MQTT TSL fingerprint ignored

[lobradov]

Just figured out that Tasmota ignores MQTT TLS fingerprint and continues connection:

00:00:03 MQT: Verify TLS fingerprint...
00:00:04 MQT: Insecure connection due to invalid Fingerprint
00:00:06 MQT: Attempting connection...
00:00:06 MQT: Connected

To add insult to injury, message about invalid fingerprint is not at INFO loglevel, but only at DEBUG which is not on by default, so it's easily missed.

I added a message to INFO and broke the connection loop to retry. Not sure if this is the best way to handle it so please propose better one.

[arendst]

In next pre-release

[issacg]

I'd actually noticed this a month or so ago and thought this was intentional, and I don't like the new behavior as-is, for a number of reasons:

1. It doesn't allow an up-to-date upgrade path to allow switching the certificate (this breaks backwards compatibility, and is the primary reason I'd argue to not leave the new patch as-is)
2. It doesn't match the security of the OTA mechanism.

I'd suggest a patch which allows for 2 fingerprints (or 2 MQTT server configurations), similar to the WiFi config, and also patching the OTA mechanism (and allowing 2 fingerprints). This would mean a bit more RAM (~200 bytes) to hold these 3 variables, but would allow for real TLS validation, something which I haven't really felt as of yet. I'm willing to start working on this now, but wanted to get this comment out there.

[issacg]

OK, maybe I'm getting in over my head by taking this on my own, as there seems to be quite a lot of touchpoints needed to add something to Settings beyond the more simplistic patch of just falling back between fingerprints, but the idea still stands

[davidelang]

using fingerprints for cert validation does mean that we need the ability to have two fingerprints. unfortunantly, fingerprint checking is the only practical way to validate certs in a device this small.

[issacg]

@arendst Any chance you could help me by letting me know what touchpoints I need to deal with to propose a patch that would support the 2 fingerprints? I haven't found any commits (yet) that would provide a clean example of what needs to be changed. For example, I would assume beyond adding the SYSCFG entry it needs to be added to user_config, and probably to the commands for set/get, but where else? Also, how does one go about updating the memory address helpers you have in the SYSCFG struct?

Thanks!