[Cryptography] Imitation Game: Can Enigma/Tunney be Fixed?

Ray Dillinger bear at sonic.net
Fri Jan 9 01:18:54 EST 2015

On 01/08/2015 03:16 PM, Jon Callas wrote:
> On Jan 8, 2015, at 1:35 PM, Ryan Carboni <ryacko at gmail.com> wrote:
>> If we're doing alternate history and second guessing the decisions of the Germans while they had limited resources.... why not use a Feistel cipher with six letter blocks in ECB mode? Such a machine would only weigh 50 kg... not a major problem?
> That's about 45kg too heavy.

It's true though.  Restricting ourselves to exactly the same
parts list they were using with the Enigma, the single most
security-enhancing adjustment I can think of would be putting
the steckerboard between the reflector and the rotors rather
than between the keyboard/lights and the rotors.  That would
defeat the cyclometric attack that gave away which rotors were
being used, thereby making the additional rotor choices that
the nazis introduced after the first year much more effective
in creating a combinatorial explosion of possibilities that
the Allies would actually have to crunch through.

Restricting ourselves to the same mass, bulk, and expense,
two dozen printed-circuit wiring slugs, plus four rotors
that could have them inserted and use them, would have been
easier and cheaper to produce, not to mention lighter and less
bulky, than the set of eight fully-wired rotors with spring-
loaded contacts etc they were using by the end of the war.
For no additional expense they could easily be made left-right
symmetric so they could be mounted in the rotors "backwards"
drastically increasing their effectiveness in creating more
possibilities to crunch through, and radially symmetric so
they could be mounted in any rotation, which would have
saved the cost of introducing the movable alphabet rings.

Both of these measures would leave the "can't map to itself"
problem in place, so the Bombe would still work to eliminate
possibilities for rotor combinations based on plaintext cribs.
But without the cyclometric attack to tell them which
possibilities were worth checking they'd have had to crunch
through a whole LOT of possibilities. The steckerboard placement
alone would have required crunching through about sixty times
as many possibilities.  And two dozen reversible wiring slugs
plus four very slightly more complex rotors in which they could
be mounted, instead of eight fully wired rotors and movable
alphabet rings, would have increased the allies' difficulties
by a factor of at least tens of thousands.

The Naval Enigma which had a papertape reader/writer attached
could have been used to re-encrypt its own output, which would
have eliminated the cant-map-to-itself problem and placed it
completely beyond analysis with WWII methods - but it would
have complicated the job of the cipher clerk and therefore
made mistakes more likely as well, so that might not be a
good idea in practice.


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