import struct


def unpack_value(format_string, stream):
    """Helper function to unpack struct data from a stream and update the signature verifier.

    :param str format_string: Struct format string
    :param stream: Source data stream
    :type stream: io.BytesIO
    :returns: Unpacked values
    :rtype: tuple
    """
    message_bytes = stream.read(struct.calcsize(format_string))
    return struct.unpack(format_string, message_bytes)