Subject: interpretation of extended key usage

From: "Heyden, Klaus" <[email-masked]>
Date: Wed, 24 Jul 2002 13:12:22 +0200

Hello,

i have a question about the meaning of extended key usage extensions in a
certificate, specialy in a root certificate.

How is the understanding of extended Key Usages in Root or CA certificates.
Specialy under the circumstance of certificate path validation. Therefor i
think two opinions are possible:

(1) the extended key usage is only usable for the use of the public key
itself and his direct usage i.e. signing of certificates etc. (view RFC 2469
4.2.1.13 first sentence). So a root certificate dont need an extended key
usage, because the public key will only be needed for path validation and
signing of certificates and CRLs.

(2) the public key is also needed for path validation, so an extended key
usage can be used to restrict the use of sub and end entity certificates. 

I have looked a bit around and found some CA certificates with extended key
usages, so its a bit confusing. Both way's are imagineable.  

Best Regards 
Klaus Heyden 

Dresdner Bank AG 
D-60301 Frankfurt/Main 
[email-masked] 
+49-(0)69-263-11126 
+49-(0)69-263-15015

---

From: "Housley, Russ" <[email-masked]>
Date: Wed, 24 Jul 2002 10:10:53 -0400

I think that RFC 3280 is pretty clear.  It says:

    If a certificate contains both a key usage extension and an extended
    key usage extension, then both extensions MUST be processed
    independently and the certificate MUST only be used for a purpose
    consistent with both extensions.  If there is no purpose consistent
    with both extensions, then the certificate MUST NOT be used for any
    purpose.

I would not expect a root certificate to contain an extended key usage 
extension.

Russ

---

From: [email-masked] (Peter Gutmann)
Date: Thu, 25 Jul 2002 16:54:26 +1200 (NZST)

"Heyden, Klaus" <[email-masked]> writes:

>(1) the extended key usage is only usable for the use of the public key itself
>and his direct usage i.e. signing of certificates etc. (view RFC 2469 4.2.1.13
>first sentence). So a root certificate dont need an extended key usage, because
>the public key will only be needed for path validation and signing of
>certificates and CRLs.
>

Both are used (see e.g. the X.509 style guide for comments).  If you need to
use one or the other, just find the appropriate standard which specifies the
one you want.  Alternatively, if you have to comply with a certain standard,
make sure that the software you're using does actually apply the same
interpretation as the standard.

Peter.

---

From: "Tom Gindin" <[email-masked]>
Date: Thu, 25 Jul 2002 06:50:50 -0400

You're right, there is no key purpose for CMP.  My question is
whether this is a good thing or not, especially if an intermediate CA wants
to publish a certificate for CMP use.  How does a remote client recognize
such a certificate?  Remember, this issue applies to all CA's, not just
root ones.

            Tom Gindin

"Housley, Russ" <[email-masked]> on 07/24/2002 04:25:49 PM

To:    Tom Gindin/Watson/IBM@IBMUS
cc:    [email-masked]
Subject:    Re: interpretation of extended key usage

Tom:

I am unaware of any extended key usage OIDs for CMP.

It is quite reasonable for a CA certificate to indicate that the key can be

used to validate OCSP or SCVP responses (if the CA handles these with the
same signing key).

I tend to think of a root CA as being primarily off-line.  This
architecture provides for physical protection of the root CA's signing
key.  I guess this bias spilled into my response.

Russ

At 03:16 PM 7/24/2002 -0400, Tom Gindin wrote:

>       Russ:
>
>       Would it be reasonable for a CA certificate to contain extended key
>usage values for CMP usage (or perhaps OCSP)?  It would be better practice
>for the CA to issue a certificate to its own DN with such values but no
>certificate signing ability, of course.  It is clear that the extended key
>usage extension in a CA certificate is not an analogue to name or policy
>constraints.
>
>             Tom Gindin
>
>
>"Housley, Russ" <[email-masked]>@mail.imc.org on 07/24/2002
>10:10:53 AM
>
>Sent by:    [email-masked]
>
>
>To:    "Heyden, Klaus" <[email-masked]>
>cc:    [email-masked]
>Subject:    Re: interpretation of  extended key usage
>
>
>