[cabfpub] Name Constraints, Auditing and EKU

Ryan Hurst ryan.hurst at globalsign.com
Tue Apr 23 21:28:38 UTC 2013


You are right, I didn't consider that case <blush>.

I am verifying if there is special handling for this case now; will update
the thread once I confirm.


-----Original Message-----
From: Rob Stradling [mailto:rob.stradling at comodo.com] 
Sent: Tuesday, April 23, 2013 2:13 PM
To: Ryan Hurst
Cc: Erwann Abalea; public at cabforum.org
Subject: Re: [cabfpub] Name Constraints, Auditing and EKU


If a Root CA issues a Name-Constrained Subordinate CA Certificate that
contains the OCSP Signing EKU, then surely that Subordinate CA Certificate
can also function as a "2. CA delegated" OCSP Response Signer _for the Root

Or are you perhaps implying that Windows will spot basicConstraints:CA=TRUE
and/or keyUsage:keyCertSign in the Subordinate CA Certificate and therefore
refuse to let the cert be used as a Delegated OCSP Response Signer cert?

On 23/04/13 21:50, Ryan Hurst wrote:
> I am just catching up on my CAB Forum mail, I see this conversation 
> regarding the OCSP signing EKU and it seems there may be a miss 
> understanding.
> There are four trust models for OCSP:
> 1. CA Signed; the same key/certificate that signed the certificate was 
> used to sign the OCSP status.
> 2. CA delegated; the same key/certificate that signed the certificate 
> was used to sign a certificate that contains the OCSP Signing EKU that 
> was used to sign a OCSP response.
> 3. VA Signed; the relying party has been configured out of band to 
> trust this specific key/certificate to sign OCSP responses for a given 
> set (usually all) of certificates.
> 4. VA delegated; the relying party has been configured out of band to 
> trust a specific key/certificate to sign OCSP responses for a given 
> set of certificates, that entity has signed a certificate/key that 
> contains the OCSP Signing EKU.
> It is only possible to trust do 1 & 2 automatically in that they are 
> in essence leverage the policy expressed in the certificate and not 
> out of band configuration. In other words even if clients support 2 & 
> 3 (which most don't in my testing) the only thing the browsers do is #1
and #2.
> Additionally notice the logic is gated by the signing key; even in CA 
> delegated the delegated responder can not sign for any other CA in the 
> hierarchy -- only those within its scope.
> I am confident Windows behaves this way, I am also confident what was 
> once the ValiCert  (now Axway) client behaved this way.
> As such I don't see a need to exclude name constrained CAs from using
> Ryan
> -----Original Message-----
>> 12. Appendix B(2)G:
>>        "id-kp-OCSPSigning MUST be present"
>> Disagree.  The OCSP Signing trust purpose is not supposed to be 
>> passed down from the Root, and AFAIK there is no way to prevent a 
>> Subordinate CA from issuing delegated OCSP Signing Certificates!  (If 
>> you have evidence to the contrary, please say).
> I think the requirement is really a "id-kp-OCSPSigning MUST NOT be
> If CA A issues a certificate to CA B with id-kp-OCSPSigning in the 
> EKU, then CA B has now a valid OCSP responder for certificates issued 
> by CA A; which is certainly NOT something wanted by CA A.
> There are limits to using an extension for something it wasn't 
> designed for... I'm not a fan of "EKU constraints".
>> 13. Appendix B(2)G:
>>        "** Extended Key Usage within Subordinate CAs is an exception to
>>         RFC 5280 that MAY be used to further protect relying parties
>>         the Name Constraints extension is supported by Application
>>         Software Suppliers whose software is used by a substantial
>>         of Relying Parties worldwide."
>> I agree that we should note that this is "an exception to RFC 5280".
>> However, I don't think Mozilla have any plans to stop requiring EKU 
>> once Name Constraints are supported more widely, since EKU and Name 
>> Constraints achieve different things.

Rob Stradling
Senior Research & Development Scientist
COMODO - Creating Trust Online
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