Re: [TLS] New draft: draft-solinas-tls-additional-prf-input-00.txt

Paul Hoffman <paul.hoffman@vpnc.org> Wed, 07 October 2009 00:15 UTC

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Date: Tue, 6 Oct 2009 17:16:38 -0700
To: Michael Gray <mickgray@au1.ibm.com>
From: Paul Hoffman <paul.hoffman@vpnc.org>
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Subject: Re: [TLS] New draft: draft-solinas-tls-additional-prf-input-00.txt
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At 9:35 AM +1000 10/7/09, Michael Gray wrote:
>Comparing this with draft-rescorla-tls-extended-random-02.txt I see the
>requirement for the size of Server response is missing and thus undefined.

Correct. Both sides send arbitrary-length values.

>From 3.1 in the above we have:
>
>   If the server wishes to use the extended randomness feature, it MUST
>   send its own "extended_random" extension with an
>   extended_random_value equal in length to the client's
>   extended_random_value
>
>Additionally, I think some limitation of the size/amount of data that can
>be requested from a server is needed as large sizes could pose an attack on
>a server by exhausting the entropy pool and/or causing server performance
>degradation.

In this proposal, server never "requests" any particular size. The client offers its value in the extended_random extension, and the server offers its value in the reply.

>  Our prototype implementation allows servers to ignore or
>respond with error, depending on configuration, requests from clients that
>are larger than 256 octets.

...which is a good reason why we abandoned the "request a size" semantics.

--Paul Hoffman, Director
--VPN Consortium