Re: [DNSOP] Whiskey Tango Foxtrot on key lengths...

Stephane Bortzmeyer <bortzmeyer@nic.fr> Wed, 02 April 2014 13:48 UTC

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Date: Wed, 2 Apr 2014 15:47:24 +0200
From: Stephane Bortzmeyer <bortzmeyer@nic.fr>
To: Olafur Gudmundsson <ogud@ogud.com>
Message-ID: <20140402134724.GA12506@nic.fr>
References: <0EA28BE8-E872-46BA-85FD-7333A1E13172@icsi.berkeley.edu> <53345C77.8040603@uni-due.de> <B7893984-2FAD-472D-9A4E-766A5C212132@pch.net> <102C13BE-E45E-437A-A592-FA373FF5C8F0@ogud.com> <474B0834-C16B-4843-AA0A-FC2A2085FEFB@icsi.berkeley.edu> <CFA0ED6F-6800-4638-90B0-CD414301C501@ogud.com>
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Subject: Re: [DNSOP] Whiskey Tango Foxtrot on key lengths...
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On Tue, Apr 01, 2014 at 10:37:54PM -0400,
 Olafur Gudmundsson <ogud@ogud.com> wrote 
 a message of 158 lines which said:

> Furthermore using larger keys than your parents is non-sensical as
> that moves the cheap point of key cracking attack.

Mostly true, but still too strong a statement, in my opinion. This is
because, if you are an attacker and managed to crack a key somewhere
between the root (inclusive) and your real target, the higher you are
in the tree, the more things you have to emulate or simulate below. If
you are after ogud.com, and you cracked the root key, you need to
either create a false DS for .com (and then the resolver will croak on
most .com responses, detecting there is something wrong) or a false
NSEC proving that .com is not signed (but the fact that .com is signed
is rapidly cached in validating resolvers).

So, yes, basically, you are right, since DNSSEC is tree-based, the
security of the weakest node is what matters. But, in practice,
exploiting a cracked key upper in the tree is a bit more difficult
than it seems.