[x500standard] Re: Lack of clarity in X.509
- From: David Chadwick <d.w.chadwick@xxxxxxxxxx>
- To: x500standard@xxxxxxxxxxxxx
- Date: Tue, 24 Jun 2008 12:02:40 +0100
Hi Santosh
thankyou for responding. Whilst you have found text that applies to
non-critical extensions, I dont believe there is any that applies
directly to critical ones. It is always dangerous to assume firstly
that people can correctly work out what the inferences are, and secondly
that they can work out the correct inferences, from a standard piece of
text. Clearly browser manufacturers today cannot. Explicit statements
are always preferred to implicit ones.
If this list can agree amongst itself what the correct inferences are
for unknown bits of critical extensions, then I would suggest a short
paragraph be inserted in the standard to explicitly state these. If, as
Santosh seems to be implying, there is no single correct inference, and
each extension has to explicitly state it on a case by case basis, then
the standards should insert a sentence along the following lines.
"If unknown elements appear in a recognised extension that is marked
critical, then the interpretation of the known and unknown elements is
extension specific and is documented in each extension".
regards
David
Santosh Chokhani wrote:
David,
I found the following in X.509:
"If unknown elements appear within the extension, and the extension is
not marked critical, those unknown elements shall be ignored according
to the rules of extensibility documented in 12.2.2 in ITU T Rec. X.519 |
ISO/IEC 9594-5".
I would say that this implies through corollary that if there are
elements that can not be processed in a critical extension, the
certificate must be rejected.
That said, you have to be logical about this stuff.
For example, for EKU, if your EKU is present, even if you do not
recognize others, you should use the certificate.
The standard has specific language regarding unrecognized name forms in
the SAN extension. I would say that the guidance is incomplete. If a
name form is not recognized and that name form is constrained via prior
certificates in the certification path, you must reject the certificate.
You must be able to process all the name forms asserted in the name
constraints extension.
In general, I think the rule in X.509 may be generic and specific fields
have rules except that SAN rule may be incomplete.
-----Original Message-----
From: x500standard-bounce@xxxxxxxxxxxxx
[mailto:x500standard-bounce@xxxxxxxxxxxxx] On Behalf Of David Chadwick
Sent: Monday, June 23, 2008 4:34 AM
To: x500standard@xxxxxxxxxxxxx
Subject: [x500standard] Lack of clarity in X.509
Dear All
A colleague has been testing how various browsers behave when faced with
SSL certificates with various extensions marked critical or
not-critical. He has found that the browsers behave in different ways.
He says that one of the problems is that the standard is not specific
enough on how the browsers should behave in all circumstances.
Here is one scenario to consider. An extension is marked critical and
the RP software understands the extension but only implements part of it
(say it has an OID for describing elements of it, or it is a set of
choices). Does the text in X.509 say how the RP should handle this? I
think not. Should the unknown parts of the extension be ignored or
should the entire certificate be rejected? e.g. the extension is Subject
Alt Names and some of the names are understood and some are not.
As a side issue, an RP can decide to accept any certificate, regardless
of what the CA says in the certificate (and some browsers allow the user
to do this). In this case the CA wont accept any liability and the RP is
acting outside the standard. But is this stated clearly enough in the
standard?
regards
David
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