ideas on clipper i have an idea as to why the encryption algorithm needs to be keep secret and some things that i think it implies of course these could all be wrong from the clipper chip a technical summary dorothy denning revised april the clipper chip contains a classified single key bit block encryption algorithm called skipjack the algorithm uses bit keys compared with for the des and has rounds of scrambling compared with for the des it supports all des modes of operation the algorithm takes clock ticks and in electronic codebook ecb mode runs at mbits per second 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 encrypting 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 suppose i knew how the algorithm worked and knew the n for my chip but did not know f then by cryptanalysis i might be able to determine f from if law enforcement field e e k u n f not knowing n would might make this much harder now suppose that i know f either legitimately or not if i know k either because i am involved in the conversation or i know u for a party in the conversation i may now be able to determine u for the other party if i know f i can also defeat the law enforcement field since i could make my own with a different k then the one i am using knowing f also allows traffic analysis to be performed so i might not know what you are saying but i could know who you are saying it too now i admit that someone trying to compute u will not have lots of messages to work from but since we have no way of knowing that the key generation method does not deliberately generate weak keys or for that matter that the published method is in fact used perhaps the u s will be chosen from know weak keys for the system obviously the compromise of f would be a disaster both to law enforcement for whom this whole thing is designed and for the people who believe that it is giving them security f is but one number and i sure that alot of people more then know what it is and if some panel of experts is going to check it over for flaws then many more will know f forget grinding chips bribery and blackmail work just fine so am i wrong or are these problems jon
