From: m5@vail.tivoli.com (Mike McNally)
I don't think the idea proposed is to reverse-engineer the Clipper. Rather, the idea is that once you know the pin-out you can make an electrically-compatible (and, in important ways, software-compatible) replacement.
While the clipper chip and its CCEP brethern have chip specifications that imply that key is supplied as long as a read flag is in a certain state. The key for the clipper chip is 10 bytes of actual key plus 3 bytes of cryptographic check word (CCW), for a total of 13 bytes. Operating in a system expecting a clipper chip potentially restricts the keyspace. Non-centrally selected keys use the clipper chip to 'fish' for the CCW, where it is re-fed. The host system (to the clipper chip) is going to try and feed 10 bytes plush 3 bytes of a constant. Utilizing IDEA, the key is supposed to be 16 Bytes. The point being that dropping an IDEA chip in is not 'plug and play'.