DES Question

[uri at watson.ibm.com]

Phil Karn says:
> >Second question: The DES code that I have (not written by me) has a
> >comment section which describes filling all 16 subkeys seperately,
> >thereby allowing a 128 byte key.  Is there any significant advantage to
> >doing this?  Is there any reason that I should not do it?
> That sounds like my code. That feature seemed like a good thing to do
> at the time. Then I learned about differential cryptanalysis. No, you
> cannot strengthen DES in this way, and in fact you could actually
> weaken it unless you are sure to use 128 completely random bytes for
> your key.

Phil is wrong and ys you can strengthen DES by choosing completely
independent subkeys, rather than generating the subkeys with known
algorithm from 56-bit "seed".

However, the additional strength will mostly go towards foiling
brute-force attacks.

Note, that it will take about 2^60 chosen plaintexts instead
of 2^47 to mount differential cryptanalysis attack, and also
linear cryptanalysis is somewhat hampered by using subkeys
independently generated.

> >What is the purpose of the initial and final permutations?
> Mainly to sabotage the performance of DES software implementations.
> Even back then the government knew it was much easier to control
> the dissemination of hardware than software.

Wrong. Pure hardware requirements - nothing so subtle as to
"complicate" software implementation, simply peculiarity of
that day hardware... Trust me! (:-)
--
Regards,
Uri         uri at watson.ibm.com      scifi!angmar!uri 	N2RIU
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<Disclamer>



>From owner-cypherpunks  Tue Mar  1 06:58:15 1994