re clipper considered harmful steve bellovin writes a well thought out and nearly persuasive article about why the clipper are we still calling it clipper today or have they figured out a non infringing word yet protocol is almost as good as one can do given their marching ordes in article apr ulysses att com smb research att com steven bellovin writes i issue this invitation assume that you were charged with implementing such a system with complete regard for civil rights and due process what would you do differently in answering this question please accept nsa s fundamental assumptions that both strong cryptography against outsiders and the ability to wiretap some domestic users is necessary i ll accept the second assumption only for the sake of argument in my view the primary remaining flaw is that the encryption algorithm is secret leading to suspicion that there is a back door without complete disclosure this suspicion cannot be dispelled no matter how many trusted experts are allowed to look at it in isolation is it possible to do this whole thing with a public algorithm the only concern i ve seen with making skipjack public is that someone could build clipperphones without registering the keys assume f can really be kept secret as the government assumes then as part of the initial connection a clipperphone executes a protocol with the pseudophone to demonstrate that they both know f for example the initiating phone picks a number q and sends e q f the receiver sends back e q f and the initiator sends back e q f to demonstrate that her first packet wasn t just a random bit block repeat in the opposite direction with another q so it can t be defeated by somebody building up a library of sequential numbers by remembering responses and using those responses as the initial numbers of subsequent challenges this way clipperphones will talk only to other clipperphones of course the pseudo company can build their own proprietary skipjack phone but the presidential fact sheet implies that they won t get approval for it without equivalent key escrow what s wrong with this picture it depends of f staying secret and on skipjack being resistant to cryptanalysis but the government appears to believe in both of these even if the particular q a i suggest has some flaw i imagine there s a zero knowledge proof protocol that doesn t smb says the third and most serious issue is whether or not clipper is a good idea even if carried out strictly according to the letter of the law agreed my view and you can quote me if it s not worth doing it s not worth doing well jim gillogly hevensday thrimidge s r
