def __add_roundkey(message, roundkey):
    """
        Input: message - List of lists (4x4 Matrix)
               roundkey - List of lists (4x4 Matrix)

        Output: message - List of lists (4x4 Matrix)

        In this method, each element of the original matrix is XORed with
        corresponding element of roundkey. This way a matrix with new values
        is created.
    """
    for i in range(4):
        for j in range(4):
            message[i][j] = message[i][j] ^ roundkey[i][j]
    return message
