319f0",
  "value": 20000000000000000,
  "data": "",
  "from": "0x0d67ffdd9b1d8858582d903bf0093e22b9da682e",
  "r": "c4b71f516c36468022d61b0e22016c6bc69bc2c99666fb7e63e3d04841c82ff5",
  "v": "29",
  "s": "3d5c687ebe43b290d03caa6766e8e957192faa6b360f4102b536c54c3318304f"
}

Which is clearly not the from address I want. Am I using the serialize function in the correct way? Is the encoding that I am sending the signature in correct? Any other way I can use this signature? Any other package?
Any help will be greatly appreciated!!!

Giuseppe Bertone · Answer

You don't sign a request with a signature, you sign a transaction. You cannot generate generic signatures to be associated to transactions later on, you need to have access the complete object you want to sign - the raw transaction in your case - before the process works correctly. Each library and wallet has its own signing implementation. With ethers.js in particular, you can use the Signer object to sign a transaction. From the [ethers.js official documentation][1]: signer.signTransaction( transactionRequest ) ⇒ Promise< string< DataHexString > > Returns a Promise which resolves to the signed transaction of the transactionRequest. This method does not populate any missing fields. Sub-classes must implement this, however they may throw if signing a transaction is not supported, which is common for security reasons in many clients. Signer usually refers to an external wallet - i.e MetaMask - not controlled by your ÐApp. If instead you control the wallet with your app - so not a ÐApp anymore ;) - below some examples of signing transactions and messages with ethers.js and a wallet. You can find them inside the [Wallet official documentation][2] also: // Create a wallet instance from a mnemonic... mnemonic = "announce room limb pattern dry unit scale effort smooth jazz weasel alcohol" walletMnemonic = Wallet.fromMnemonic(mnemonic) // ...or from a private key walletPrivateKey = new Wallet(walletMnemonic.privateKey) walletMnemonic.address === walletPrivateKey.address // true // The address as a Promise per the Signer API walletMnemonic.getAddress() // { Promise: '0x71CB05EE1b1F506fF321Da3dac38f25c0c9ce6E1' } // A Wallet address is also available synchronously walletMnemonic.address // '0x71CB05EE1b1F506fF321Da3dac38f25c0c9ce6E1' // The internal cryptographic components walletMnemonic.privateKey // '0x1da6847600b0ee25e9ad9a52abbd786dd2502fa4005dd5af9310b7cc7a3b25db' walletMnemonic.publicKey // '0x04b9e72dfd423bcf95b3801ac93f4392be5ff22143f9980eb78b3a860c4843bfd04829ae61cdba4b3b1978ac5fc64f5cc2f4350e35a108a9c9a92a81200a60cd64' // The wallet mnemonic walletMnemonic.mnemonic // { // locale: 'en', // path: 'm/44'/60'/0'/0/0', // phrase: 'announce room limb pattern dry unit scale effort smooth jazz weasel alcohol' // } // Note: A wallet created with a private key does not // have a mnemonic (the derivation prevents it) walletPrivateKey.mnemonic // null // Signing a message walletMnemonic.signMessage("Hello World") // { Promise: '0x14280e5885a19f60e536de50097e96e3738c7acae4e9e62d67272d794b8127d31c03d9cd59781d4ee31fb4e1b893bd9b020ec67dfa65cfb51e2bdadbb1de26d91c' } tx = { to: "0x8ba1f109551bD432803012645Ac136ddd64DBA72", value: utils.parseEther("1.0") } // Signing a transaction walletMnemonic.signTransaction(tx) // { Promise: '0xf865808080948ba1f109551bd432803012645ac136ddd64dba72880de0b6b3a7640000801ca0918e294306d177ab7bd664f5e141436563854ebe0a3e523b9690b4922bbb52b8a01181612cec9c431c4257a79b8c9f0c980a2c49bb5a0e6ac52949163eeb565dfc' } // The connect method returns a new instance of the // Wallet connected to a provider wallet = walletMnemonic.connect(provider) // Querying the network wallet.getBalance(); // { Promise: { BigNumber: "42" } } wallet.getTransactionCount(); // { Promise: 0 } // Sending ether wallet.sendTransaction(tx) [1]: https://docs.ethers.io/v5/api/signer/#Signer-signTransaction [2]: https://docs.ethers.io/v5/api/signer/#Wallet--methods

Signing a request with a signature gives the wrong from address

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