re how does it really work was re text of white house announcement and q as on clipper chip encryption marc vanheyningen mvanheyn cs indiana edu writes the main thing i just don t get is whether this chip implements symmetric or asymmetric cryptographic techniques anybody know i don t know but i m as willing to speculate as anyone several people have suggested that the chips use public key cryptography another possibility is to use diffie hellman key exchange or some other algorithm which has a similar effect dh allows both ends to agree on a session key which they use with symmetric cryptography something like des for the encryption how could the back door keys work in this system i can see a few possibilities one is that the dh like algorithm has the property that the session key can be deduced by an eavesdropper who knows the back door keys for one or possibly both communicants perhaps the random numbers used in the dh are seeded by the back door key or some such another possibility as was suggested here earlier is that the chips simply broadcast the session key encrypted with the chip s own internal secret key in this system the back door keys are secret keys usable for decrypting this session key broadcast actually the chip s secret key doesn t need to be a public key in this usage but can be identical to the secret back door key this proposal seems most straightforward to me still another possibility is that the encryption algorithm used for the message itself has a dual key property that possession of either of two keys can be used to decrypt it one key would be the session key from the dh exchange and the other would be the back door key for the chip it s worth noting that one of the proposals dorothy denning raised during her amazingly prescient on line discussion last november and december was a variation on diffie hellman in which a third party would participate in deriving the session keys this doesn t quite fit into what we know of how clipper works but it hints that those who had early knowledge of clipper or whose thinking was somehow synchronized with clipper designers may have had diffie hellman on their minds hal finney
