[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]


   Frequently when discussing a cypher the
   question of whether it is a group arises.  In the absence of further
   definition, is it safe to assume that the set of elements for this
   group is the cyphers with each possible key and that the operation for
   this group is composition?

Yes, this is exactly how what this "is X a group" mean when applied to
ciphers.  It's an attempt to get a handle on just how much extra
scrambling happens under composition, i.e. double, triple, multiple

The useful question is, however, not whether it's actually a group,
but just how close to a group is it?  If it were only lacking one
element, it wouldn't be a group, but double encryption would be
statistically speaking a waste of effort for such a hypothetical
cipher.  The work on DES showed that DES is very far away from being a
group.  There are interesting questions about the semigroup that DES
encryptions generates.  Does it contain the identity, i.e. does it
even generate a group?  Put yet another way, does some combination of
encryption (not decryption) operations eventually generate the
identity function?  If so, how long is the shortest such combination?

The goal is to estimate the size of the keyspace for a theoretical
exhaustive search attack.  The result is a greatest lower bound on the
keyspace entropy.

These techniques are not really well developed.  I expect that these
issues will lead to some extremely interesting developments in
mathematics.  In analogy I point out the stochastic stability theorem
for vector fields.  It turns out that strictly topological
classification of vector fields doesn't work for a variety of reasons.
But add a small amount of "diffusion" to the flows and you get a
really nice classification theorem in terms of Morse functions and
elementary catastrophes.  (See Chapter Two of Casti's _Reality Rules_.)

For groups the situations seems similar.  You've got a situation where
a small deletion removes huge amounts of structure, which,
nevertheless, the stochastic version has.  In fact these two areas may
be connected, by considering discrete and finite subgroups of these
flow and turning the diffusion into a discrete Markov process.