Helper method that verifies if the signature part of a signed transaction is valid

verifySignedTxn.ts

/**
* VERIFY SIGNED TXN
* Helper method that verifies if the signature part
* of a signed transaction is valid
*
* REQUIREMENTS
* Install the required packages first
* Run: npm i algosdk @noble/ed25519 @noble/hashes
*
* USAGE
* import verifySignedTxn from 'verifySignedTxn';
* const isValid = await verifySignedTxn( mySignedTxnBtyes );
* ==================================================
*/

import algosdk, { SignedTransaction } from "algosdk"; 
import * as ed from '@noble/ed25519';
import { sha512 } from '@noble/hashes/sha512';
ed.etc.sha512Sync = (...m) => sha512(ed.etc.concatBytes(...m));
const NODE_API_URL = 'https://mainnet-api.algonode.cloud';


/**
* Verify Signed Transaction
* ==================================================
*/
export default async function verifySignedTxn(txnBuffer: Uint8Array) {
  const signed = algosdk.decodeSignedTransaction(txnBuffer)
  if (!signed || !signed.txn || !signed.sig || !signed.sgnr) return false;

  const signer = signed.sgnr;
  const signerAddr = algosdk.encodeAddress(signer);
  const from = signed.txn.from.publicKey;
  const fromAddr = algosdk.encodeAddress(from);
  const authAddr = await getAuthAddress(fromAddr);
  if (signerAddr !== authAddr) return false;

  const sig = Buffer.from(signed.sig);
  const txnBytes = algosdk.decodeUnsignedTransaction( 
    algosdk.encodeUnsignedTransaction( signed.txn )
  ).toByte();
  const txnMsg = Buffer.from(
    concatUint8Arrays( signed.txn.tag, txnBytes)
  ).toString('hex');

  const isVerified = ed.verify(sig, txnMsg, signer);
  return isVerified;
}



/**
* Helpers
* ==================================================
*/
// get the auth address of an account
async function getAuthAddress(addr: string) {
  try {
    const response = await fetch(`${ NODE_API_URL }/v2/accounts/${ addr }?exclude=all`);
    const account = await response.json();
    return account['auth-addr'] || account.address || 'NOT_FOUND';
  }
  catch {
    return 'NOT_FOUND';
  }
}

// https://github.com/algorand/js-algorand-sdk/blob/7965d1c194186e5c7b8a86756c546f2ec35291cd/src/utils/utils.ts#L83
function concatUint8Arrays(...arrs: ArrayLike<number>[]) {
  const size = arrs.reduce((sum, arr) => sum + arr.length, 0);
  const c = new Uint8Array(size);
  let offset = 0;
  for (let i = 0; i < arrs.length; i++) {
    c.set(arrs[i], offset);
    offset += arrs[i].length;
  }
  return c;
}