Add support for signing configuration for separate MAVLink channels

[IdoBuchman]

Feature request

I want to activate signing on my MAVlink communication channel between the FCU and the Mission Planner. When I activate signing, all the mavlink ports get activated, including the mavlink port that communicates with my companion computer. Usually, there's no need to protect a wired Point to Point serial communication, and it requires ongoing support to keep the companion computer signing-key updated (If we want to change it from time to time).

Solution Proposal

I suggest adding a new bitmask parameter that states which of the mavlink serial ports should work with signing. For keeping backward compatibility, we will make sure the default is 11111 (All ports activated).

Specifically, I found this code snippet under libraries/GCS_MAVLink/GCS_Signing.cpp lines 93-100, in updated master (commit 4b040ee):

    // activate it immediately on all links:
    for (uint8_t i=0; i<MAVLINK_COMM_NUM_BUFFERS; i++) {
        GCS_MAVLINK *backend = gcs().chan(i);
        if (backend == nullptr) {
            return;
        }
        backend->load_signing_key();
    }

I assume that adding here a simple condition can do the job 😄 .

Alternatives

• Using another existing parameter with a similar meaning to this I want to add, and use it instead of introducing a new one.
  • I searched through all the parameters in the copter documentation and did not find a good enough candidate.
• Adding a way to easily update my secret key on my companion computer
  • I found that it makes the operations and ongoing support inconvenient, and would be happy to have a better option.

Platform
[X] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine