[Onion Services][SSL][Vanity] FR: writeup about authentication for onion sites. SSL certs, vanity addresses, etc.
Context - I was reaching out to the owners of securityinabox.org about their onion address https://bpo4ybbs2apk4sk4.onion/, which presents a cert for a completely different domain. I looked for but couldn't find authoritative docs about best practices around SSL for onion sites.
It'd at least be nice to have a short writeup about the recent movement about not requiring EV certs (https://cabforum.org/pipermail/servercert-wg/2020-February/001637.html).
At the risk of scope creep it'd perhaps be even better to have a more comprehensive writeup about best practices around proving authenticity for onion addresses. e.g. perhaps also mention why vanity addresses aren't helpful, alternatives to certs you can do (link from something else already securely tied to your identity), etc.
FWIW here's what I sent to the securityinabox folks:
FYI the onion address (http://bpo4ybbs2apk4sk4.onion) linked from your 'about' page (https://securityinabox.org/en/about) appears to be broken. It presents a certificate for common-name "api-test.ttc.io", which results in browser warnings. Unfortunately even if the user clicks through the warnings, the server then just returns a 502 error.
I wanted to mention a few things about the cert in particular, but I should preface with: I'm a developer at the Tor Project; I'm somewhat familiar with this subject but to be clear I'm new and this is outside my primary area
The Tor protocol itself already provides encryption and authentication. Most of the potential value in a certificate would be to link the onion address to your clear-web domain name, but a cert for some other domain, as your server is presenting, doesn't do that either.
A cert for "securityinabox.org" might be a little better - it'd still cause a warning, but at least on inspection would prove that this onion address really belongs to the owner of that domain. OTOH simply having a link to your onion site from an SSL/TLS clear-web page you own, which you already do, already does that in a less obscure way.
A cert that includes the onion address itself would get rid of the warning. Until recently this required getting an expensive EV cert, but this is changing now (https://cabforum.org/pipermail/servercert-wg/2020-February/001637.html).
Assuming you don't have much resources to dedicate to this, the best short-term course of action might be to just drop the cert (and hence SSL/TLS) for now to get ride of the warnings (and thus not either scare people away or train them to click away the warnings).