is a self signed certificate always invalid the first time?

[Frank-Ulrich Sommer]

I can't see any security advantages of a self signed cert. If the keypair is generated locally (which it should) a certificate signed by an external CA can't be worse just by the additional signature of the external CA.

Better security can only be gained if all users are urged to remove all preinstalled trusted CAs from their mail clients (which seems impractical). Else an attacker could still use a fake cert signed by one of those CAs. Public key pinning could be an (academic) alternative and would still work with a cert signed by an external CA without restrictions.

If someone tells me to add security exceptions this rings all alarm bells. Users who are not experts should not get used to doing this as they soon will accept everything.

Am 10. August 2017 21:40:25 MESZ schrieb Doug Hardie <bc979 at lafn.org>:
>
>
>> On 10 August 2017, at 04:37, Alef Veld <alefveld at outlook.com> wrote:
>> 
>> I completely agree (having said that I'm pretty new to all this so I
>might be full of it). 
>> 
>> You should run your own CA if you have an active financial interest
>in your company (say your the owner). No added benefit to have your
>certificate certified by a third party, why would they care about that
>one client). Ofcourse people would say "but ofcourse you would verify
>your own certificate" but in that case they probably don't understand
>how it all works.
>> 
>> Ofcourse once your own company grows large you run the same risk of
>entropy (incorrect documentation or records, no trained staff, no up to
>date procedures etc.) large companies have to deal with. Maybe if you
>had one person working full time on it, or an automated process
>handling things it would be more secure and reliable.
>> 
>> Was diginotar the Dutch company, I think I remember that one.
>> 
>> Sent from my iPhone
>> 
>>> On 10 Aug 2017, at 08:18, Stephan von Krawczynski <skraw at ithnet.com>
>wrote:
>>> 
>>> 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
>>> 
>
>
>If you use a self-signed certificate, your users either have to accept
>the certificate when requested, or install your root certificate. 
>Installing the root certificate is not easy to explain to non-tech
>users even with step-by-step instructions with screen shots attached. 
>I have gone this approach ever since the RSA patents expired and it can
>be a pain at times.  Users just don't understand the obnoxious warning
>(panic) messages the browsers put out that are intended to keep them
>from accepting self-signed certificates.  The browser developers don't
>understand the certificate trust issues either.  Several Microsoft
>versions did not provide a way to accept the certificates.  Those users
>were forced to install your root certificate.  However, as stated
>before, if you are only certifying your own certificates, then that is
>the most appropriate approach.
>
>-- Doug

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