From: "James A. Donald" <jamesd@netcom.com>
Date: Thu, 12 Jan 95 10:04:26 PST
To: "L. Todd Masco" <cactus@seabsd.hks.net>
Subject: Re: Multiple symetric cyphers
In-Reply-To: <199501120607.BAA19021@bb.hks.net>
Message-ID: <Pine.3.89.9501120931.A15243-0100000@netcom10>
MIME-Version: 1.0
Content-Type: text/plain


>
> >Strength is not right aspect.  Global risk is reduced, simply because
> >the aggregate cost of a breach is reduced.
 
On Thu, 12 Jan 1995, L. Todd Masco wrote:
> Isn't it?  If an attacker does not know what cipher is used and breaking
> each is computationally expensive (though not prohibitively so) doesn't
> that add extra complexity? 

The increase in strength, if each cypher was roughly equal,
is merely order n, where n is the number of cyphers.

If, as is likely, one of the cyphers required a billionfold
less power to break than the others, you have decreased
strength by an enormous factor.  

The way to increase strength is to use a cypher, such
as IDEA, which has a large key.  Key size will increase
strength by a factor of billions, not a factor of n.

Current key sizes are such that computationally expensive
attacks do not work on symmetric cyphers.  An attack has to 
be clever.

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