input: Write a function to implement an encryption and decryption application in Python. The application should take a string as input and use the AES (Advanced Encryption Standard) algorithm to encrypt and decrypt the string. The key for encryption and decryption should be "Sixteen byte key". The function should return the encrypted or decrypted string based on the operation requested.
  assert encrypt("Hello, World!")==b'bicngWHtMAwhchrtHlEBdA=='
  assert decrypt(encrypt("Hello, World!"))==b'Hello, World!'
  assert encrypt("")==b''
  assert decrypt(encrypt(""))==b''
  assert encrypt("Secret message")==b'Et23Wp3sd5LLCbYSBoEDzw=='
  assert decrypt(encrypt("Secret message"))==b'Secret message'
output: from Crypto.Cipher import AES
  from Crypto.Util.Padding import pad, unpad
  import base64
  
  def encrypt(plaintext):
      key = b"Sixteen byte key"
      cipher = AES.new(key, AES.MODE_ECB)
      padded_plaintext = pad(plaintext.encode(), AES.block_size)
      encrypted_text = cipher.encrypt(padded_plaintext)
      return base64.b64encode(encrypted_text)
  
  def decrypt(ciphertext):
      key = b"Sixteen byte key"
      cipher = AES.new(key, AES.MODE_ECB)
      decoded_ciphertext = base64.b64decode(ciphertext)
      decrypted_text = cipher.decrypt(decoded_ciphertext)
      return unpad(decrypted_text, AES.block_size)