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 is the u used in the law enforcement field from the phone which placed the call from the unit whose start secure session button was pressed first or does each phone transmit its own law enforcement field even assuming one of the first two choices the fbi is going to get a fresh n u for its own database about every other phone call eventually accumulating keys for all the phones used to connect to the line they are monitoring not just the suspects key assuming the ever thrifty fbi doesn t forget each key after its wiretap permission has expired not quite a pyramid but not bad either carl ijames ijames helix nih gov more worried every day
