coretasks, irc: implement CertFP / SASL EXTERNAL authentication

[dgw]

Description

Tin. Followed Libera's instructions for generating a certificate and added its fingerprint to our official Sopel bot's account in NickServ, then stuck around in my human-operated client to observe. Flawless victory:

[00:53:18] -NickServ- SopelTest!~sopel@68.168.def.abc has just authenticated as you (Sopel)
                                       ^ my home IP ^

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

Note to self

@dgw pls update IRCv3 support info for Sopel when this is merged

Future plans

This lays some groundwork for expanding into SCRAM challenges, as implied by a new TODO comment, but implementing those will take some more research. I had some pseudocode for it in there, but obviously removed it before committing (otherwise linting would probably fail).

As far as actually getting SCRAM into Sopel goes, the options are:

• choose a library — possibly scramp or tinysasl (no kidding; the whole library is ~110 lines of code), but I've barely searched for alternatives
• implement SCRAM ourselves (oy, IRC specs are hard enough amirite)
• steal someone else's implementation (e.g. BitBot has its own, but under GPLv2)

Using a library could be our first excuse to give Sopel setuptools extras. (pip install sopel[scram], anyone?)

Meanwhile, CertFP is what was explicitly requested on May 29 (May 30 UTC) in our IRC channel, and it turns out that was fairly easy to get done, so it might as well get reviewed and merged without waiting for SCRAM nonsense.

[half-duplex]

LGTM with nits.
Is there a sane way in stock sopel to have a conversation through the bot? Notably, to command the bot to PRIVMSG NickServ :CERT ADD for enrollment without needing to configure a regular client or ZNC as the bot to do that. I've always just loaded a hackjob .exec plugin.

[half-duplex]

References:
IRCv3.2 SASL spec
SASL EXTERNAL RFC

[dgw]

@half-duplex re: sending commands as Sopel, the .say and .me commands (in admin) could use a sibling that handles raw commands. Maybe .sendraw or just .raw?

[half-duplex]

Heh, this isn't the first time this has come up, I guess it is needed.

2019-03-20 06:41:51      ma1    idk if there's a .quote or .raw or something
2019-03-20 06:43:31     +dgw    hmm, feels like there should be

[half-duplex]

Actually, thinking about it... .say NickServ CERT ADD is entirely sufficient, no need for a raw message. The half of the thought that isn't currently handled gracefully is that there's no way to have the bot send NickServ's responses back to you, either over IRC or in the terminal... 🤔

[half-duplex]

The docstrings for auth_target and server_auth_sasl_mech should also have EXTERNAL added as a valid option. Maybe an example in configuration.rst too?

[dgw]

@half-duplex Both of those comments should be addressed by the newest push.

[half-duplex]

Well, here's a fun one: sasl is considered server-based, so that example won't work.
Does SASL make sense as server-based? Why do we distinguish between "server-based" and "nick-based"? Could we just allow an arbitrary set of authentications?
If you want to leave that for another time, I guess I'd just use a single-auth example.

[dgw]

Hmm, I should probably just leave only the single-stage examples for CertFP.

Why do we distinguish between "server-based" and "nick-based"? Could we just allow an arbitrary set of authentications?

This is something Exi and I talked about fairly recently. At some point, it would make a ton of sense to rewrite Sopel's auth system to be a bit more flexible. The context was more about adding pluggability, but I imagine that simplification of the settings would come along with it. No one's going to promise that for any particular timeframe, though, so I think it's best to ship CertFP as-is once we iron out the doc details.

True, this doesn't fix the confusing auth crap, but it also doesn't make things appreciably worse. client_cert_file does its thing regardless of the SASL config, unless the network chooses to require SASL EXTERNAL before authenticating the user.