[cabfpub] A conflict about EKU with PKIX

[王文正]

On Mar 7, 2017 at 12:46 AM, Ryan Sleevi via Public <public at cabforum.org<mailto:public at cabforum.org>> wrote:
> For those new to the discussion, this seems to have cropped up every few years in the IETF - Here's a
> selection of messages going back over a decade on this topic.
>
> https://www.ietf.org/mail-archive/web/pkix/current/msg27104.html
> https://www.ietf.org/mail-archive/web/pkix/current/msg32409.html
> https://www.ietf.org/mail-archive/web/pkix/current/msg33507.html

These discussion threads in IETF PKIX WG clearly pointed out that "using EKUs as constraints in intermediate CA certificates" is a mistake. I hope this mistake would not be inherited into the Google's S/MIME profile.

If the purpose is to make sure only those EE certificates issued by those CAs which comply to some kinds of S/MIME certificate policies (or requirements), please consider to define an OID for S/MIME certificate policy and use Certificate Policy chaining for that purpose.

> With respect to Google's program requirements, the current operating thought is that certificates
> should be dedicated for the appropriate purpose, and that reusing the same certificate across different
> application spaces - such as document signing versus S/MIME - is not a recommended practice.

There might already be billions of eID certificates issued by governments all over the world. I believe that in most eID programs, governments not only want their citizens can use eID certificates for general-purposed e-authentication, e-signature, and encryption in e-government applications or e-commerce applications but also want to let their citizens can use eID certificates for S/MIME secure emails. If Google's S/MIME profile prohibits reusing the same certificate across the S.MIME usage and general-purposed e-government or e-commerce usages, I am afraid it will reject billions of eID certificates from being used S/MIME secure emails.

> That said, should a CA wish to apply such logic to the certificates they issue, they MUST issue from
> an intermediate that explicitly lists the purposes for which it is authorized. As Li-Chun notes, this
> means that such intermediates used to issue these end-entity certificates MUST note all the purposes
> for which the certificate is intended.
>
> The logical consequence of this is that the anyEKU generally SHOULD NOT appear in end-entity
> certificates.

In practice, it is not possible to enumerate all the purposes for which eID certificates is intended. If a government has ever defined a private EKU with its Key Purpose ID indicating that the certificates can be used in any kinds of  e-government or e-commerce applications in that jurisdiction or country,  the private EKU will be in fact similar to the anyEKU.

Best Regards,
Wen-Cheng Wang


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