[Cryptography] Variable length rotor machines: was: how to detect breakage -- lures etc.??

[Peter Fairbrother]

On 08/01/2020 02:43, Ray Dillinger wrote:
> On Mon, 2020-01-06 at 11:54 +0000, Peter Fairbrother wrote:

>> I don't think anybody has ever built a rotor machine with variable
>> length permuting rotors and reuse of the unused rotor inputs and
>> outputs, either with or without intervening permutations. If you did
>> some input characters would necessarily be treated differently to
>> others, so eg a message of zzzzz's might not involve the first rotor
>> at all.
> 
> Point.  I hadn't considered some of the "bad idea" configurations this
> makes possible. As you point out there's some amazing ones.  

There are also some design limitations - the number of pins in each 
rotor position must be constant, by which I mean that the first rotor 
must always have x pins, the second y, and so on. You can't use a 25-pin 
rotor in a 26-pin slot, and afaict you can't move slots around.

This makes the rotors non-interchangeable. If you want to change rotor 
actions then you must either change their internal wirings or have a set 
of rotors for each position - I do not think it is practical to change 
the number of pins on a physical rotor.

As the number of pins in each rotor position is fixed, it must be 
assumed that it is known to the enemy - it will be as soon as they pinch 
an example of a machine.


Another, perhaps lesser, consideration is that for a stacked rotor 
system like Enigma there is only one set of moving connections per 
rotor, whereas with your system there are two - not insurmountable, but 
not an aid to reliability in a mechanical system.



Peter Fairbrother