Re: Key usage. No, wait, *extended* key usage

Blake,

Thank you for being conscientious about asking the group about the use of
these extensions.  

In an earlier message, I recommended that the S/MIME v3 Cert Handling I-D,
Sec 4.2, 1rst para should be modified as follows:  Please add "as per
[KEYM]" to "Certificate, CRL, and chain validation MUST be performed...".
If this comment is incorporated, then I believe that the S/MIME Cert I-D
doesn't need to discuss the vast majority of the keyUsage checks that you
list in your message because it will refer to the PKIX X.509 Cert and CRL
Profile which provides sufficient info for the implementor to determine
which keyUsage bits need to be checked in most of the situations that you
describe in your message.  

The only keyUsage-related text that I believe is appropriate for inclusion
in the S/MIME Cert Spec is:  "Prior to verifying the signature of an S/MIME
message, if the keyUsage extension is present in the signer's certificate
and is indicated as being critical, then the verifying software MUST ensure
that the nonRepudiation bit is set to 1."

I agree with your suggestion to add wording like:  "Prior to using the
public key included in a certificate to support S/MIME security services, if
the extendedKeyUsage extension is present in the certificate and is
indicated as being critical, then the S/MIME software MUST ensure that the
id-kp-emailProtection OID is present.  This check is only required for the
end-entity certificates."

- John Pawling