How to send Signature (r,s,v) to smart contract external function? - hash

Question

I need to execute an external function at a deployed smart contract. This function receives a "Signature" struct as a parameter.
struct Signature {
uint8 v;
bytes32 r;
bytes32 s;
}
I've checked at https://medium.com/#angellopozo/ethereum-signing-and-validating-13a2d7cb0ee3
, the following code:
const Web3 = require('web3')
const provider = new Web3.providers.HttpProvider('http://localhost:8545')
const web3 = new Web3(provider)
function toHex(str) {
var hex = ''
for(var i=0;i<str.length;i++) {
hex += ''+str.charCodeAt(i).toString(16)
}
return hex
}
let addr = web3.eth.accounts[0]
let msg = 'I really did make this message'
let signature = web3.eth.sign(addr, '0x' + toHex(msg))
console.log(signature)
What should we set at "msg" in this case to call the function?

Answer 1

The article you shared is about verifying the account which does the transaction:
Signing is a nice way to know something is being done by the correct
person/contract. This means we can trust that someone is actually
doing what they say they are.
Especially in NFT marketplaces before you upload metadata or images you are required to verify that your metamask account will the one who really does the transactions. To verify you just sign a message on the front-end and sent this signed message to the server. This msg can be anything, its value is not important, its signature is important. Once you receive the msg, you can verify it. I will show you the verification process, so you will extract the v,r,s and then pass them to your contract function if the verification process is successful. You should be also building the contract on the server as well.
import * as util from "ethereumjs-util";
import { ethers } from "ethers";
// you could send this with session as well
const message = req.body.message;
// getting contract on serverside. I am setting with ganache
const provider = new ethers.providers.JsonRpcProvider(
"http://127.0.0.1:7545"
);
// import your abi and deployed smart contract address
const contract = new ethers.Contract(
contractAddress,
abi,
provider
)
// ****** Verification Process Starts ********
// we need to get the unsigned message
// nonce is the representation of something that we are going to sign
let nonce: string | Buffer =
"\x19Ethereum Signed Message:\n" +
JSON.stringify(message).length +
JSON.stringify(message);
// create a new buffer
nonce = util.keccak(Buffer.from(nonce, "utf-8"));
// signature: signedData, from signature we will get the address
const { v, r, s } = util.fromRpcSig(req.body.signature);
// matching signature with the unsigned message
const pubKey = util.ecrecover(util.toBuffer(nonce), v, r, s);
const addressBuffer = util.pubToAddress(pubKey);
const address = util.bufferToHex(addressBuffer);
if (address === req.body.address) {
// here call your contract external address passing "v,r,s"
} else {
reject("Validation Failed");
}