[Cryptography] Hypothetical WWII cipher machine.

Peter Fairbrother peter at m-o-o-t.org
Sat Jul 18 03:12:24 EDT 2015

On 18/07/15 06:56, Ray Dillinger wrote:
> 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.

A smallish suggestion, perhaps one change - the device is an updated, 
secure rotor machine, but it has been designed so as to be defeat the 
methods used to break Enigma - unfortunately, it's design is also such 
that if it fell into Nazi hands they would know for sure that Enigma had 
been broken ..

> 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?


Your first mistake is in the first sentence: there are 13 triangular 
parts, 24 boat-shaped parts (and with four input and four output 
contacts, there are only ten possible different varieties of each).

> 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.

-- Peter Fairbrother

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