Using root CAs as a trusted 3rd party

[Gregor Zattler]

Hi Aaron, gnupg users,
* Aaron Toponce <aaron.toponce at gmail.com> [21. Jan. 2012]:
> On Sat, Jan 21, 2012 at 10:50:11PM +0100, Gregor Zattler wrote:
>> IMHO by signing a key you make a statement about the connection
>> between a person or owner and the user id you sign, saying "I
>> somehow convinced myself that user owns this key".  This only
>> makes sense if you have some insight into the matter that a
>> person which is confronted with the key only cannot have.  Your
>> signature should add some information.  Merely saying I'm
>> convinced that the user is the owner/originator of the key
>> because someone else already signed this key, does not make much
>> sense to me.  I think you should have added a notation explaining
>> you reasoning.
> 
> I trust the encrypted connection between my browser and my bank, because
> the certificate they present to by browser is signed by a root CA that is
> installed in the browser. 

I do the same since my bank refuses unwaveringly to send me their
certificate by snail mail.  Yes I actually asked them to send me
their certificate but they explicitly refused to do so and told
me I am free to quit my account.  In this dispute I learned I'm
the only customer ever to ask for their certificate.

> It seems possible to make a valid corollary with
> OpenPGP keys. I trust a key belongs to a specific user, because that key is
> presented to be to be owned by a specific person is signed by a root CA.
> 
> Esentially, I'm using a CA as a 3rd party to casually establish identity.
> At this point, I can rest assured that the key this person claims is theirs
> is actually theirs.

Sure.  Nothing wrong with that.  You look at the key, see it's
signed by the CA, you check the signature and decide 
*for yourself* that this is proof enough, that this is the users
key.  You take the risk.

But don't use this as an argument to sign the key because then
you are making a public statement instead of a private
reasoning:


Next time I use the very same key: I see the signature of the
CA.  Now there are two possibilities:

a) I trust the CA.  Then I check their signature, see it's good
   and I'm convinced it't the valid key of the user.
   What does your signature help me in this instance?


b) I do not trust the CA.  Therefore I don't even bother to check
   their signature.  So I can't trust the validity of the key.
   But stop: There is a signature of Aaron Toponce.  For the sake
   of the argument, let's assume we met at a key signing party,
   signed our respective keys and had a nice talk then.  Now I
   see the users key is signed with a fully trusted key (yours)
   and therefore I might consider it valid -- but only because
   you trust a CA I don't trust.

   In my opinion that's the wrong outcome.


Please sign keys only because of your own judgement on some facts
not present with the key alone, not others (the CA).

Ciao, Gregor
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