re the clipper chip a technical summary in article apr guvax acc georgetown edu denning guvax acc georgetown edu writes prof denning s description of skipjack mostly omitted chip structure the clipper chip contains a classified 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 throughput is mbits a second f an bit family key that is common to all chips n a bit serial number u an bit secret key that unlocks all messages encrypted with the chip the key k and message stream m i e digitized voice are then fed into the clipper chip to produce two values e m k the encrypted message stream and e e k u n f a law enforcement block three questions it looks like each bits of input gives you bits of output e m k bits e k u e bits bits e e k u n f e bits bits do you really need to transmit all bits each time or do you only transmit the bits of wiretap block at the beginning all would be really obnoxious for bandwidth limited applications like cellular phones or even regular phones over how do the des modes interact with the two part output do the various feedback modes only apply to the message block or also to the wiretap block or if the wiretap block is only transmitted at the beginning does it get incorporated into everything through feedback modes but not during ecb mode does the clipper chip check the wiretap block itself does the block have to be present at all since the receiving chip doesn t know the transmitter s u it presumably can t check the validity of e k u so it s limited to checking the form of the wiretap block and maybe checking the serial number for reasonableness unless there s some sort of back door structure that lets it recognize a valid e k u in that case can you replace the wiretap block with a different wiretap block presumably an old valid one to avoid attracting attention the chip won t do it so you postprocess the output regular people can do one with their own serial number and a dummy key paranoid people can use someone else s serial number on the other hand if i could think of that solution so easily presumably the nsa could too have they done something to block it like use message encryption that s really e m k u n thanks pray for peace bill bill stewart wcs anchor att com at t bell labs m holmdel nj no i m from new jersey i only work in cyberspace white house commect line fax
