Re: [TLS] New Cached info draft

Martin Rex <mrex@sap.com> Tue, 30 March 2010 19:08 UTC

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From: Martin Rex <mrex@sap.com>
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To: brian@briansmith.org (Brian Smith)
Date: Tue, 30 Mar 2010 21:08:49 +0200 (MEST)
In-Reply-To: <002201cad028$d20eb950$762c2bf0$@briansmith.org> from "Brian Smith" at Mar 30, 10 11:48:26 am
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Cc: tls@ietf.org
Subject: Re: [TLS] New Cached info draft
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Brian Smith wrote:
> 
> Adam Langley wrote:
> > One last point, I think I'd like to see the ability to cache the
> CertificateStatus:
> 
> Optimally, the client only really wants a certificate status update when a
> previously-cached one has expired and/or the server's certificate has
> changed. Server certificates very rarely change. Consequently, the client
> should be able to get by with sending the certificate status request
> extension only when it doesn't have a non-expired certificate status for
> that server, right? AFAICT, this would work equally well for servers that do
> and don't support the cached-info extension.
> 
> Please let me know if I am overlooking something.

The cached info uses a fail-safe approach in that the real data will
be sent by the server when the information by the client does not
match (based on the server comparing the hash values supplied by the
client).

You can not do that with the TLS extension for OCSP.  The client only
knows what certificate the server from the reply to the ClientHello
handshake message.  IMHO, the robust approach for a TLS handshake
that has the highest likelihood to succeed, requires the use of
the TLS extension for OCSP plus the caching extension for
the actual extension contents (the OCSP responses).

I would not want to go through a handshake failure when the
OCSP responses that the client has cached are still valid, but
the server has switched to a different server certificate
since the last connect.


-Martin