[rc5] rc5 crack schemes

Fowler, Robert RFowler at ins-server.jpl.nasa.gov
Mon Aug 25 13:14:37 EDT 1997

>From: 	Eric Gindrup[SMTP:gindrup at okway.okstate.edu]
>Reply To: 	rc5 at llamas.net
>Sent: 	Monday, August 25, 1997 12:17
>To: 	rc5 at llamas.net
>Subject: 	Re[2]: [rc5] rc5 crack schemes
>     Well, it may be that any such "neural system" is the same sort of 
>     black box that the Swiss neural tank seekers are.  Nobody knows how 
>     they work, but they were trained on a significant sampling of a 
>     population and seem to work really well.

Are these the tank seekers that were really looking for a cloudy sky?

One of the classic warnings about neural nets, and the fact that you
don't really know what they are doing, is the story about the tank
seeker that had been trained on a set of photos, and was really good
(faster and more accurate than human gunners) at finding the ones that
had tanks.

Then they tested it with a new set of photos and it was horrible.  In
the training set, all the pictures with tanks had clouds, and all the
ones w/o tanks had blue sky.  

It wasn't looking for tanks at all.
>     Similarly, one could imagine training a neural net to guess key bits 
>     given the cipher-text.  Perhaps there are effective cryptanalytic 
>     techniques.  If we don't know them , then there is some non-zero 
>     chance that a trained neural net might stumble on one or part of one.  
>     Then the neural net has a better idea of what is the key than the 
>     people who trained it.  However, guaranteeing that the training is any 
>     good would be many more times more compute-intensive than the Bovine 
>     effort.  Of course, if any agency were to attempt it, the NSA would be 
>     the most likely to succeed.
>     BTW: Neural and genetic algorithms (to pick two silver bullets at 
>     random) don't magically solve all of our problems.  Just like 
>     simulated annealing they occasionally make exceptionally good guesses 
>     about the right form of solution.  Cryptography is slightly kinder to 
>     the digital neural net because the inputs and outputs are entirely 
>     discrete.  This does not mean that running a few 10^15s of encrypted 
>     messages past one and training it to produce the key is necessarily 
>     going to work.  It's still a black art to make a neural net do better 
>     than the programmers at some task...
>            -- Eric Gindrup ! gindrup at okway.okstate.edu
>______________________________ Reply Separator
>Subject: Re: [rc5] rc5 crack schemes
>Author:  <rc5 at llamas.net> at SMTP
>Date:    1997/08/25 10:25
>     [elide]
>If each key has an equal probability, how could this magic neural 
>net work?  
>The only better way is if one could come up with a cryptanalytic 
>attack that is less costly than brute force.
>If there is substance to this rumour, then bring it to light so that 
>the world might be warmed and enlightened.
>     [elide]
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