Re: [TLS] New Cached info draft

"Brian Smith" <brian@briansmith.org> Tue, 30 March 2010 20:24 UTC

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From: "Brian Smith" <brian@briansmith.org>
To: <mrex@sap.com>, <tls@ietf.org>
References: <002201cad028$d20eb950$762c2bf0$@briansmith.org> from "Brian Smith" at Mar 30, 10 11:48:26 am <201003301908.o2UJ8n4K013702@fs4113.wdf.sap.corp>
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Date: Tue, 30 Mar 2010 15:24:10 -0500
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Subject: Re: [TLS] New Cached info draft
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Martin Rex wrote:
> 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).

My concern is that a hash of the client's cached value compared against a
hash of the value the server would otherwise send would not result in
optimal caching, because the server will often have an OCSP response that is
different bitwise, but which is equivalent for verifying that the
certificate was valid at the time of the handshake. Ideally, most
applications in that situation you would rather not have the server return
the newer OCSP response, because the client's older OCSP response is good
enough at the time of the request, and by the time that old response
expires, the server will probably have an even newer response. 

Instead, it would be better to have an extension that said "Don't send me a
CertificateStatus unless there was a cache miss for the certificate and/or
the certificate status has changed from X," where X is the client's cached
value of the certificate status (usually "good"). Practically, that would be
the same as " Don't send me a CertificateStatus unless there was a cache
miss for the certificate" because a good server wouldn't carry on with the
handshake if it knows it has a bad certificate, and a malicious server
wouldn't bother telling you that its certificate has been revoked.

- Brian