Re: [tcpm] WGLC: draft-ietf-tcpm-tcpsecure-10.txt

"Anantha Ramaiah (ananth)" <ananth@cisco.com> Mon, 29 September 2008 17:41 UTC

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Thread-Topic: [tcpm] WGLC: draft-ietf-tcpm-tcpsecure-10.txt
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From: "Anantha Ramaiah (ananth)" <ananth@cisco.com>
To: "Ted Faber" <faber@ISI.EDU>
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Cc: rrs@cisco.com, tcpm@ietf.org, David Borman <david.borman@windriver.com>, "Mitesh Dalal \(mdalal\)" <mdalal@cisco.com>
Subject: Re: [tcpm] WGLC: draft-ietf-tcpm-tcpsecure-10.txt
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> 
> The paragraph I'm commenting on reads:
> 
>    A third type of attack is also highlighted by both the RST and SYN
>    attacks.  It is also possible to inject data into a TCP 
> connection by
>    simply guessing a sequence number within the current receive window
>    of the victim.  The ACK value of any data segment is 
> considered valid
>    as long as it does not acknowledge data ahead of the next 
> segment to
>    send.  In other words an ACK value is acceptable if it is 
> ((SND.UNA-
>    (2^31-1)) <= SEG.ACK <= SND.NXT).  The (2^31 - 1) in the above
>    inequality takes into account the fact that comparisons on TCP
>    sequence and acknowledgement numbers is done using the 
> modulo 32 bit
>    arithmetic to accommodate the number wraparound.  This 
> means that an
>    attacker has to guess two ACK values with every guessed sequence
>    number so that the chances of successfully injecting data into a
>    connection are 1 in ((2^32 / RCV.WND) * 2).
> 
> I'm saying that, assuming the math is correct, that last 
> expression should be (2^33 / RCV.WND).  I didn't check the 
> math.  If you also believe the math is wrong, you have a 
> larger edit. :-)
> 
> On rereading the argument, it does sound like a data 
> injection attack should be harder by a factor of 2 and that 
> they multiplied by the wrong expression (2^32 rather than 
> 2^31).  So I think you're correct about the math, and if so 
> the final expression would be 2^32 / RCV.WND.
> 
> I'd be happier if someone else also looked at this paragraph 
> for technical accuracy.

I have updated the document with the correct math. I agree that it would
be nice if a few more folks take a look.

Thanks,
Anantha

> 
> --
> Ted Faber
> http://www.isi.edu/~faber           PGP: 
> http://www.isi.edu/~faber/pubkeys.asc
> Unexpected attachment on this mail? See 
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> 
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