Subject: [Ipsec] Purpose of sequence numbers

From: Ulrich Weber <[email-masked]>
Date: Thu, 04 Nov 2004 15:16:18 +0100

Hi all,

at the moment I'm writing a thesis about a IPSec takeover solution. One crux
are sequence numbers. IKEv2 plans to add sequence numbers even for IKE SA's.

But I don't get the meaning of sequence numbers in IPSec at all. What are
their purpose ?

Most of the encrypted traffic is tcp, which has its own sequence number and
is invulnerable against Ipsec replay attacks.
Secound biggest amount of traffic is udp. Ok its vulnerable against replay
attacks, but what harm could someone do with dns replay attacks, where no
data can be modified within the udp packet ?

So the only possibilty I can think of are DOS attacks to consume as much cpu
power for decrypting ipsec packets as possible.

Arent the SPI numbers sufficient enough to prevent third party attackers (who
are not able to sniff the ipsec traffic) from dos attacks ?

So any DOS attack has to come from one of the two sides and has to be able to
sniff the packets. However a local 100MBit connection should be sufficient
enough to DOS a IPSec system even with wrong seq numbers.

Best regards
~ Ulrich

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From: Michael Bowler <[email-masked]>
Date: Thu, 04 Nov 2004 09:49:49 -0500

> is invulnerable against Ipsec replay attacks.

TCP sequence numbers wrap (quiet quickly on high speed links).  By 
replaying the same packet over and over again, an attacker will likely 
hit the TCP window on the next sequence number wrap, corrupting the 
stream which is supposed to be secure.

> data can be modified within the udp packet ?

Total DOS against the UDP transmission.  Replay the same packet over and 
over again on a secure UDP video/audio channel will completely garble 
the reception.  Again, this channel is supposed to be secure.

> power for decrypting ipsec packets as possible.

Yup, another reason for early detection of replays.  The processing 
involved in replay detection is trivial compared to the 
decryption/authentication processes.

> are not able to sniff the ipsec traffic) from dos attacks ?

A replay attack assumes that the traffic can be sniffed, as bogus 
packets cannot be arbitrarly created by an attacker without the hash keys.

Cheers,

Michael

-- 
Michael Bowler                                [email-masked]
IC Design Engineer                                  (613) 254-5456x107
Elliptic Semiconductor                            www.ellipticsemi.com

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From: "Steven M. Bellovin" <[email-masked]>
Date: Thu, 04 Nov 2004 11:58:05 -0500

In message <[email-masked]>, Ulrich Weber writes:
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>Hi all,
>

See http://www.research.att.com/~smb/papers/badesp.ps (or .pdf), 
Steven M. Bellovin, "Problem Areas for the IP Security Protocols," in
Proceedings of the Sixth Usenix Unix Security Symposium, pp. 1-16,
San Jose, CA, July 1996.

		--Steve Bellovin, http://www.research.att.com/~smb