From: Rick Smith <smith@sctc.com>
Date: Wed, 24 Apr 1996 06:14:51 +0800
To: karl@geoplex.com
Subject: Re: DES as a stream cipher
Message-ID: <199604231630.LAA14179@shade.sctc.com>
MIME-Version: 1.0
Content-Type: text/plain


karl@geoplex.com ("Karl A. Siil") asks:

>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.

The right answer depends on the types of attacks you're interested in
countering. The classic reference is probably Voydock and Kent's
"Security Mechanisms in High Level Network Protocols," from Computing
Surveys in 1983. I think Stallings recently put together collection of
paper reprints for IEEE Press that included this one.

This paper is particularly nice since they present various streaming
modes and then talk about the vulnerabilities associated with them.
So it's not crypto algorithms so much as how to use them.

Rick.
smith@sctc.com        secure computing corporation