Impossible Analysis Paper at Crypto98

There's talk of a paper given at Crypto98 on "Impossible
Differential Analysis" which got the NSA people scribbling
like mad taking notes as though this was something that
had never come up at the agency and they'd better get
right on it.

Roughly, as I heard it (and I may be way off), the premise is 
that instead of using differential analysis for finding weaknesses 
in a cipher, to flip that to determine what could not possibly be 
a weakness in a cipher and build one with just those attributes.

Is this report correct, and is there a source for that paper?

a proof that you have an unbreakable cypher has a lot
of implications to the P=?NP question. if the analysis
is novel and elegant, than it might very well leads to
insights into P=?NP of computer science.