... is the one you don't ask. So here I go... #1 Isn't "part of the deal" of patent granting a requirement that the details of the thing being patented be revealed. If so, why isn't IBM required to reveal the details of s-box design? After all, they hold the patent on DES. They revealed the values of the S-box. Unless the patent included claimes relating to its design criteria, they didn't have to disclose them. Of course, then they wouldn't be protected if someone else were to reinvent and use those criteria in a cipher that isn't covered by other parts of the DES patent. #2 Skipjack has a 80 bit key which is proposed to be "escrowed" in two parts. Now considering that the likeliest attack on DES is a brute force key search of 2^55 keys, isn't it true that a compromise of one half of a "Clipper key" would allow a brute force attack to "discover the remaining 40 bits. If 2^55 is possible, then 2^40 is even easier, no? As several people have pointed out today, the two halves are 80 bits apiece, and they're XORed together to make the full key. You can't do a brute-force search on 80 bits.