RE: [LinkSec] Factoid: MIC size
Russ and Marcus,
I am confused by this discussion. Let n be the size of the MIC in bits, and assume that the MIC has the standard properties. Then the probability that a randomly generated message will have a valid MIC is 1/2^n.
To apply the birthday attack, the adversary must create and send O(2^(n/2)) different messages, i.e., the adversary must generate and send about 2^(n/2) random messages before there is a 50% probability for one to succeed. This does not imply you have to change keys frequently. Indeed, changing keys does not increase immunity to the attack. All it says is how often to expect a successful forgery, independent of the MIC key used.
What am I missing?
-- Jesse
> -----Original Message-----
> From: Russ Housley [mailto:housley@vigilsec.com]
> Sent: Tuesday, September 02, 2003 12:36 PM
> To: stds-802-linksec@ieee.org
> Subject: Re: [LinkSec] Factoid: MIC size
>
>
>
> IPsec uses a MIC of 96 bits for similar reasons.
>
> Russ
>
> At 03:09 PM 9/2/2003 -0400, Leech, Marcus (EXCHANGE:FITZ:8M86) wrote:
>
> >Under the standard birthday attack collision assumptions,
> the probability
> > of collision becomes significant when the number of
> messages under a given
> > key approachs sqrt(2^size-of-mic).
> >
> >That means that for a 64-bit MIC, you need to change keys
> before 2^32 messages
> > have been sent under a key. For a 10gbit link, under
> assumption of average
> > 1000-bit messages, that's a key change every 7.15 minutes
> or so. That
> > impacts
> > the efficiency of the key-exchange algorithm, and the
> decision-point for
> > key-change (if you algorithm for key-changing is slow,
> you want to
> > start trying
> > to do a key-change rather early, so that there's NO
> possibility of
> > exceeding
> > the collision limit).
> >
> >Since we seem to be in the "how many bits can we
> realistically add to the
> >header"
> > phase, I'd vote for more MIC bits than 64.
> >
> >--
> >-------------------------------------------------------------
> ---------
> >Marcus Leech Mail:
> >Advisor Phone: (ESN)
> 393-9145 +1 613 763
> >9145
> >Security Architecture and Planning Fax: (ESN)
> 393-2754 +1 613 763
> >2754
> >Nortel Networks mleech@nortelnetworks.com
> >-----------------Expressed opinions are my own, not my
> employer's------
>
>