Subject: [Ipsec] More of ESN

From: Markku Savela <[email-masked]>
Date: Tue, 22 Feb 2005 10:39:57 +0200

One attack does occur to me...

*IF* an attacker can create a packet with sequence number that is
below the window and which passes the authentication check with the
next higher sequence space, then attacker has practically shut down
the connection (because all subsequent real packets will have
"incorrect" high ESN part).

Because high ESN is mostly 0, all that is needed is to be able to
generate a single bit change that passes the authentication algorithm.

I'm no expert on hashes and HMAC, so I have no idea how easy or
difficult it would be.

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From: Bill Sommerfeld <[email-masked]>
Date: Wed, 23 Feb 2005 21:03:10 -0500

> generate a single bit change that passes the authentication algorithm.

if you have that sort of attack capability, you could most likely be
able to tweak other bits within the packet payload, which if anything
is a far more serious attack.  the correct response to a real attack
of that form would be to retire the auth algorithm...

						- Bill

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From: Stephen Kent <[email-masked]>
Date: Wed, 23 Feb 2005 21:20:54 -0500

>I'm no expert on hashes and HMAC, so I have no idea how easy or

HMAC should detect such changes very reliably. It is very sensitive 
to a one-bit change anywhere in the covered data.

Steve