Re: [TLS] One approach to rollback protection

Nikos Mavrogiannopoulos <nmav@gnutls.org> Tue, 27 September 2011 15:10 UTC

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Date: Tue, 27 Sep 2011 17:13:34 +0200
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Subject: Re: [TLS] One approach to rollback protection
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On 09/27/2011 04:51 PM, Dan Winship wrote:
> On 09/26/2011 07:44 PM, Eric Rescorla wrote:
>> http://svn.resiprocate.org/rep/ietf-drafts/ekr/draft-rescorla-tls-version-cs.txt
>
>>     The natural defense against this attack is to have a "maximum
>>     version" indicator which the client can safely send even to downrev
>>     servers but which upgraded servers can check and will thus allow
>>     downgrade detection.
>
> You don't even need to indicate maximum version. The client just needs
> to indicate "I've downgraded". Clients will never downgrade against a
> server that implements version negotiation correctly, so if a compliant
> server sees that flag, it knows there's an attack in progress.

The problem is that many servers support TLS 1.0 but they are so buggy 
(e.g. fail when seeing extensions or fail if padding in record packets 
exceeds the cipher block size) that clients downgrade in order to talk 
to them. In that case the server would see that as an attack. Adding 
such a protection could add more compatibility issues.

I think the goal should be to eliminate the need for downgrade and use 
the protocol negotiation of TLS.

regards,
Nikos