From: wcs@anchor.ho.att.com (bill.stewart@pleasantonca.ncr.com +1-510-484-6204)
Date: Sun, 13 Mar 94 19:21:20 PST
To: paul.elliott@hrnowl.lonestar.org
Subject: Re:  Question about the Clipper Chip
Message-ID: <9403140320.AA22125@anchor.ho.att.com>
MIME-Version: 1.0
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> 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