From: Ray Arachelian <sunder@dorsai.dorsai.org>
Date: Wed, 24 Jul 1996 21:13:51 +0800
To: Matt Thomlinson <mattt@microsoft.com>
Subject: RE: Brute Force DES
In-Reply-To: <c=US%a=_%p=msft%l=RED-77-MSG-960723153957Z-593@tide19.microsoft.com>
Message-ID: <Pine.SUN.3.91.960723173252.13554C-100000@dorsai>
MIME-Version: 1.0
Content-Type: text/plain


On Tue, 23 Jul 1996, Matt Thomlinson wrote:

> why not put together (a LOT of) disk space and we can build a table
> (read: "a cryptanalytic time-memory tradeoff") for cracking DES? Using
> the table, we could brute-search the DES keyspace in less time than it
> would take to do an exhaustive search of a 38 bit keyspace, according to
> the paper. 4 gigs is what, a couple of hundred nowadays?
> 
> Making DES equivalent to a 40-bit crack would take approx. 500Gig, but
> publishing the table would push DES out usefulness. Certainly we could
> scale back (make DES equivalent to a 45-bit crack?) if we don't have
> enough disk...

IMHO it's more expensive to go this route than to build a machine with
dedicate DES cracking chips. 2^40 = 1,099,511,627,776 or about 1 terabyte
worth of space, not 500G. 2^39 would be 500Gb. A 4Gb drive these days is 
$800, hardly a couple of hundred dollars. :)

Even so, that's a ton of hard drives. Further you need machines to hook 
these drives up to. Infact, you need a farm of machines.  Why?  You can 
only put 7 on a chain, and maybe if you're lucky four chains in a machine 
using four controllers.

A better idea might be to make small cheap computers, say based on 8086's 
or 68000's that replace the drive's controller card, or if that drive 
controller card is intelligent enough to be a CPU or contain one, burn 
EEPROMs.  Have the EEPROMs be able to generate DES (or any other 
cypher's) keyspace given a range, and then have them able to search the 
whole drive for a match.

Even so, if you build these drive boxes, all you've accomplished is to 
create a nice huge big searchable array.  You still will need some sort 
of logic to figgure out when it finds the right key, and you still can't 
do 3DES or recusively encrypted files, nor know when you've found the key 
for data you can't recognize - or rather have these drives recognize.

However: Reading a 4Gb drive end to end takes less than 2 hours.  I know 
this because I have a RAID array of them, and it takes 2 hours to 
rebuild, so since rebuilding an array requires reading from two drives 
and writing to one drive, reading a whole 4Gb drive at full speed would 
be something like maybe 1 to 1.5 hours(???)

You might be better off with 9.0Gb drives if you can afford them because 
you then have less controller logic cards to build.

The drives alone will cost $204,800.  $800*(2^40/(4*1024*1024*1024)).  
You could get a nice big discount if you buy that many, but this will 
also mean however much it will cost you to build the cpu cards for 
multiplied by 256 drives, plust the R&D cost, plus the network connection 
between all the CPU boards.

At that point you also run into the MTBF of the drives which means that 
your drives will fail quite often.

If you want to go dirt cheap on the CPU's while using this huge space
method, you could just buy something like 37 Mac IIsi's, hook each up to 7
of the drives (you'll have to partition the drives as they won't support
volumes that huge.) and network the machines using localtalk.  You won't
need a faster connection because all you need for networking is keyspace 
distribution and success reporting.  But then IIsi's are sloooow machines 
and your searches will suffer a hit from the lack of the machine's speed, 
plus all the overhead of having an operating system and using the SCSI 
chain to talk to the drives.

IIsi's go for $350-$500 nicely loaded... $14800 for 37 at $400 a pop, 
add the drives to that, plus the cost of writing the program and hooking 
all of this crap together and that'll be  $219,600.  Ya got that kinda 
dough to spare?

==========================================================================
 + ^ + |  Ray Arachelian |FL|       KAOS KERAUNOS KYBERNETOS      |==/|\==
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===================http://www.dorsai.org/~sunder/=========================
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