sign forwarded messages

when forwarding an unsigned message from a downstream link to an
upstream link that has signing enabled then re-pack the message to
sign it.

This allows for tools like DroneCAN GUI tool to be used more easily

Author: tridge

MAVProxy/mavproxy.py

@@ -256,6 +256,7 @@ def __init__(self):
             MPSetting('mavfwd_disarmed', bool, True, 'Allow forwarded control when disarmed'),
             MPSetting('mavfwd_rate', bool, False, 'Allow forwarded rate control'),
             MPSetting('mavfwd_link', int, -1, 'Forward to a specific link'),
+            MPSetting('mavfwd_signing', bool, True, 'Sign forwarded messages'),
             MPSetting('shownoise', bool, True, 'Show non-MAVLink data'),
             MPSetting('baudrate', int, opts.baudrate, 'baudrate for new links', range=(0, 10000000), increment=1),
             MPSetting('rtscts', bool, opts.rtscts, 'enable flow control'),
@@ -902,12 +903,18 @@ def process_mavlink(slave):
     if allow_fwd:
         for m in msgs:
             target_sysid = getattr(m, 'target_system', -1)
-            mbuf = m.get_msgbuf()
             if mpstate.settings.mavfwd_link > 0 and mpstate.settings.mavfwd_link <= len(mpstate.mav_master):
-                mpstate.mav_master[mpstate.settings.mavfwd_link-1].write(mbuf)
+                output = mpstate.mav_master[mpstate.settings.mavfwd_link-1]
             else:
                 # find best link by sysid
-                mpstate.master(target_sysid).write(mbuf)
+                output = mpstate.master(target_sysid)
+            if (mpstate.settings.mavfwd_signing and
+                    output.mav.signing.sign_outgoing and
+                    (m._header.incompat_flags & mavutil.mavlink.MAVLINK_IFLAG_SIGNED) == 0):
+                # repack the message if this is a signed link and not already signed
+                m.pack(output.mav)
+
+            output.write(m.get_msgbuf())
             if mpstate.logqueue:
                 usec = int(time.time() * 1.0e6)
                 mpstate.logqueue.put(bytearray(struct.pack('>Q', usec) + m.get_msgbuf()))