input: from Crypto.Cipher import AES
  
  key = b'Sixteen byte key'
  
  def encrypt_decrypt_strings(plaintext: str) -> str:
      """Encrypts and then decrypts the input string using AES encryption in ECB mode.
      Returns the original plaintext string after decryption.
      >>> encrypt_decrypt_strings('Hello World')
      'Hello World'
      >>> encrypt_decrypt_strings('This is a test string.')
      'This is a test string.'
      """
      cipher = AES.new(key, AES.MODE_ECB)
      encrypted_text = cipher.encrypt(plaintext.encode())
      decrypted_text = cipher.decrypt(encrypted_text).decode()
      return decrypted_text
output:     cipher = AES.new(key, AES.MODE_ECB)
      padded_plaintext = plaintext.encode() + b'\0' * (AES.block_size - len(plaintext.encode()) % AES.block_size)
      encrypted_text = cipher.encrypt(padded_plaintext)
      decrypted_text = cipher.decrypt(encrypted_text).rstrip(b'\0').decode()
      return decrypted_text