input: Write a function that encrypts and decrypts a string using an AES cipher. The function should take a plaintext string as input and return the encrypted ciphertext. Another function should take the ciphertext as input and return the decrypted plaintext. Use a constant 16-byte key for both encryption and decryption.
  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'\xa6\xaaMtg{\xc9\xec@\xcb\xd8\x15\xbb\x0b\x91\x19'
  assert decrypt(b'\xa6\xaaMtg{\xc9\xec@\xcb\xd8\x15\xbb\x0b\x91\x19') == b'Hello World'
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)