Subject: Re: [pkix] Name constraints on domain name in common name

From: Jean-Marc Desperrier <[email-masked]>
Date: Thu, 01 Apr 2010 10:08:08 +0200

Stefan Santesson wrote:
> It IS actually common practice to apply dNSName constraints on CN when the
> certificate has server auth EKU set. This practice is implemented by the
> Microsoft path validation logic in MSCAPI. It is considered the best
> security practice considering how server certificates are issued in
> practice.

I suppose that remark is with your implementer hat well fixed on your head.

I'm worried a naive implementation of this can entrench even more the 
habit of including the DNS name in the CN.

I think a smart implementation of this is to take into account the CN as 
a possible location for a DNS name *only* when it respect the dNSName 
constraints and *not reject* a certificate where the CN does not match 
the constraint.

I would sound nice to make this more generic by applying the same logic 
to all the names that are found in the certificate, ignoring the values 
that do not conform, but taking into account the ones that do, but I'm 
afraid the correct interpretation of the RFC requires to fully reject a 
certificate that does not respect a name constraints, and not take into 
account only the values that do conform. So this needs to be special cased.

---

From: Stefan Santesson <[email-masked]>
Date: Thu, 01 Apr 2010 13:26:16 +0200

> I suppose that remark is with your implementer hat well fixed on your head.

No, I'm just the messenger. I have worked for the CAPI team for several
years though but that is no longer so.

> habit of including the DNS name in the CN.

You are absolutely right about that. That is a big problem. If the CA would
enter something else in the CN, they risk the certificate being rejected.

I don't remember if the path processing made any attempt to verify whether
the CN actually contains a legal dns name or not (before applying name
constraints), but I would guess not.

> account only the values that do conform. So this needs to be special cased.

If a CA certificate contains name constraints, then no certificate can be
valid under that CA certificate unless it meets the name constraints.

Unconstrained name forms can still be present but constrained name forms
must match.

/Stefan

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