Re: Real-time sound cyphering algorithm

David Eather wrote:
> Correct - to be impossible to follow the signal would have to be 20
> db below the ambient noise floor (just over 3 bits) and a cipher
> output does sound like white noise.
>
> In audio engineering the 20db figure was widely quoted. +20db(spl)
> above the ambient noise to be easily distinguished (by an audience).
> +0db compared to the ambient noise can be followed for a short time
> by people highly interested in the broadcast. -20db is considered
> inaudible.

So, correct me if I'm wrong: your solution is to transmit the
sum of payload-sound plus a 20dB larger crypto-pseudo-random
noise signal. Then the receiver subtracts off the crypto signal,
to get the plaintext plus any noise on the transmission. For the
sound to "be easily distinguished", we want the plaintext sound
to be 20dB above the noise on the transmission channel. To get
the payload 20dB above the noise, and the crypto-stream 20dB
above the payload, we need an audio carrier with a signal-to-
noise of 40dB, which is 10,000-to-1 in sound power.

Modern digital methods offer vastly better performance. We can
transmit easily-intelligible speech at a fraction of the power,
or high-fi stereo music at the same power. Google "turbo codes"
along with "dB" for more.

The digital method also offers better security than does adding
pseudo-random noise. Both rely on the security of a
cryptosystem, but the analog method also assumes that no useful
information is available in a signal 20dB below the noise.
"Considered inaudible," "in audio engineering," does not imply
the level of confidentiality required in cryptography. -20dB
certainly limits the bit-rate, but leaking slowly isn't good
enough.

> The constrains given by the OP don't give much room for a "perfect"
> result - processing in real time and no feedback and it is probably
> safe to assume it is a one time project for an individual.

Cheap mass-market (or used) PC's can do the digital processing,
and cheap commodity audio systems can handle the analog I/O. In
terms of software, the modern methods are sophisticated, but the
hard parts are already done.

-- 
--Bryan