Modem Based Crypto Telephones

[thug@phantom.com (Murdering Thug)]

jim@tadpole.com writes:
> thug@phantom.com writes:
> > Even higher speeds are available if you're willing to go that far.  Zyxel
> > v.32bis modems have proprietary 16.8 kbps and 19.2 kbps full duplex raw
> > modulation rates, but they use DSPs instead of modem chips like the
> > ones from Rockwell, AT&T, and Intel.  I believe there are some v.FAST
> > (not CCITT compliant) modems like the one's from Motorola (Codex) that
> > can do 21.6 kbps and 24.0 kbps.  I believe the final speed of v.FAST once
> > standardized by the CCITT will be 28.8 kbps.
> 
> I don't see how a 28.8kbps (raw) data rate is possible, as the Shannon
> limit for a POTS line is 22kbps.  Certainly parts of the phone system
> no longer impose the narrow bandwidth that are part of the 'spec', but
> one can not always depend on getting a line that exceedes the published
> parameters of the phone system.

Then how come Hayes demonstrated their v.FAST modems at Fall Comdex '92 in
Las Vegas.  According to the report on Comdex I am reading, the Hayes
modem dialed up another modem at Hayes headquarters in Atlanta and set
up a perfect 28.8 kbps full duplex raw data link.  With v.42bis the two
modems were exchanging text at close to 115.2 kbps.

> The Rockwell (RC96AC/RC96ACL/RC144AC/RC144ACL) modem chip (set) has an
> on-board codec that does ADPCM in hardware.  It makes for a very nice
> programable answering machine.  Interestingly, the designers guide has
> someting to say about bit rates.
> 
> At a sampling rate of 7.2 KHz, (the only sample rate this chipset
> supports) 8 bit samples are presented at a bit rate of 57600 bps.  Add
> in a start/stop bit, and you now need a DTE rate of 72,000 bps.  Most
> UARTS don't support this rate, and thus you will need to find one that
> will run at 115.2 Kbps.

This is way more than what is needed for telephone quality audio.  I
have programmed voice mail systems based on Dialogic hardware.  They use
a simple ADPCM codec and 6,000 4-bit samples/second.  This gives you
a audio bandwidth of 3khz, basically telephone quality.  At this rate,
we're talking about 6,000 x 4 bits = 24,000 bps.  And this is WITHOUT 
any kind of advanced compression.  A v.FAST modem doing 24.0 kbps
like the Motorola Codex can handle this now, and 28.8 kbps modems
can handles this and provide a 4kbps digital subcarrier for carrying
data with voice.  For instance, I could be having an encrypted conversation
with you and at the same time, I can send you a spreadsheet file at 4 kbps.
Obviously since the entire 28,800 bps stream would be encrypted, the
spreadsheet file would be encrypted as well.

You can also get excellent quality using 4800 samples/second using 3-bit
ADPCM samples.  This would give you 14,400 bps and an analog bandwidth
of 2400hz.  This is lower than phone quality which is 3000hz, but anything
above 2400hz is really useless for transmitting a male speaking voice
which hardly ever goes past 2000hz.  A female voice on the other hand might
sound somewhat distorted if everything above 2400hz is chopped off.  However,
using a DSP, one may shift the 0-2400hz bandwidth to 300-2700hz using a
toggle switch.  Thus all a female would have to do is toggle a switch on
the cryptophone to tell the other side about the shift.
 
> I'm also fairly sure that recent Zyxel modems (e.g. the U-1496) use
> this Rockwell chip(set), and not a dedicated DSP.

No, Zyxel uses a DSP. They are always updating their DSP roms to provide
new features.  Not only do Zyxel modems provide v.32/v.32bis/v.42/v.42bis,
and MNP 1-5, but also MNP 10, Caller ID, Voice Mail, and proprietary
16.8 kbps and 19.2 kbps full duplex modes. 

As soon as ISDN hits the streets, we won't have to worry about bandwidth
since it will be quite easy to build an all-digital crypto-phone that
provides end-to-end encryption based on a public key system.

Picture this: an ISDN phone that can operate in normal or encrypted mode,
that has a small 20mb 1.8" hard disk or flash eprom card to store the public
keys of all the people that you converse with who have similar phones.  In
fact, it is possible to set up a trusted centralized public key directory
assistance like service, which would contain perhaps everyone's public key,
and could be queried automatically at the beginning of each call.  The 20mb
storage could be a public key storage cache for people you call frequently,
while the public key directory assistance is used for people who you only
plan to call once.  On the other hand, a centralized authority is always
bad when it comes to security.  A PGP-like scheme of decentralized public
key distribution is much safer.  If Bob wants to give Mike's public key
(which is stored in Bob's phone) to Joe, all Bob has to do is call up
Joe, tell Joe that he that he wants to give him's Mike's phone number and
public key. Bob then presses a button on his phone and instantly uploads
Mike's public key to Joe's phone, either via a digital subchannel, or
via the main channel (and interrupt the conversation for a few seconds),
like the old video phones used to do to transmit still frames. 
 

Thug