[Cryptography] Hypothetical WWII cipher machine.

[Ray Dillinger]

Someone on wattpad (where I do some recreational writing) noticed
that I've done some cryptography, and read my 'fixing Enigma'
articles, and asked me to design a Plot Device.

He's writing a WWII spy thriller, and the MacGuffin is a
super-duper cipher machine that the protagonist and his little
group have to prevent from falling into the hands of the Axis.
Having broken Enigma, the Allies don't want them adopting
something that's actually secure.

He asked me for a detailed design for something that (A) could
realistically have been built around WWII, (B) is not a rotor
machine, (C) is very much more secure than Enigma, (D) that he
could do a cool, interesting, understandable illustration of,
(E) whose basic operation could be explained in one page or
less, and (F) would not make real cryptographers laugh if they
read his book.

So I thought pretty hard about what's wrong with Enigma again -
you can read my blog posts from times I've thought about it
before. I admit I've obsessed somewhat on the topic of old
cipher machines.

One of the problems with Enigma, and most rotor machines, is
that too much of their state is static.  While their setup can
have a lot of crucial bits, most of them are things that don't
change during encryption/decryption, and most of them make
the machines very prone to related-key attacks (which was how
Bletchley Park worked). In fact the only thing that changes
during encryption is the rotor positions, and those only
describe a small fraction of the total state.

So I obsessed for a couple of days and came up with something
whose state is almost entirely dynamic state, as well as
having a much larger state space than Enigma.  It has what's
effectively a 154-bit keyspace and it's chaotic, so it will
fall into repeating cycles of about 2^76 states, with each
state representing a different monoalphabetic cipher, and the
relationships between adjacent states being far more chaotic
and obscure than Enigma ever made them.

And it does have a cool diagram and one-page basic explanation.

Now, the final test:  Will it make real cryptographers laugh?

Have a look, if you feel like thinking about the security of
a Plot Device and possibly having a laugh and correcting my
mistakes if I've made any obvious ones. The diagram and one-
page explanation are at

http://www.sonic.net/~bear/DiabolusCipherMachine.png

My only objections to it are practicality, not security; key
setup would be a big pain in the butt, and during the
process small removable parts could become lost making the
machine useless.


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