Re: [TLS] One approach to rollback protection

Dan Winship <dan.winship@gmail.com> Tue, 27 September 2011 14:48 UTC

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Date: Tue, 27 Sep 2011 10:51:13 -0400
From: Dan Winship <dan.winship@gmail.com>
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Subject: Re: [TLS] One approach to rollback protection
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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.

-- Dan