const crypto = require('crypto');
const fs = require('fs');
const path = require('path');

// Generate a key pair for this example
const { privateKey, publicKey } = crypto.generateKeyPairSync('rsa', {
  modulusLength: 2048,
  publicKeyEncoding: {
    type: 'spki',
    format: 'pem'
  },
  privateKeyEncoding: {
    type: 'pkcs8',
    format: 'pem'
  }
});

// File to sign and verify
const filePath = 'example_file.txt';
const signaturePath = 'example_file.sig';

// Function to sign a file
function signFile(filePath, privateKey, algorithm = 'SHA256') {
  return new Promise((resolve, reject) => {
    const sign = crypto.createSign(algorithm);
    const stream = fs.createReadStream(filePath);
    
    stream.on('data', (data) => {
      sign.update(data);
    });
    
    stream.on('end', () => {
      const signature = sign.sign(privateKey, 'hex');
      resolve(signature);
    });
    
    stream.on('error', (error) => {
      reject(error);
    });
  });
}

// Function to verify a file
function verifyFile(filePath, publicKey, signature, algorithm = 'SHA256') {
  return new Promise((resolve, reject) => {
    const verify = crypto.createVerify(algorithm);
    const stream = fs.createReadStream(filePath);
    
    stream.on('data', (data) => {
      verify.update(data);
    });
    
    stream.on('end', () => {
      const isValid = verify.verify(publicKey, signature, 'hex');
      resolve(isValid);
    });
    
    stream.on('error', (error) => {
      reject(error);
    });
  });
}

// Create a test file if it doesn't exist
if (!fs.existsSync(filePath)) {
  const content = 'This is a test file for signature verification.\n'.repeat(100);
  fs.writeFileSync(filePath, content);
  console.log(`Created test file: ${filePath}`);
}

// Main function
async function main() {
  try {
    // Sign the file
    console.log('Signing file...');
    const signature = await signFile(filePath, privateKey);
    
    // Save the signature for later verification
    fs.writeFileSync(signaturePath, signature);
    console.log(`Signature saved to: ${signaturePath}`);
    
    // Verify the file with the saved signature
    console.log('Verifying file...');
    const isValid = await verifyFile(filePath, publicKey, signature);
    console.log('Is signature valid?', isValid);
    
    // Tamper with the file and try to verify again
    console.log('\nTampering with file...');
    fs.appendFileSync(filePath, '\nThis line was added after signing!');
    
    try {
      const isTamperedValid = await verifyFile(filePath, publicKey, signature);
      console.log('Is tampered file valid?', isTamperedValid);
    } catch (error) {
      console.log('Error verifying tampered file:', error.message);
    }
    
  } catch (error) {
    console.error('Error:', error.message);
  }
}

// Run the example
main();