Re: fixed block size

"Antony Clements" <antony.clements@xxxxxxxxxxxxxxx> wrote in message
news:4630682e$0$11540$afc38c87@xxxxxxxxxxxxxxxxxxxxxxx
given my realisation, on top of every other weakness people have found in
my layer, is a fixed block size in this instance yet another avenue of
attack?

While I can see where the idea is coming from, and I can also see where
mathematically there is an argument that having multiple block sizes can
offer some extra protection, I do not see an advantage of a variable block
size over using the largest block size of the system. The reasoning is
fairly straight-forward, by using the variable block length you are in
effect consuming keyspace for selecting that blocksize, by doing this you
are limiting the possible {domain, range, key} combinations and leaking
information about the key in every block boundary, by fixing the blocklength
no information has to be leaked at the block boundary. A fairly
straightforward example is to look at the Viginere cipher, whose block
length is the key length, this provides extra leverage on the key in that if
the block length can be found the keylength is immediately discovered.

The core problem comes down to providing leverage to the attacker, it should
be fairly obvious that giving the attacker any leverage is a bad idea.
Joe


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