Re: Real-time sound cyphering algorithm

From: Jan Panteltje (pNaonStpealmtje_at_yahoo.com)
Date: 05/25/05 

Date: Wed, 25 May 2005 18:56:24 GMT

On a sunny day (Wed, 25 May 2005 13:21:16 +0000 (UTC)) it happened Astier
Anisse <astier@ece.fr> wrote in <slrnd98uu9.nff.astier@gandalf.ece.fr>:

>Hello,
>My problem is quite simple:
>I'am willing to find a simple algorithm to cypher a sound:
>I need the cyphered sound to be audible, but not understandable,
>and that we can decypher it from a recorded sound (verbatim copy not needed).
>
>This seems simple, but in fact, i tried a few algorithms and i had no results.
>The real problem is that the input data is a 8-bits encoded buffer
>(or more, depending on the encoding).
>It represents the movement of the speaker membrane.
>
>I tried classical cypher or modifications on the stream:
> -XOR-ing it with a 1 or 4 bytes key. It is or understandable with the 1 byte
>keys, or impossible to decypher (because we may not have the same values
>due to numerical/analogical conversion, and that we need to know how to sync
>the key with the data)
> -permutation of amplitude ranges (still understandable)
> -modifying harmonics after a fast fourier transform. Increasing the level
>of each harmonic (still understandable but strong noise), multiplying all
>harmonics by a factor (inverse fourier transform goes out of sound range).
>
>The objective is to have an output sound that seems to be a random sound.
>And that we can decypher after recording. "real-time" is achevied by a small
>buffer (0.5 sec for example).
hehe, fun project.
Why not play it backwards?
It is a signal processing issue, you could make a single for double sideband
signal, reverse sideband, shift... the works.
Maybe also ask in a comp dsp newsgroup or something.
Analog based encryption systems like that have been used for example for police.

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