Re: 3DES

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