input: Write a function to create an application in Python for encryption and decryption of strings. The application should take a string as input, and use an AES cipher algorithm in ECB mode to encrypt and decrypt the string.
  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'secret message')==b' \x1e\xff\xf1\xe3`\x89\xb9\xe3\x8fP\xae\xf3&\x9d\xb8'
  assert decrypt(b' \x1e\xff\xf1\xe3`\x89\xb9\xe3\x8fP\xae\xf3&\x9d\xb8')==b'secret message'
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)