From: jim@mentat.com (Jim Gillogly)
Date: Thu, 29 Jan 1998 08:34:07 +0800
To: rfarmer@HiWAAY.net
Subject: Re: Predicting cipher life / NSA rigged DES? / Destroying encrypted data (Tangent to Re: Burning papers)
Message-ID: <9801290026.AA13035@mentat.com>
MIME-Version: 1.0
Content-Type: text/plain



Randall Farmer skribis:
> Been thinking, most applications for ciphers assume solely based on cipher x's
> keysize that data will be secure for a certain length of time. ...
> My idea ... is averaging the remaining lifetimes in
> analysis-hours of broken ciphers which survived as many person-hours of attack
> as the one in question.

Doesn't seem terribly likely.  Typically ciphers will look strong until
someone discovers a chink.  The chink will sometimes lead to a serious
break, but not always, and not always quickly -- but at that point the
cipher looks weak.  Your best chance at encrypting stuff that needs a
long shelf life is with a cipher that's had a lot of analysis and
plenty of intrinsic key, like 3DES.

> Am I just going crazy, or is it kind of obvious that NSA knew the s-boxes they
> provided for DES weren't secure?

The former.  The S-boxes they replaced were bogus, and the ones they
came up with were good against differential cryptanalysis -- better
than random ones.  There's no a priori reason to believe they knew
about linear cryptanalysis, and in any case Matsui's l.c. attack on DES
is better than brute force only in situations where you have a great
deal of known or chosen plaintext.  So how come you claim they aren't
secure?  DES isn't suitable for long-archived info, but is still OK
for short-lifetime data against a not-too-motivated attacker: its
only known weakness for this application is its key-length, not its
S-boxes.

> Anyhow, these two (or three) values are XORed together to form the key used to
> encrypt the volume. When your adversaries, armed with their trusty rubber
> hoses, come knocking at and/or down your door, you hit a hotkey to start
> destroying those 24 bytes on disk, which can be done faster and more
> effectively than a wipe of every sector in the volume. The folks with the

I like it!

 	Jim Gillogly
	Trewesday, 8 Solmath S.R. 1998, 00:27
	12.19.4.15.17, 8 Caban 15 Muan, Second Lord of Night