Subject: Re: part1-11

From: "Housley, Russ" <[email-masked]>
Date: Thu, 01 Nov 2001 14:22:22 -0500

Peter:

First, I will provide some background, then I will respond to the issue 
that you raise.

The extended key usage extension was invented to meet Internet community 
requirements.  Then, X.509 included the extension in the next edition.

RFC 2459 and Son-Of-2459 contain the following text in the discussion of 
extended key usage:

    If the extension is flagged non-critical, then it indicates the
    intended purpose or purposes of the key, and MAY be used in finding
    the correct key/certificate of an entity that has multiple
    keys/certificates.  It is an advisory field and does not imply that
    usage of the key is restricted by the certification authority to the
    purpose indicated.  Certificate using applications MAY nevertheless
    require that a particular purpose be indicated in order for the
    certificate to be acceptable to that application.

In your earlier message, you said:

    ... it seems to me that there has been a change in X509. See defect
    record 244 and also 275.  It is essentially: When you support an
    extension, You don't treat is differently depending on whether the
    critical bit is set.

X.509 4th Edition Draft V8 includes the following text in the discussion of 
extended key usage:

    This extension may, at the option of the certificate issuer, be either
    critical or non-critical.

    If the extension is flagged critical, then the certificate shall be used
    only for one of the purposes indicated.

    If the extension is flagged non-critical, then it indicates the intended
    purpose or purposes of the key, and may be used in finding the
    correct key/certificate of an entity that has multiple 
keys/certificates. If
    this extension is present, and the certificate-using system recognizes
    and processes the extendedKeyUsage extension type, then the
    certificate-using system shall ensure that the certificate shall be used
    only for one of the purposes indicated.  (Using applications may
    nevertheless require that a particular purpose be indicated in order
    for the certificate to be acceptable to that application.)

    If a certificate contains both a critical key usage field and a critical
    extended key usage field, then both fields shall be processed
    independently and the certificate shall only be used for a purpose
    consistent with both fields. If there is no purpose consistent with both
    fields, then the certificate shall not be used for any purpose.

It is pretty clear that this is an area where are out of alignment.  I 
spoke with Tim Polk about this situation today.  We think that the revised 
extended key usage extension no longer meets the original Internet 
community requirements.  In particular, we are concerned that the inclusion 
of a non-critical extended key usage extension in a certificate could 
prevent its use in two otherwise compatible applications.  This would occur 
if one of those applications required the presence of a particular extended 
key usage OID and the other application does not define any extended key 
usage OIDs (such as S/MIME).

With all of this background, Tim and I do not think the Son-Of-2459 
extended key usage text should be changed.

Russ