

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.