is a self signed certificate always invalid the first time?

[Stephan von Krawczynski]

On Wed, 9 Aug 2017 08:39:30 -0700
Gregory Sloop <gregs at sloop.net> wrote:

> AV> So i’m using dovecot, and i created a self signed certificate
> AV> with mkcert.sh based on dovecot-openssl.cnf. The name in there matches
> AV> my mail server.  
> 
> AV> The first time it connects in mac mail however, it says the
> AV> certificate is invalid and another server might pretend to be me etc.  
> 
> AV> I then have the option of trusting it.  
> 
> AV> Is this normal behaviour? Will it always be invalid if it’s not signed
> AV> by a third party?  
> 
> Yes.
> The point of a trusted CA signing your cert is that they have steps to
> "verify" who you are and that you're "authorized" to issue certs for the
> listed FQDNs. Without that, ANYONE could create a cert, and sign it and then
> present it to people connecting to your mail server [perhaps using a MITM
> style attack.] The connecting party would have no way to tell if your cert
> vs the attackers cert was actually valid.
> 
> It would be like showing up at the bank and having this exchange: 
> 
> You: "Hey, I'm Jim Bob - can I take money out of his account?"
> Bank: "Do you have some ID?"
> You: "Yeah! See, I have this plastic card with my picture and name, that I
> ginned up in the basement."
> 
> Now does the bank say: "Yeah, that looks fine." or do they say "You know we
> really need ID [a certificate] that's authenticated and issued [signed] by
> the state [third-party/trusted CA.]."
> 
> I think it's obvious that accepting your basement produced ID would be a
> problem. [Even if we also admit that while the state issued ID (or trusted
> CA signed certs) has some additional value, it isn't without potential
> flaws, etc.]
> 
> The alternative would be to add your CA cert [the one you signed the server
> cert with] to all the connecting clients as a trusted CA. This way your self
> signed cert would now be "trusted."
> 
> [The details are left as an exercise to the reader. Google is your friend.] 
> 
> -Greg

This was exactly the global thinking - until the day DigiNotar fell.
Since that day everybody should be aware that the true problem of a
certificate is not its issuer, but the "trusted" third party CA.
This could have been known way before of course by simply thinking about the
basics. Do you really think your certificate gets more trustworthy because
some guys from South Africa (just an example) say it is correct, running a
_business_? Honestly, that is just naive.
It would be far better to use a self-signed certificate that can be checked
through some instance/host set inside your domain. Because only then the only
one being responsible and trustworthy is yourself. And that is the way it
should be.
Everything else involving third party business is just bogus.

-- 
Regards,
Stephan