I am interested for Encryption Algorithms (in C Language). The
algorithms encrypt the received data (binary data)from an Analog to
Digital Converter and send the encrypted data to a Digital to Analog
Converter. The same process with the same algorithm happens at the
receiver, for the decription of the data.

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Request for Question Clarification by maniac-ga on 01 Mar 2004 18:32 PST

Hello Isidoros2004,

Do you have any specific constraints on the processor that will do the
encryption / decryption of the data?

Also, how many A/D values do you expect to encrypt with each message?

A number of algorithms exist that can be adapted for your use,
however, they tend to do processing in 64 bit blocks and require 32
bit arithmetic. If this is not feasible in your system, please let us
know before answering the question.

  --Maniac

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Clarification of Question by isidoros2004-ga on 03 Mar 2004 06:16 PST

Hi Maniac,

Firstly, I use the TMS3206711 DSP processor. 
The steps of the procedure are:
 Audio Signal => A/D => Audio Encyption => D/A => Receiver => Audio Decryption

I want to encrypt data of a few KBs and have in mind that there is noise as well.

I transmit 16bit blocks and they require 16 bit arithmetic.

I want the code to be as simple as possible (C Language).

Tnx
Isidoros

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Request for Question Clarification by maniac-ga on 09 Mar 2004 18:37 PST

Hello Isidoros2004,

I've looked at this problem several times but finding C source code
that uses only 16 bit arithmetic has been the problem. Does your C
compiler support "long" arithmetic (32 bit)? I found several programs
that work with 16 bit ints and 32 bit longs and can produce a proper
answer with one of those if that is acceptable.

  --Maniac

Since the output of an A/D converter is essentially no different from
any other data, you could theoretically use any common encryption
algorithm. My suggestions would be:

Arcfour - an easy-to-implement stream cipher.
http://lists.samba.org/archive/samba-ntdom/1998-February/000064.html

Blowfish - a block cipher; slightly more complicated, but possibly
more secure. http://www.schneier.com/blowfish-download.html