def check_signature(self, msgbuf, srcSystem, srcComponent):
            '''check signature on incoming message'''
            if isinstance(msgbuf, array.array):
                msgbuf = msgbuf.tostring()
            timestamp_buf = msgbuf[-12:-6]
            link_id = msgbuf[-13]
            (tlow, thigh) = struct.unpack('<IH', timestamp_buf)
            timestamp = tlow + (thigh<<32)

            # see if the timestamp is acceptable
            stream_key = (link_id,srcSystem,srcComponent)
            if stream_key in self.signing.stream_timestamps:
                if timestamp <= self.signing.stream_timestamps[stream_key]:
                    # reject old timestamp
                    # print('old timestamp')
                    return False
            else:
                # a new stream has appeared. Accept the timestamp if it is at most
                # one minute behind our current timestamp
                if timestamp + 6000*1000 < self.signing.timestamp:
                    # print('bad new stream ', timestamp/(100.0*1000*60*60*24*365), self.signing.timestamp/(100.0*1000*60*60*24*365))
                    return False
                self.signing.stream_timestamps[stream_key] = timestamp
                # print('new stream')

            h = hashlib.new('sha256')
            h.update(self.signing.secret_key)
            h.update(msgbuf[:-6])
            sig1 = str(h.digest())[:6]
            sig2 = str(msgbuf)[-6:]
            if sig1 != sig2:
                # print('sig mismatch')
                return False

            # the timestamp we next send with is the max of the received timestamp and
            # our current timestamp
            self.signing.timestamp = max(self.signing.timestamp, timestamp)
            return True
check signature on incoming message