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From: phil@nike.UUCP
Newsgroups: net.crypt
Subject: Re: Naieve Inquiry: Streaming Cyphers (?)
Message-ID: <373@nike.UUCP>
Date: Mon, 14-Jul-86 00:49:15 EDT
Article-I.D.: nike.373
Posted: Mon Jul 14 00:49:15 1986
Date-Received: Mon, 14-Jul-86 07:28:54 EDT
References: <1065@ogcvax.UUCP> <5786@alice.uUCp>
Reply-To: phil@nike.UUCP (Phil Lapsley)
Organization: NASA Ames Research Center, Moffett Field, CA
Lines: 28
Keywords: ECB
Summary: "extended" DES can support n bit ciphering

> DES takes a 56 bits of key and 64 bits of plaintext and
> emits 64 bits of cyphertext.

Plain old DES (POD?  Like Plain Old Telephone Service (POTS)?)
does do this, using "electronic code book mode", or ECB.
However, there are (proposed?) extentions to DES which permit
stream ciphering.

One such is Cipher Feedback Mode (CFB).  To quote from the
AMD data ciphering processor handbook,

     "Cipher Feedback operates on n-bit data blocks, 'n' being
     any value from 1 to 64.  The content of the IV register is
     processed by the DES algorithm.  The most significant n-bits
     of the result are exclusive-or'ed with the n-bit input data
     block.  The result is the n-bit ciphered output block.  This
     output block is shifted into the 'n' least significant
     bits of the IV register [hence the name 'chaining']."

The book goes on to say that this works well for 8 bit data
streams, but the resulting throughput is lower due to only
using 8 of 64 available bits.

I still have no relation to AMD, and the opinions expressed herein do
not necessarily reflect those of NASA or the contractor to NASA for
which I work.

					Phil