Re: Neural Nets

This comes up once in a while --- it appears an inappropriate approach,
they say,
since the solution space for the problem consists of exactly one spike, in
the vast sea of 
all possible solutions ... there is no smooth contour over which to
minimize the net's 
error function, and finding the one spike which is the correct result is no
more efficient
in such a case than any other exhaustive search.

This argument would break down if there were detectable biases in the
crypto algorithm
that you could exploit.  But then whether a nn would be the tool of choice
in such a case
may be uncertain.


At 01:00 PM 12/12/96 GMT, you wrote:
>Hi Folkd
>
>Just sat through a lecture on an itroduction to  Neural Nets. 
>
>Thought. Does anybody know of any tools that use Neural Nets to break
>ciphers?
>
>Cheers
>
>Ste
>
>

geeman@best.com wrote:

> This comes up once in a while --- it appears an inappropriate approach,
> they say, since the solution space for the problem consists of exactly
> one spike, in the vast sea of all possible solutions ... there is no
> smooth contour over which to minimize the net's error function, and
> finding the one spike which is the correct result is no more efficient
> in such a case than any other exhaustive search.
> 
> This argument would break down if there were detectable biases in the
> crypto algorithm that you could exploit.  But then whether a nn would
> be the tool of choice in such a case may be uncertain.

Detecting and exploting biases of encrypted bits vs plaintext bits is the
basis of many well-known techniques which are known as differential and
linear cryptanalysis.  I think it would certainly be possible to perform
differential cryptanalysis via an evolutionary algorithm which looked for
correlations and favored those which were statistically more likely.
Of course, any well-designed algorithm should make it take an impractically
large number of iterations to discover any useful relations, but the
technique would probably work pretty well against common snakeoil.