Subject: NXT - authenticated non-existence - design questions

From: Reza Curtmola <[email-masked]>
Date: Tue, 3 Aug 2004 09:59:01 -0400 (EDT)

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I have posted first on the bind-users mailing lists, but was advised to
ask it in namedroppers, since this is where the experts in the area hang
out, so here it goes:

I am wondering about the design considerations for choosing the NXT record
to provide authenticated non-existence, as part of the DNSSEC security
extensions (I know, NXT has been changed to NSEC). There are several
reasons why this was preferred versus just returning a signed negative
answer. Some of these reasons include:

1) too expensive to generate a public-key signature on the spot (also the
signing key would have to be online)
2) possibility of a replay attack: if an attacker asks for www.domain.com,
and gets back a signed negative answer (that www.domain.com does not
exist), then the attacker could later use this signed answer, as long as
the signature hasn't expired yet, even if the name www.domain.com has been
added meanwhile.

I have two questions:

1) Isn't it possible to use the same replay attack also when a NXT record
is used? The attacker can return an older NXT record, with a corresponding
SIG record which is still valid. This old NXT record might not reflect the
new configuration, where www.domain.com had been added meanwhile.

2) what are other reasons why the NXT record was chosen versus simply
returning a signed negative answer?

Thank you,
-Reza

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