The basic scheme of digitizing voice, scrambling the bits, then sending the
bits is the most straightforward voice encryption scheme.  You can't, of
course, accomplish much by just XOR'ing with a constant, you need to XOR
with a pseudo-random sequence, then descramble using the same sequence at
the receiving end.

Several problems occur:
1) Any simple scheme for modulating the radio (FSK, ASK, PSK, etc.) to carry
the bits will have much greater bandwidth than the original voice.  4kHz
bandwidth * 2 for Nyquist, * 8 bits = 64Kb/second.  If you have the
bandwidth to burn, that makes the problem a whole lot easier.  Simply
modulating the carrier with the scrambled bits amounts to 256-level
amplitude- (or frequency-) shift keying, which would be extraordinarily
difficult to demodulate with any accuracy.  Your receiver would have to
correctly pick out very small differences from one byte to the next --
differences that are probably much smaller than the noise.

2) You need some way to synchronize the receiver's sequence with the
transmitter's.  This is most easily achieved by sending the data in
fixed-length blocks with synchronizing sequences of some kind between them.
That adds yet more to the bandwidth, but that's part of the cost of sending
the voice digitally anyway.

3) For real security, you need to use a cryptographic pseudo-random sequence
generator, such as DES.  (You can make any block cypher algorithm into a
pseudo-random sequence generator by feeding the output from one round into
the next round.)

Real systems (like military ones) usually deal with the bandwidth problem by
compressing the audio up front, which reduces the number of bits, and then
using complex and sophisticated modulation techniques to the point that the
data stream can be sent in about the same bandwidth as the original voice.
This takes more number-crunching horsepower than a PIC can muster, but is
within the capability of relatively cheap DSP chips.

SO if you can use more than 64kHz to send your 4kHz voice signals, you can
probably do it with a PIC.  The most difficult thing to build will be the
demodulator that picks the bits out of the stuff arriving at the receiver
end.  There are a number of techniques like Manchester encoding that will
make this easier, but most of them will increase your bandwidth some more
again (double, as I recall, for Manchester).

You might note that digital cell phones arrived long after analog ones, but
that when they arrived, they were encrypted (sort of, but it was politics
not technical difficulty that made it very weak encryption).  Encrypting a
digitized voice stream is a much easier problem than getting the bits from
one end of the radio to the other.

-----Original Message-----
From: Wesley Moore [mailto:wmoore@CS.RMIT.EDU.AU]
Sent: Thursday, January 06, 2000 12:26 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: ADC and voice sampling


Hello all,
I am interested in making a PIC based project in which I need to encrypt
voice. The voice is encrypted for transmission using a UHF radio.
Basically the person speaks into the microphone normally, the PIC samples
the analog data from the microphone at a minimum rate of 8kHz, XOR's it
with a preset 'key' and converts it back to analog for transmission. At
the other end the opposite is done. My question is does you run of the
mill ADC0831 8-bit ADC have what it takes to sample a voice signal?

Wesley

___________________________________________
Wesley Moore
RMIT - BEng/BApp.Sc. 2nd Year

wmoore@cs.rmit.edu.au
http://wmoore.tsx.org/