input: Write a function to implement an encryption and decryption application in Python. The function should take a string as input, encrypt it using the AES algorithm with a 16-byte key, and return the encrypted string. Conversely, the function should also be able to take an encrypted string and decrypt it back to the original plaintext. The key used for both encryption and decryption should be "Sixteen byte key".
  
  from Crypto.Cipher import AES
  
  key = b'Sixteen byte key'
  
  def encrypt(plaintext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.decrypt(ciphertext)
  
  assert encrypt(b'HelloWorld') != b'HelloWorld'
  assert decrypt(encrypt(b'HelloWorld')) == b'HelloWorld'
output: def encrypt(plaintext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.encrypt(plaintext)
  
  def decrypt(ciphertext):
      cipher = AES.new(key, AES.MODE_ECB)
      return cipher.decrypt(ciphertext)