From: karn@qualcomm.com (Phil Karn)
Date: Tue, 28 Sep 93 23:31:39 PDT
To: ld231782@longs.lance.colostate.edu
Subject: Clipper specifics
In-Reply-To: <9309290546.AA27506@longs.lance.colostate.edu>
Message-ID: <9309290628.AA16515@servo>
MIME-Version: 1.0
Content-Type: text/plain


As I recall from a note from Denning some months back, the bits of the
LEEF (Law Enforcement Exploitation Field, its original and far more
descriptive name) are spread out among the ciphertext in some
unspecified way precisely to make it difficult or impossible to
remove.

Damn. Now I remember one of the points I meant to make in my NIST
comments, but forgot: if the LEEF is added periodically to the
ciphertext stream, that implies that the ciphertext data rate must be
greater than the plaintext rate. And that precludes just dropping the
Clipper chip into existing synchronous communication systems such as
our CDMA digital cellular telephone system without *major* system
redesign. Everything in our system is designed around four specific
fixed frame "rates", specifically 16, 40, 80 or 171 bits every 20 ms:
the vocoder, which generates these "frames", the CDMA modem, the
Viterbi decoder, everything.

Encryption that simply performs a 1-to-1 mapping between plaintext and
ciphertext would be easy to add to this system. But an encryption chip
that has to add something to each frame to encode an LEEF is useless
to me.

Anybody know if there is a "reply comments" cutoff date for the
Clipper proposal?  Under the rules that usually govern this sort of
thing, if you can find someone else's comments on file that address
the point you make, you can usually file "reply comments" that address
this point beyond the original due date -- as long as it arrives by
the "reply comments" date (usually a month or two later).

Phil