I have heard that the Clipper Chip is only supposed to be used for voice applications. Does anyone know if the inputs and outputs of the Clipper Chip digital or analog? That is does the clipper chip input/output an analog signal with some sort of internal analog to digital converter or does have purely digital inputs and outputs?

No, it's strictly digital; any analog circuitry is the responsibility of the equipment designer, as are details like key exchange. I assume the motivation for "voice only" are issues like - Makes Clipper easier to get away with politically - Avoids stepping on the toes of data standards processes - Avoids any federal rules about how to make data standards - Data standards are tougher, since the cyphertext is bigger than the plaintext by the length of the Law Enforcement Abuse Field; for voice this mainly means it takes a bit longer to start up.

The reason I ask is, if the chip does analog IO, then it would be possible for the chip to be designed to deliberately leak its own key disguised as a measurement error in the low bits of the analog to digital/analog conversion.

It's still possible to leak data - the LEAF includes a checksum field, which could easily be leaking part of the key if they wanted it to. Leaking the chip's master key in 8 pieces would still leave enough real checksum bits to make sure you were using a real clipper. Bill