Validate onion URLs before provisioning

[heartsucker]

The sd-whonix VM will fail to start the tor process if the onion address is malformed.

[eloquence]

Is this still occurring, and if so, does it cause make all to hang when it does?

[redshiftzero]

also we should validate the auth cookie (cc #321 as we'll need to do the same for v3), the expected format for v2 is:

Valid onion addresses contain 16 characters in a-z2-7 plus
".onion", and valid auth cookies contain 22 characters in
A-Za-z0-9+/

[sssoleileraaa]

So I think make all used to hang here but now a Tor Config Check error window pops up with a message that says the onion address has the wrong format. I think validating the auth cookie would be an improvment, but also I wonder if there is a better error we could show that says the config.json file contains an error that points to the malformed json value?

[conorsch]

we'll need to do the same for v3

Nearest I can figure for the v3 validation options is:

• v3 Onion URL regex: ^[a-z2-7]{56}\.onion$
• v3 Onion auth regex: ^[A-Z2-7]{52}$

That's based on my read of https://github.com/torproject/torspec/blob/master/rend-spec-v3.txt. Since both values are base32-encoded, that's the target charset to match against, with a known length for each value, to boot. Will update #379 accordingly, now that #321 is resolved.