const crypto = require('crypto');
const { generateKeyPairSync } = crypto;

const { privateKey, publicKey } = generateKeyPairSync('rsa', {
  modulusLength: 2048,
  publicKeyEncoding: {
    type: 'spki',
    format: 'pem'
  },
  privateKeyEncoding: {
    type: 'pkcs8',
    format: 'pem'
  }
});

const data = "apiCode=0001&entCode=1&nonce=999&timestamp=1565244098737";
const sign = crypto.createSign('RSA-MD5');
sign.update(data);
const signature = sign.sign(privateKey, 'base64');
console.log('Signature:', signature);