[cabfpub] Technically Constrained SubCAs

Dimitris Zacharopoulos jimmy at it.auth.gr
Thu May 12 01:04:11 MST 2016


On 12/5/2016 10:59 πμ, Rob Stradling wrote:
> On 12/05/16 05:38, Dimitris Zacharopoulos wrote:
>> On 11/5/2016 4:30 μμ, Rob Stradling wrote:
>>>>
>>>> So, let's assume that there is an Intermediate CA with EKU
>>>> id=kp-serverAuth + NameConstraints extension (all non-critical),
>>>> and an
>>>> Intermediate CA with no EKU but with NameConstraints extension. Is
>>>> there
>>>> a difference regarding the technical constraints? Browsers and
>>>> implementations that follow RFC5280 will treat them both exactly the
>>>> same way.
>>>
>>> Unfortunately, the "Browsers and implementations that follow RFC5280"
>>> don't have 100% market share, hence why the BRs permit non-critical
>>> Name Constraints.
>>>
>>> Also, most browsers don't "follow RFC5280" when it comes to processing
>>> a CA certificate that contains the EKU extension!
>>>
>>>> Implementations that completely ignore the Name Constraints
>>>> extension will also treat them exactly the same way.
>>>
>>> That's not true.  Implementations that process Name Constraints will
>>> reject cert chains that don't fit within those constraints, whereas
>>> implementations that ignore Name Constraints won't do that.
>>
>> Perhaps I did not describe it clearly enough. Implementations that do
>> not handle name constraints, will treat a subCA with or without the
>> "kp-server-auth" EKU, in exactly the same way and probably allow the
>> verification of the SSL certificate. This is not what we discuss here
>> though. If an implementation ignores the nameConstraints, there is
>> nothing the CA/B Forum or the BRs can do about it.
>>
>> I believe the nameConstraints extension alone, with the three limits
>> described in 7.1.5 of the BRs is sufficient for an intermediate CA to be
>> considered "technically constrained" for SSL certificates. An exception
>> to this rule would be to _include_ an EKU that _does not_ contain
>> "kp-server-auth" or "anyEKU". In the latter case, you don't need a
>> nameConstraints extension for dNSName, iPAddress and DirectoryName
>> because the EKU is the blocking factor.
>
> Let's look at an actual example:
> https://crt.sh/?id=12967730
>
> Was it your intent for this CA to be capable of issuing
> id-kp-codeSigning end-entity certificates that Windows would trust?
>
> Assuming it wasn't your intent...
> What constraint prevents this CA from issuing id-kp-codeSigning
> end-entity certificates that Windows would trust?
>

This intermediate CA is technically constrained for SSL certificates,
not code Signing. Section 7.1.5 tries to technically constrain
intermediate CAs in issuing SSL certificates.

Dimitris.




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