re revised technical summary of clipper chip in article apr guvax acc georgetown edu denning guvax acc georgetown edu writes each chip includes the following components the skipjack encryption algorithm f an bit family key that is common to all chips n a bit serial number this length is subject to change u an bit secret key that unlocks all messages encrypted with the chip to see how the chip is used imagine that it is embedded in the at t telephone security device as it will be suppose i call someone and we both have such a device after pushing a button to start a secure conversation my security device will negotiate an bit session key k with the device at the other end this key negotiation takes place without the clipper chip in general any method of key exchange can be used such as the diffie hellman public key distribution method once the session key k is established the clipper chip is used to encrypt the conversation or message stream m digitized voice the telephone security device feeds k and m into the chip to produce two values e m k the encrypted message stream and e e k u n f a law enforcement field which are transmitted over the telephone line the law enforcement field thus contains the session key k encrypted under the unit key u concatenated with the serial number n all encrypted under the family key f the law enforcement field is decrypted by law enforcement after an authorized wiretap has been installed one very interesting thing i notice about this is that the only use of the chip key is to encode the session key for the law enforcement field it is not used at all by the encryption algorithm it seems like it would be possible to create a device that would be otherwise compatible but would send out a bogus law enforcement field this might render the device unusable with normal devices if there s some sort of validation involving the law enforcement field but it could certainly be used with other such modified devices of course this is irrelevant if the nsa has a backdoor in the algorithm but it does make it possible to defeat the key escrow system wilt thou seal up avenues of ill pay every tom weinstein debt as though god wrote the bill emmerson tomw orac esd sgi com
