Subject: Re: [saag] PKIX - TLS Feature Extension

From: Nikos Mavrogiannopoulos <[email-masked]>
Date: Fri, 03 Jun 2016 05:41:22 +0200

I am not sure I understand your reply. My understanding of this draft
is that it will cause future compatibility issues because it assumes
that a TLS "feature" is equivalent to a TLS extension. The TLS
extensions "Multiple Certificate Status Request Extension" (17) and
"Certificate Status Request" (5) are a prominent example. They are the
same feature (a method to provide OCSP staples) but utilize different
TLS extensions.

If any of the intermediate certificates in a chain is restricted the
way the draft describes (with tls-feature=5), then the deployment of
the multiple certificate status request extension becomes impossible,
even though the intention the original restriction was to require
servers to provide OCSP staples. 

On Tue, 2016-05-31 at 13:15 -0400, [email-masked] wrote:
> Yes, that is exactly what is done and the reason that the spec is
> written the way it is.
> 
> > 
> > On May 30, 2016, at 6:26 AM, Nikos Mavrogiannopoulos <n.mavrogianno
> > [email-masked]> wrote:
> > 
> > Hi,
> > RFC7633 describes a PKIX certificate extension to indicate
> > mandatory
> > to be present TLS extensions (called feature extensions in the
> > text).
> > In particular this draft while generic, is mainly about requiring
> > an
> > OCSP staple from the connecting server if the certificate mandates
> > it.
> > 
> > However, the rfc text doesn't go into details on how to do that. My
> > understanding and as far as I've seen in practice certificates will
> > include a "Feature" extension containing integer 5 which is the
> > OCSP
> > status request.
> > 
> > If the reading above is correct, how does this scale with
> > status_request_v2 (which has TLS extension ID 17)? How could the
> > certificate be used to indicate that the server can provide any of
> > these TLS extensions? Should both the numbers 5,17 be listed as TLS
> > features (which can also mean that both TLS extensions must be
> > used),
> > or having 5 listed is sufficient?
> > 
> > regards,
> > Nikos

---

From: "[email-masked]" <[email-masked]>
Date: Fri, 3 Jun 2016 08:06:51 -0400

No, "Multiple Certificate Status Request Extension" (17) and "Certificate Status Request" (5) are different features implemented differently on the client and server. There are many servers that support one but not the other. Hence the need to advertise them differently.

If a certificate specifies feature 5, that does not prohibit the server offering 17. But a certificate that specifies 17 cannot be used by a server that does not support 5.

Replacement of a certificate is usually straightforward. There may be a fee for the very cheapest but but most CAs seem to replace expensive ones at no cost.

> On Jun 2, 2016, at 11:41 PM, Nikos Mavrogiannopoulos <[email-masked]> wrote:
>