17 Dec
2003
17 Dec
'03
5:17 p.m.
pstemari@bismark.cbis.com (Paul J. Ste. Marie) writes: 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?
OK -- if you want to retain compatibility with old 56-bit DES chips in your same network, you can set a = b = c and get: Eaaa(x) = Ea(Da(Ea(x))) using the first form, which reduces to Ea(x), or a single 56-bit DES encryption instead of the 168 bits your TripleDES chip can handle. The second form doesn't have this property or any other useful property other than standalone security, since DES isn't a group. Jim Gillogly Highday, 7 Winterfilth S.R. 1994, 22:03