[gnutls-devel] [PATCH V3] Check for all error conditions when verifying a certificate

[Armin Burgmeier]

Hi Nikos,

On Wed, 2014-09-17 at 13:12 +0200, Nikos Mavrogiannopoulos wrote:
> On Tue, Sep 16, 2014 at 8:02 PM, Armin Burgmeier <armin at arbur.net> wrote:
> > This allows to check for all possible flaws with a certificate chain with a
> > single call to gnutls_x509_crt_list_verify and friends.
> 
> I've noticed that this causes some discrepancies between
> pkcs11-chainverify and chainverify in the verification. That is not
> much of your concern of course as it is about the PKCS#11 module
> verification and I'll handle it,

Okay.

>  but it uncovered the following issue:
> The verification of a chain works like that:
> CA -> ICA1 -> ICA2 -> ENDCERT
> 
> To prevent a DoS (e.g. by a server or client that sends a list of 1000
> certificates) it starts like (see _gnutls_verify_crt_status()):
> 1. verify: CA -> CA1
> if (fail return status)
> 
> 2. verify: CA1 -> CA2
> if (fail return status)
> 
> 3. verify: CA2 -> ENDCERT
> if (fail return status)
> 
> So with your patch you'll get the status up to the point of first
> failure. If the failure is in step 1 you'll get the full status for
> CA->CA1 verification, but no flag will apply on ENDCERT. In your case
> I think you verify against the scenario: CA -> ENDCERT, so you get
> some reasonable flags. I don't know how reasonable these would be if
> you are in a multiple CA scenario. Still it may make sense to do that
> (in that case I should document that correctly), and I'm not sure
> whether getting the flags of the 3 steps combined would offer much of
> an advantage as they refer to multiple certificates. What do you think
> of that? Is the current situation reasonable for your use case?

Yes, I think it is reasonable. As you say in the scenario with
intermediate CAs, the verification flags would be for multiple
verifications combined, and therefore still lack some information. I
think it is fine if it is documented that:

a) the verification procedure stops if a failure has been found with one
certificate in the chain.
b) in that case all issues with that particular certificate are
reported.
c) the verification order starts from CA->CA1, then CA1->CA2, ..., then
CAn -> ENDCERT
d) if people need to know more details, they should run the verification
for each certificate in the chain individually.

Does it sound good to you?

Cheers,
Armin