def aes(key, data, counter=False):
    """ encrypt data with aes, using either pycryptopp or PyCrypto.
        Args
            key: The encryption key
            data: plain text data
            counter: a callable, usually not needed
    """
    # using either pycryptopp...
    if hasattr(AES, "process"):
        a = AES(key)
        return a.process(data)
    # ... or PyCrypto
    counter = counter or Counter()
    a = AES.new(key, AES.MODE_CTR, counter=counter)
    rest = len(data) % 16
    if not rest:
        return a.encrypt(data)
    # Data length must be a multiple of 16
    # Pad with bytes all of the same value as the number of padding bytes
    pad = (16 - rest)
    data += chr(pad) * pad
    return a.encrypt(data)[:-pad]