Re: [TLS] An SCSV to stop TLS fallback.

mrex@sap.com (Martin Rex) Sat, 07 December 2013 04:38 UTC

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To: Adam Langley <agl@google.com>
Date: Sat, 7 Dec 2013 05:38:43 +0100 (CET)
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From: mrex@sap.com (Martin Rex)
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Subject: Re: [TLS] An SCSV to stop TLS fallback.
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Adam Langley wrote:
> Martin Rex <mrex@sap.com> wrote:
>>
>> For this feature to work at all, the prerequisite is that both communication
>> peers must be updated.  When looking at a protocol feature that requires
>> both communication peers to be updated as a prerequisite, wouldn't it be
>> preferable to fix those protocol features of SSL/TLS that we deem necessary
>> for reliable security to become negotiable even in the "most conservative"
>> ClientHello that clients are willing to fallback to?
> 
> Fallback behaviour is needed for wide variety of buggy servers,
> including those that are intolerant to unknown extensions.
> 
> I don't believe that there's a ClientHello format that is both devoid
> of compat issues and sufficiently expressive to be able to negotiate
> what we need now and in the future.


If you *NEED* a certain feature, then there would not be a point in
a reconnect fallback to a ClientHello that doesn't have that feature.
So you would not need an SCSV to cover this, methinks.

-Martin