From: Eric Young <eay@mincom.oz.au>
Date: Tue, 23 Apr 1996 12:49:48 +0800
To: "Karl A. Siil" <karl@geoplex.com>
Subject: Re: DES as a stream cipher
In-Reply-To: <2.2.32.19960422214547.006b636c@geoplex.com>
Message-ID: <Pine.SOL.3.91.960423093853.1018F-100000@orb>
MIME-Version: 1.0
Content-Type: text/plain


On Mon, 22 Apr 1996, Karl A. Siil wrote:
> As this sounds like a previously solved problem, I wanted to find out about
> using DES (or any block cipher) as a stream cipher, i.e., in a manner that
> keeps input and output data length equal. I don't want to use a true stream
> cipher, as I want to use the same key for multiple messages and stream
> ciphers tend to place the bulk of their overhead in the re-key. Since stream
> ciphers have "memory," I would have to "re-key" to the same key for each of
> my messages. I would rather key something like DES once and run it in CBC
> mode or use some other form of IV.

Have a look at cipher feed back mode.  I have functions I call cfb64 in 
my DES library that give a 'single' character interface to cfb mode DES 
using 64bit feedback.

This should be what you want.  I also have a triple DES version of cfb64.
In my SSL library also has cfb64 mode for IDEA.

eric

ftp://ftp/pub/Crypto/DES
ftp://ftp/pub/Crypto/SSL
http://www.psy.uq.oz.au/~ftp/Crypto
--
Eric Young                  | Signature removed since it was generating
AARNet: eay@mincom.oz.au    | more followups than the message contents :-)