From: Derek Atkins <warlord@MIT.EDU>
Date: Tue, 23 Apr 1996 11:17:13 +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: <9604222228.AA01072@bart-savagewood.MIT.EDU>
MIME-Version: 1.0
Content-Type: text/plain


> 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.

Well, it all depends on what encryption mode you are using.  You can
always use cfb or ecb or ofb modes to get a stream-like cipher.
However you have to beware the security ramifications of using these
encryption modes.

FYI: PGP uses IDEA in cfb mode (albeit in a strange way).

-derek