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Re: CISCO PIX Vulnerability
From: Damir.Rajnovic () EUROCERT NET (Damir Rajnovic)
Date: Fri, 5 Jun 1998 10:36:53 +0100


Hi there,

At 10:19 -0700 4/6/98, Mat Butler wrote:
On Thu, 4 Jun 1998, Damir Rajnovic wrote:

Hi there,

At 19:25 -0700 3/6/98, David Wagner wrote:
Either the sci.crypt folks were confused, or I am.  With only 48
unknown bits in the DES key, you can break the encryption 2^8 = 256
times faster than you can break DES.  This is a serious weakness.

Probably I was unclear. What I want to say is that it does not matter
what bits inside key are known. It is the same if you know that first
8 bits are 0 or middle or end bits. In all cases you must put the same
effort to break encryption. In that sense there is no 'additional gain'
knowing WHAT bits are fixed it does matter only that some are fixed.

If you know the bits in the key that are fixed, you create a program to
generate all possible combinations with those bits fixed.  (If nothing
else, you create a list of every possible combination of the number of
bits that aren't fixed, then insert the bits that are fixed before using
the strings as keys.)

It -does- matter if you know what bits are fixed.  We're talking the -key-
here.  Not the output of the encryption.

Yes, but what I was trying to say is that if you know that first 8 bits
are fixed you can break encryption in X time units, so it will take again
X time units to break it if last 8 bits are fixed or any other 8 bits.
It will always take X time units no matter what 8 bits are known. There
is no, allegedly, 8 'preferred' bits that will allow you to break it in
less than X time units.

Cheers,

Gaus

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