Subject: [MSEC] draft-ietf-msec-ipsec-tesla-00: regarding anti-replay protection

From: Adrian-Ken Rüegsegger <[email-masked]>
Date: Mon, 24 Sep 2007 17:45:56 +0200

Hello all,

I have a question concerning draft-ietf-msec-ipsec-tesla-00:

In the document it is stated that TESLA Authentication is protected from
DoS attacks by non-group members through the usage of an external
authentication transform using a symmetric-key MAC.

It is my understanding that this is not entirely true since it leaves
the system vulnerable to replay-attacks to some extent (even from
outsiders). All operations that have to be performed before evaluating
the TESLA MAC and checking the sequence number, will have to be done
even for replayed packets. This would also include the steps incured by
the external authentication transform. Could this lead to a *true* DoS
attack or is the computational overhead negligible?

For smaller groups it is possible to use the IPsec anti-replay
protection (ESP sequence numbers) and keep track of each senders state
but as noted in other documents this does not scale to larger groups.
draft-ietf-msec-ipsec-extensions-06, Appendix 3 says that IPsec
anti-replay protection should be disabled in large-scale any-sender
multicast scenarios.

Can TESLA help mitigate the replay-attack problem in situations where
ESP sequence numbering is not feasible and if so in what way?

Regards,
Adrian

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