Warn when SIM card is suspected to not be working

[untitaker]

Prerequisites

• I have read CONTRIBUTING.md

What problem does this feature solve or what does it enhance?

This is a followup to #345, #875 is related.

We should find a way to warn the user if the SIM card appears not to be working. In the past we have given somewhat handwavy answers like "unactivated sim cards might work" that weren't always accurate, but it seems much more likely to me that we can give a more definitive answer with that sim card inserted.

Unfortunately the IP route heuristic in #345 has not worked out for this. Apparently there are SIM cards in the US that do not give out an IP, but do get valid RRC messages.

In some EU countries it's also much harder than I thought to get unactivated SIM cards anonymously, and so people will be tempted to insert dead sims that don't work at all.

Proposed Solution

I have one more idea we can try, but all feedback is wanted:

1. We change the test heuristic to throw warnings when no new cell towers (or RRC messages in general, not sure yet) have been seen in a while.
2. We enable that heuristic by default.
3. Now the line will turn orange if the SIM is useless.

I think step 3 is somewhat useful, the IP heuristic was only alerting within the web interface (since it isn't implemented as analyzer), but really we should warn on the devices' display as well.

We can roll this out more carefully by copying the test heuristic into a new one, implementing the changes there and letting people beta-test it there.

Note that there's indications that the current test heuristic triggers without a SIM at all (#833), we should probably investigate if that is the case for other devices.

Alternatives Considered

No response

[cooperq]

I think to properly test this we need to build our own IMSI catcher, but for now I think inverting the test heuristic is probably a good way to go (though I suspect this will always work even without a SIM card.

[untitaker]

@cooperq what other things are you looking for to see if a capture is valid?

[cooperq]

So I think most IMSI Catchers would need to send NAS messages to operate. So we could just look for NAS messages BUT what I wonder is whether a SIM card that's not receiving NAS messages from real towers would still receive NAS messages form a fake base station. This is something I would like to test. But the naive version of this would be to just test for any nas messages received in a certain time window.