the great voice-over-data protocols and products introduced by Intel, Rockwell, ZyXEL and others at Comdex which will make Voice-PGP so much easier

As I understand these voice-over-data products, the voice goes over analog, added to the modem signal. The modem signal is interpreted, and then reconstructed and subtracted from the incoming signal, leaving voice. Very clever, but insufficient for secure phones.

I can't quite see how this would work unless the voice was run at a very low level relative to the data. In order to subtract the modulated version data coming from the other end you have to know exactly what it is, and in order to do that you need sufficient signal to noise of data over everthing else to reliably demodulate it or you need some means of reliably predicting it. Now I recognize that some of the time there is little or no entropy in the information in one direction (it is completely predictable - such as flags during LAPM idle intervals) and it is possible that one could contruct a syllabic gizmo that would turn off the entropy in the data when talkspurts happened by doing flow control and stopping information transmission during periods that speech was loud. But all of this seems a bit much, and certainly would be subject to lots of kinds of degradation depending on the speech content and any nolinearity in the channel. In general the modulations used in modems require at least 12-15 db of SNR for decent BERs - this would imply that if the line was not timeshared with voice in talkspurts that the peak level of the voice would have to be about 20 db below the modem tones. I guess that this would still result in intelligible speech even though it would effectively be transmitted by only a couple of bits per 8 khz sample. And yes I guess that by using forward error correction on the data at a variable coding rate one could allow the voice to creep up on the data a bit more by using a heavier duty error correction during talk spurts. I had assumed that these products digitized voice and multiplexed it with the data stream however. Certainly there are technologies to do this including variable rate vocoding (ala Qualcomm) that would allow almost the full bandwidth of the line to be used for data. [ And yes I'll bring up premail tommorow so I can sign things if I can get around to it without interrupting serious work work - but I'm left wondering who in the hell would want to forge posts from me ? ] Dave Emery N1PRE

From: "Dave Emery"

I can't quite see how this would work unless the voice was run at a very low level relative to the data.

I believe there are also maximum rates on the data when used with voice (4.8 kbps?) and the modulation doesn't use echo cancellation.

This whole thing has an interesting security consequence - retrieving the voice under data off a crude alligator clip type 2 wire wiretap may be difficult because it requires knowing the data going in both directions. So if one sends random or cryptographically secure pseudo random data one might be able to do a reasonable job of hiding the voice channel from simple wiretapping technology even though it is analog rather than digital. And generating random data with a noise diode and a UART is easy to do and very secure. It is already known that retrieving both streams of data from a two wire tap (voltage only) when neither data stream is known or predictable is difficult or even nearly impossible to do with modern near-end-echo cancelling modem modulations such as V.32 and V.34, and without this capability the voice would almost certainly not be intelligable over the roar of the modem tones. Granted this is not the kind of security one gets from strong encryption, but it is easy to do and makes wiretapping a lot harder.... If I were concerned with securing my calls from most threats other than the TLA's I think spending 640 bucks for two of these modems and a little box with a UART and a noise diode might be a cheap and dirty voice privacy hack. Certainly it would defeat the idly curious and the private eye divorce investigator types... Dave Emery N1PRE (PGP signature out for repair)

From: "Dave Emery"

I can't quite see how this would work unless the voice was run at a very low level relative to the data.

I believe there are also maximum rates on the data when used with voice (4.8 kbps?) and the modulation doesn't use echo cancellation.

This whole thing has an interesting security consequence - retrieving the voice under data off a crude alligator clip type 2 wire wiretap may be difficult because it requires knowing the data going in both directions. So if one sends random or cryptographically secure pseudo random data one might be able to do a reasonable job of hiding the voice channel from simple wiretapping technology even though it is analog rather than digital. And generating random data with a noise diode and a UART is easy to do and very secure. It is already known that retrieving both streams of data from a two wire tap (voltage only) when neither data stream is known or predictable is difficult or even nearly impossible to do with modern near-end-echo cancelling modem modulations such as V.32 and V.34, and without this capability the voice would almost certainly not be intelligable over the roar of the modem tones. Granted this is not the kind of security one gets from strong encryption, but it is easy to do and makes wiretapping a lot harder.... If I were concerned with securing my calls from most threats other than the TLA's I think spending 640 bucks for two of these modems and a little box with a UART and a noise diode might be a cheap and dirty voice privacy hack. Certainly it would defeat the idly curious and the private eye divorce investigator types... Dave Emery N1PRE (PGP signature out for repair)

From: "Dave Emery"

I can't quite see how this would work unless the voice was run at a very low level relative to the data.

I believe there are also maximum rates on the data when used with voice (4.8 kbps?) and the modulation doesn't use echo cancellation.

This whole thing has an interesting security consequence - retrieving the voice under data off a crude alligator clip type 2 wire wiretap may be difficult because it requires knowing the data going in both directions. So if one sends random or cryptographically secure pseudo random data one might be able to do a reasonable job of hiding the voice channel from simple wiretapping technology even though it is analog rather than digital. And generating random data with a noise diode and a UART is easy to do and very secure. It is already known that retrieving both streams of data from a two wire tap (voltage only) when neither data stream is known or predictable is difficult or even nearly impossible to do with modern near-end-echo cancelling modem modulations such as V.32 and V.34, and without this capability the voice would almost certainly not be intelligable over the roar of the modem tones. Granted this is not the kind of security one gets from strong encryption, but it is easy to do and makes wiretapping a lot harder.... If I were concerned with securing my calls from most threats other than the TLA's I think spending 640 bucks for two of these modems and a little box with a UART and a noise diode might be a cheap and dirty voice privacy hack. Certainly it would defeat the idly curious and the private eye divorce investigator types... Dave Emery N1PRE (PGP signature out for repair)