On 9/25/07, Roger Hui <[EMAIL PROTECTED]> wrote:
> Suppose
>
> c=: ?256
> d=: 16+?16
> e=: a.{~ c (d b.) i.256
> NB. e is used as: map=: e {~ a. i. ]
>
> Can you recover c and d given e? c and d are not
> necessarily unique so it suffices to compute c1 and d1
> such that e -: a.{~ c1 (d1 b.) i.256 .
The possibilities are few enough that it's reasonable to exhaustively
test all candidates for c1,d1.
If ?256 were replaced with ?~256 in the above this would become
more challenging (though I imagine the hard part would be
constructing the search-for-candidate-permutation routine against
candidate d1 values which which are only partially invertible).
--
Raul
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