nobody said:
Eric Michael Cordian, emc@wire.insync.net, writes:
The concerns are generally that we will experience an unexpected "combinatoric explosion" in the higher round problems
Unexpected by you, perhaps, but expected by everyone else. The complexity of the expressions should increase exponentially with the number of rounds. Extrapolating from two and four round results to eight and sixteen is the wrong model. ...
Can't you come up with a back-of-the-envelope estimate for the number of terms in your sixteen round expression? Even without fully optimized S-box expressions this information would be useful. If it is greater than the number of atoms on Earth then it would be a strong hint that this approach won't work.
In the early 1980's I started trying this approach. I did the back-of-the-envelope estimate and realized it was too big, but I thought it worth trying, since if there were a back door in DES it might manifest itself by a massive collapse in the complexity of these expressions. I didn't get far enough into it to decide one way or the other, since I didn't have a good tool for reducing the expressions to minimal form. Jim Gillogly