input: Write a function to implement a string encryption and decryption application in Python using the AES cipher algorithm. The function should take a plaintext string as input and return the encrypted string. Additionally, the function should be able to decrypt the encrypted string back to the original plaintext.
  
  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'Hello, World!') == b'\xaa\xa4\x96\xb4{\xd8\x82\xcc\x8c\xfa\xec+\xfd\xc8Z'
  assert decrypt(b'\xaa\xa4\x96\xb4{\xd8\x82\xcc\x8c\xfa\xec+\xfd\xc8Z') == b'Hello, World!'
  assert encrypt(b'') == b'\xb0\x0b\xd7\x9fRb\xa7i\x9b\x056\xe8\xcdS\xea#'
  assert decrypt(b'\xb0\x0b\xd7\x9fRb\xa7i\x9b\x056\xe8\xcdS\xea#') == b''
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)