From: mccoy@io.com (Jim McCoy)
Date: Wed, 28 Sep 94 15:31:29 PDT
To: psmarie@cbis.com
Subject: Re: 3DES
In-Reply-To: <9409282126.AA00174@focis.sda.cbis.COM>
Message-ID: <199409282231.RAA05505@pentagon.io.com>
MIME-Version: 1.0
Content-Type: text/plain


> From: pstemari@bismark.cbis.com (Paul J. Ste. Marie)
> 
> Quick question.  There's a brief mention in Applied Cryptography that
> triple DES uses:
> 
> 	Eabc(x) = Ea(Db(Ec(x)))
> 
> as opposed to:
> 
> 	Eabc(x) = Ea(Eb(Ec(x)))
> 
> in order to preserve some symmetry properties.  Can anyone give a
> better explanation?

If a=b=c, a=b, or b=c  then the first operation is Ea(Da(Ea(x))) which is
just Ea(x).  This method allows one to support 56bit (single DES) and
168bit (triple DES) keys on the same function, basically making the system
backward compatible with those just using DES.

jim