input: Write a python function to encrypt a plaintext using AES encryption for a given user.
  assert aes_enc("Alice") == b'abcdefghijklmnop' + b'\xd3\x91\x83\x1b\x02\x12\x90\x16\xba\x1d\x97\x9d\x94\x94\x95\xa3'
  assert aes_enc("Bob") == b'abcdefghijklmnop' + b'\x9f\xa6\xa8\x13\x8c\x1d\xfa\xc5\xb9\x1d\x0a\x1e\x9c\x1d\x0a\x1e'
  assert aes_enc("Carol") == b'abcdefghijklmnop' + b'\x1c\x4d\x1b\x01\x98\x11\x9b\x3e\x8d\x98\x1b\x3e\x9d\x18\x1b\x3e'
output: from Crypto.Cipher import AES
  from Crypto.Util.Padding import pad
  import hashlib
  
  def aes_enc(user):
      # Define a fixed 16-byte key (for example purposes, normally this should be kept secret)
      key = b'abcdefghijklmnop'
      
      # Use SHA-256 hash of the user name as the message to encrypt
      message = hashlib.sha256(user.encode()).digest()
      
      # Create AES cipher object using the key in ECB mode
      cipher = AES.new(key, AES.MODE_ECB)
      
      # Encrypt the message and concatenate it with the key
      ciphertext = cipher.encrypt(pad(message[:16], AES.block_size))
      
      return key + ciphertext