[125] in cryptography@c2.net mail archive
Re: PCS Encryption?
daemon@ATHENA.MIT.EDU (Douglas C. Merrill)
Sat Feb 1 16:14:45 1997
Date: Sat, 01 Feb 1997 13:04:44 -0800
To: Steven Bellovin <smb@research.att.com>,
Bill Stewart <stewarts@ix.netcom.com>
From: "Douglas C. Merrill" <Doug_Merrill@rand.org>
Cc: William Knowles <erehwon@c2.net>, cryptography@c2.net
In-Reply-To: <199702012030.PAA00519@raptor.research.att.com>
At 03:30 PM 2/1/97 -0500, Steven Bellovin wrote:
>And an illegal wiretap besides, most likely -- with a warrant, they could
>simply put the tap at the base station. The story may be true, but it
>doesn't sound quite right to me.
Indeed -- phone tapping need not happen via the RF spectrum, it can happen
quite nicely where it enters the PSN wired network. Additionally, think
for a second about the logistical problems inherent in tapping a cell phone
-- they're *moving* -- you're going to need to move with them. Not
impossible, but extremely person intensive (need tappers to remain
anonymous, thus need multiple chase cars, etc.) -- it's a lot easier to tap
land lines, as the switches can be programmed to do this automagically.
CDMA is described in detail in a variety of sources . The most famous
reference is Andy Viterbi's book -- which I seem to have loaned to someone,
so I can't give the ref right now :-(
Another source is _Wireless Information Networks_ by Pahlavan and
Levesque, which is more of a general ref on wireless transmission,
particularly digital.
CDMA is, as mentioned earlier in the thread, a spread-spectrum digital
solution -- it works by assigning bits of your conversation to different
parts of the spectrum allocated. This is in contrast to TDMA -- Time
Division Multiple Access -- which allocates part of the time per channel to
each user, or FDMA -- Frequency Division Multiple Access -- which allocates
a set frequency to each user. CDMA in theory at least has higher total
user volume possible, since other people's signals appear as noise that can
be cancelled out.
Working from my not-so-great memory, CDMA uses a fixed spreading code that
is determined by a pretty simple "cipher-like" process. I believe the
details are covered by an NDA, sorry. At any rate, it isn't "encryption"
by any means -- and tapping it requires little more than building a new
base station (again, given $ and the fact that you're going to have to
follow the bloody thing around as it moves).
Phil Karn is, of course, the expert on this -- I hope he'll chime in soon
-- Phil, you out there??
A brief digression re: the marketing speak of phone providers:
Many folks think that "digital = secure" because you can't use radio
shack(TM) listening devices. This much is true. However, you *Can* build
other devices to listen in, and computer hardware is so cheap it's almost
feasible -- though I haven't built one...
More importantly, if someone really wants to listen in on your
conversation, they will bribe a telco engineer to tape your calls, or put a
recorder in your car ....
There are no lasting technical solutions to social problems -- and most
problems are social problems. (I say this so often to our clients that my
colleagues have started calling this Merrill's Law -- I think it's more a
corollary to Murphy's Law, but whatever).
Cheers, DCM
----
Dr. Douglas C. Merrill
RAND
P.O. Box 2138
1700 Main Street
Santa Monica, CA 90407-2138
Phone: (310) 393-0411, ext. 7876
Fax: (310) 451-7039
E-mail (preferred): Doug_Merrill@rand.org
----
My opinions are my own, and often bear no obvious relationship to those of
my employer, any sponsor of my research, or anything human.
