input: Write a function to create an application in Python that takes a string as input and uses the AES encryption algorithm in ECB mode to encrypt and decrypt the string. The function should return the encrypted and decrypted versions of the input string.
  assert encrypt("Hello World")!=b"Hello World"
  assert decrypt(encrypt("Hello World"))==b"Hello World"
  assert encrypt("")==b""
  assert decrypt(b"Invalid")!=b"Invalid"
output: from Crypto.Cipher import AES
  import base64
  
  def encrypt(plaintext):
      key = b'Sixteen byte key'
      cipher = AES.new(key, AES.MODE_ECB)
      padded_plaintext = plaintext.encode() + b'\0' * (AES.block_size - len(plaintext.encode()) % AES.block_size)
      encrypted = cipher.encrypt(padded_plaintext)
      return base64.b64encode(encrypted)
  
  def decrypt(ciphertext):
      key = b'Sixteen byte key'
      cipher = AES.new(key, AES.MODE_ECB)
      decoded_ciphertext = base64.b64decode(ciphertext)
      decrypted = cipher.decrypt(decoded_ciphertext)
      return decrypted.rstrip(b'\0')