Triple encryption...

[Carl Ellison]

>Date: Fri, 15 Jul 1994 01:14:52 -0400 (EDT)
>From: DAVESPARKS at delphi.com
>Subject: Re: Triple encryption...

>Carl Ellison (cme at tis.com) wrote:
>
>> have you considered
>>
>>        des | tran | des | tran | des ?
>
>That one's sort of your "trademark", isn't it? <g> 

yup :-)

>clever, BTW.)  One scheme that seems to make even more sense, though, is:
>
>         des | tran | IDEA | tran | des
>
>You get the benefits of 112 bits worth of DES keyspace along with 128 bits
>of IDEA keyspace, and thus don't stake your total security on the strength
>of EITHER algorithm.

good, too.  Of course, it leaves open the question of which should be
inside and which outside.

I'd be most concerned about any ciphertext-only attack which is improved by
having purely random bits as input.  Whichever algorithm is more resistant
to such an attack should be on the outside.  (No, I'm not aware of such an
attack, yet....)


>As I recall, last time we discussed this over on sci.crypt you also
>advocated an additional step of "PRNGXOR".  Is that still the case?  Have
>you had the opportunity to read the Eurocrypt '94 paper by Eli Biham on
>triple DES modes, yet?

Yes, it's in response to Eli's paper that I advocated prngxor, as in:


         des | prngxor | tran | des | tran | des

with the DES instances in ECB mode (in acknowledgement of Eli's attack).
The prngxor destroys any patterns from the input, which was the purpose of
CBC, without using the feedback path which Eli exploited.

	 - Carl

p.s.  tran.shar is available at ftp.std.com:/pub/cme