Re: SSL server certificate that has an intermediary certificate in the chain

[David Hawes <dhawes@vt.edu>]

On 2011-08-01 13:07, Howard Chu wrote:
> David Hawes wrote:
>> On 2011-07-30 14:14, Howard Chu wrote:
>>> Erwann ABALEA wrote:
>>>> Actual OpenLDAP configuration would be to place B (and maybe B1)
>>>> certificate in TLSCACertificateFile/Path element, to be able to verify
>>>> the Client certificate. The Server must also place A1 in this element,
>>>> so it can be sent to the Client to help it verify the Server
>>>> certificate.
>>>
>>>> So, during the TLS negotiation, B, A1, maybe B1, maybe A are sent to
>>>> the Client for it to verify the Server certificate? (remember, this
>>>> certificate is signed by A1, and A must already be known by the
>>>> Client). That's a waste of bandwidth.
>>>
>>> The TLS library doesn't do anything so stupid, it only sends the
>>> certificates that are part of the chain from the subject's cert up to
>>> the root CA.
>>
>> I think the real question here is if the CA chain that the server sends
>> and the CA chain that the server uses to verify client certificates can
>> be different.
>>
>> In Apache, this is done with the SSLCertificateChainFile and
>> SSLCACertificateFile directives.  This makes it possible to have the
>> server send a certificate chain that differs from the CAs it uses to
>> verify clients.  This is useful if you want to have a server certificate
>> signed by one CA, but only accept client certificates signed by
>> another CA.
>>
>> This sounds like an OpenLDAP feature request to me.
> 
> Think about why you would configure such a setup, and what it actually
> means. When you have a certificate of your own, signed by a particular
> CA, that obviously means that you must trust that CA. If you're going to
> accept a cert from another party that is signed by a different CA that
> obviously means that you must also trust the other CA. There is
> absolutely nothing gained from isolating these two CAs, on either side
> of the session.

What is gained is that the server can be explicit about what client
certificates it will accept.  This is useful if you want to use a
separate CA for client auth and do not want to accept certs from the CA
that signed the server's cert.

Suppose my server has a Verisign cert.  If I add the Verisign CA chain
to TLSCACertificateFile, I have just allowed all certs signed by any CA
in that chain to do TLS client auth.  If I want to allow clients signed
by my own CA and not those signed by Verisign, I'm out of luck.

Again, Apache does this by having separate directives for assembling the
certificate chain and specifying which CAs to trust for clients.

> If there were indeed anything to be gained by such a feature, it would
> also need to be implemented on clients. Look around - do any web
> browsers allow you to isolate CAs like this?

In my example above it is not necessary to do anything on the client
side.  I am strictly talking about the server and what CAs it trusts for
TLS client auth.