Node.js AES file encryption with authentication promise

encrypt.js

'use strict'
/**
 * AES file encryption with authentication (using Promises)
 * (C) Habib Rehman 2016
 ******************************/
 
const encrypt = function (origpath, destpath, key) {
  // decrypts any arbitrary data passed with the pass
  return new Promise(function (resolve, reject) {
    // readstream to read the (unencrypted) file
    const origin = fs.createReadStream(origpath)
    // writestream to write (encrypted) file
    const dest = fs.createWriteStream(destpath)
    // generate a cryptographically secure random iv
    const iv = scrypto.randomBytes(defaults.ivLength)
    // create the AES-256-GCM cipher with iv and derive encryption key
    const cipher = scrypto.createCipheriv(defaults.algorithm, key, iv)

    // Read file, apply tranformation (encryption) to stream and
    // then write stream to filesystem
    origin.pipe(cipher).pipe(dest)

    // readstream error handler
    origin.on('error', (err) => {
      // reject on readstream error
      reject(err)
    })

    // cipher error handler
    cipher.on('error', (err) => {
      // reject on cipher error
      reject(err)
    })

    // writestream error handler
    dest.on('error', (err) => {
      // reject on writestream
      reject(err)
    })

    // writestream finish handler
    dest.on('finish', () => {
      // get the generated Message Authentication Code
      const tag = cipher.getAuthTag()
      // return all the credentials used for encryption
      resolve({
        key,
        iv,
        tag
      })
    })
  })
}