Virtual Wartime Bletchley Park
by Tony Sale

An explanation of "Rodding"

You need to frequently Click to Enigma Rodding.

You may wish to print this to have it alongside the Enigma Rodding screen.

In this Rodding example the crib "DANZIGVONMANNHEIM" has been re-enciphered on virtual Enigma set to wheel order 132, Ringstellung ZZZ, wheel start AAG and no Steckers. The resultant cipher text is "JEEWDZPXJBBLIDMWJ".

Remember that the Enigma machine enciphers AFTER the key is pressed so all rodding operations and comic strips are set to start AAH.

The purpose of Rodding is to discover the right hand wheel and its start position. The magic and power of rodding is that this can be achieved without knowing the wheel order or start positions of the middle and left hand wheels.

For some time I could not understand how this was achieved and Turing's Treatise did not help much, but now I think I understand it and here is my explanation.

If you go to Enigma Rodding, the default settings are for wheel order 132, Rings ZZZ and start AAA. Change start to AAH and click on "Enigma Rodding Squares". Now scroll down to the Comic Strips.

Scroll so that the stared "Z*" is in the centre of the screen. You can now trace through the right hand wheel the encipherment of the first crib/cipher pair, DJ. Enter the right hand comic strip at entry letter D. This connects to internal wire "l" which exits on the other side a Z. What this says is that when the German Enigma operator pressed the D key current flowed into the middle wheel at entry Z. Similarly if you enter at J follow the "r" wire you come to Y. This means that however the middle and left hand wheel were select and set, the result was that a current entering the combination at Z came out at Y, went back through the right hand wheel and the J lamp lit.

So it is as if there was a reflector on the left hand side of the right hand wheel with Y connected to Z.

Now the revelation is that if at any subsequent encipherment by the German operator the key pressed and the position of the right hand wheel results in current getting to either the Z or Y entry point on the middle wheel then Y or Z will come out of the middle wheel.

You can verify this at the click point for the Y and Z rods at offset "s" on the crib/cipher pair when N keyed in went to Y on the middle wheel and Z coming out of the middle wheel went to lamp L.

So how do the rods achieve this? The rods for a wheel give every current flow path back through that wheel for one entry point, the rod letter, and for every possible wheel position.

The rod square for the wheel shows all 26 rods stacked up.

Thus by taking the rods for Z and Y and laying them under the cipher/crib pair and "moving" (selecting rod start) to H, they show J and D at the start, as they should because that's how you have selected them, but they also show the "click" at NL as we have just verified.

But this will only occur if the ZS combination is enforced by the middle left hand and reflector combination. I think this is what Turing means by the "coupling" of the rods.

All this means that by reversing the logic, if you take a pair of rods which start, at the selected offset, with a cipher/crib pair, then if they are "coupled" by being one of the 13 possible letter pairs on the face of the middle wheel, they will probable show "clicks" within the length of the cipher/crib pair. If the rods are not in the middle wheel face set, they probably won't show clicks.

You can verify all of this by producing an Alphabet square for just the middle and left wheels and the reflector for the correct Enigma configuration.
In Enigma Rodding set wheel selection to LM and click Alphabet Square.

This shows for all positions of the middle wheel the encipherments of the input letters down the right hand side. Our middle wheel was set at A so the left hand column shows the 13 letter pairs, and ZY shows quite clearly.

It is the pairs from this set which have been found in the default Rodding example. Happy rodding!

This page is created by Tony Sale
(tsale@qufaro.demon.co.uk) the original curator of the Bletchley Park Museum, and Secretary of the Bletchley Park Heritage Society.

Virtual Wartime Bletchley Park by Tony Sale

An explanation of "Rodding"

Virtual Wartime Bletchley Park
by Tony Sale