On certificates and validation

[Björn Wärmedal]

> > * How does one even validate a self-signed cert? If ew0k.example.com
> > serves a certificate with the Common Name "ew0k's Awesome Site" and no
> > Subject Alternative Names, is that less valid than one that includes
> > the hostname? Or the IP? Does expiration time matter at all for a
> > self-signed cert?
>
> Yes, test the common name and/or alternative names, and the expiration
> time.

So basically at least one of Common Name and Subject Alternative Names
should match the hostname provided? What if it doesn't, but it matches
the IP?

I guess I could press this issue even more, really. There's a
technical difficulty in parsing wildcards, for example, depending on
the libs available. If I request some.subdomain.example.com and the
certificate lists "some.*.example.com" the ssl._dnsname_match() method
in python3 won't be able to match it despite it being a valid wildcard
(afaik). I don't really know what the certificate specification says
here, but with self-signed certs we can write pretty much anything. My
example of a Common Name that doesn't have a hostname would fail under
your validation algorithm, but is it objectively *wrong*?

Under a TOFU scheme I could also encounter a situation where a cert I
have accepted has now expired, but the server still provides it. Is it
*wrong* to still accept it?

> You may find this article I wrote on TOFU helpful:
>
> gemini://drewdevault.com/2020/09/21/Gemini-TOFU.gmi

Thank you for this!