Re: Real-time sound cyphering algorithm

In article <slrnd98uu9.nff.astier@gandalf.ece.fr>, Astier Anisse wrote:
> 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).
>
> Thank you.

Thanks for all your comments.
Unruh, Jon, Joe you synthetised the problem very well.

I'll try to find a way to sync the signal on order to know when the key begins.
I tryed the playing backwards technique earlier, it works fine : when you
hear it, you know someone is speaking, but do not understand anything.

I can't use any modem, because the purpose is to send the sound
through the air.

I'm working on it, so thank you very much.

Regards, Anisse
Received on Thu Sep 29 21:38:46 2005