verifyDocument.js

import OTS from 'javascript-opentimestamps';
import { SHA256, getOtsFromCertificate, decodeOts } from './helpers.js';

const publicKey = Buffer.from('fTrOd0HG8opCEgb+dXvNUXQmzPQqyMGC/IQinDJof1g=', 'base64');

const verifyDocumentOnBlockchain = async (certificate: string, ots: string, docUrl: string) => {
  const verifiedCertificate = sign.open(Buffer.from(certificate, 'base64'), publickKey);
  if (!verifiedCertificate) {
    throw new Error('WARNING: Certificate is invalid');
  }

  const metadata = JSON.parse(Buffer.from(verifiedCertificate).toString());
  const sha256 = await SHA256(await (await fetch(docUrl)).arrayBuffer());
  if (sha256 !== metadata.sha256) {
    throw new Error('WARNING: Document has been modified');
  }

  const detachedOts = decodeOts(ots);
  const { bitcoin } = await OTS.verify(detachedOts, getOtsFromCertificate(certificate));
  if (!bitcoin) {
    throw new Error('WARNING: Verification on Bitcoin blockchain failed');
  }

  return { otsAttestedAt: new Date(Number(bitcoin.timestamp) * 1000) });
}