From: C445585@mizzou1.missouri.edu (John Kelsey)
Subject: Re: Clipper considered harmful
Nntp-Posting-Host: mizzou1.missouri.edu
Organization: University of Missouri
Lines: 32

 
   The clipper chip's User key is formed by:
 
           R1 = E[D[E[N1;S1];S2];S1]
           R2 = E[D[E[N2;S1];S2];S1]
           R3 = E[D[E[N3;S1];S2];S1]
 
   Why is the triple-encrytion used?  Is it just to gain an effective
increase in keyspace to defeat a potential keysearch?  (If so, why use
80 bit keys?)  Not knowing anything about the Skipjack algorithm, it's
not really possible to guess whether this makes it harder or easier to
guess S1,S2.
 
   Why are N1, N2, and N3 formed as they are?  It would be facinating to
see the Skipjack algorithm, to look for ways of attacking it that require
three ciphertext blocks formed in that odd way.
 
   Where do the 34-bit constant values that are concatenated with the
serial number to form N1,N2,N3 come from?  Are they changed from chip to
chip, or session to session?  (Even if they're published in the NY Times,
if SkipJack is resistant to known-plaintext attacks, when using triple-
encryption, then there's no break in security.  But why allow that kind
of weird format?  If those three 34-bit values are truly-random bits, then
maybe it's used to ensure that a known-plaintext attack on SkipJack, if
it exists, can't be easily used to derive S1 and S2 for a whole production
run of these chips....)
 
   Does Dorothy Denning read this group?  If not, is someone on the group
forwarding questions like these to her, or Martin Hellman, or anyone else
who's seen more details about the chip?
 
   --John Kelsey
