Virtual Wartime Bletchley Park
by Tony Sale

For instance R->N at g, N->S at p and S->R at q making a loop. A diagram showing such loops was known as a menu.But if Steckers are being used this is actually:

R Steckered to S1 enciphers to S2 Steckered to N at g, N Steckered to S2 enciphers to S3 Steckered to S at p, S Steckered to S3 enciphers to S1 Steckered to R at q.

The problem now is to find the core positions S1, S2 and S3. If these can be found then they are the Steckers of the menu letters.

But Turing realised that there was another way of looking at interconnected opened out Enigmas and that this way found Stecker connections.

Take the loop example above of R->N->S->R. Three opened out Enigmas are connected serially one to the other and the bottom drums are turned to the offsets g, p and q. If the correct drum order is being used then there will be some start position of the top, middle and bottom drums which corresponds to the actual original Enigma core rotor positions having allowed for the difference between the original Ringstellung and ZZZ. At this point the core rotor positions will be the same as the original Enigma core rotor positions and the encipherements will then be the same.

This means that a voltage placed onto the S1 input of the first opened out Enigma, which is the Stecker of the input R, will come out on the S2 terminal which is the Stecker of N. Since this is connected to the next opened out Enigma, this goes in on its S2 terminal and comes out on the S3 terminal which is the Stecker of S. This S3 input now goes through the third opened out Enigma and comes out at S1 which is the Stecker of R. Thus the drum positions correspond to the original Enigma positions where S1->S2->S3->S1.

The magic trick is now to connect the output terminals of the last opened out Enigma back to the input of the first Enigma. There is now a physical wired connection through the opened out Enigmas from the S1 input terminal to the S1 output terminal which is now connected to the S1 input terminal. This forms a loop of wire not connected to any other terminals on any opened out Enigma.

Thus if a voltage is placed on S1 at the input it goes nowhere else, just appears on the S1, S2 and S3 terminals. If a strip of 26 lamps is connected at the joins between opened out Enigmas then the S1, S2 and S3 lamps will light confirming the voltage path through S1, S2 and S3.

Now comes Turing's really clever bit. If S1 is not known and the voltage is placed on, say, A then this voltage will propagate through the opened out Enigmas because they are joined around from output to input, but CANNOT reach the S1, S2, S3 loop because it is not connected to any other terminals. The voltage runs around the wires inside the opened out Enigmas until it reaches a terminal which already has the voltage on it. The complete vastly complex electrical network has then reached a steady state.

Now if the lamp strip is connected at the joins of the opened out Enigmas, lots of lamps will light showing where the voltage has reached various terminals, but the appropriate S1, S2 and S3 lamps will not light. In favourable circumstances 25 of the lamps will light. The unlit lamp reveals the core letters, S1, S2 or S3. These are interpreted as the Steckers of the letters on the menu.

When the drum order and drum positions are correct compared to that of the original core Enigma encipherement there is just the one wired connection through the opened out Enigmas, at connections S1, S2 and S3. But Turing also realised that such a system of joined opened out Enigmas could rapidly reject positions of the drums which were not the correct ones.

If the drums are not in the correct position then the loop S1, S2, S3 does not exists and the voltage can propagate to these terminals as well. Thus it is possible for the voltage to reach all 26 terminals at the join of two of the opened out Enigmas. This implies that there is no possible Stecker letter and therefore this position of the drums cannot be correct. But because of the way the cross wiring inside real Enigma rotors is organised, closed loops of connections can occur which are not the loops corresponding to the actual Stecker connections being looked for. The configuration of opened out Enigmas cannot distinguish between these spurious loops and the correct Stecker loop.

The test for a loop of possible Steckers at a particular drum order and rotor position is to see if either only one or 25 of the lamps are lit. If all 26 lamps light then this position can be rejected and this rejection can occur at very high speed. The voltage flows around the wires at nearly the speed of light so that the whole complex network stabilises in a few microseconds. What was required was some way of automating the changes of drum position for all the drums in synchronism and for rapidly sensing any reject situation.

The Bombe

In 1939 the only technology available for achieving electrical connections from rapidly changing drum positions was to use small wire brushes on the drums to make contact with fixed contacts on the Test Plate. This was a proven technology from punched card equipment. High speed relays were initially the only reliable devices for sensing the voltages on the interconnections. Thermionic valves were tried but were not reliable enough in 1939. Later, thyratron gas filled valves were used successfully and these were about 100 times faster than the high speed relays.

The British Tabulating Machine Co (BTM) had designed the opened out Enigmas and built the Test Plate. The project to now build a complete search engine, which became known as a Bombe, came under the direction of H H (Doc) Keen. The machine, known as Victory, was completed by March 1940 and delivered to Bletchley Park. It was first installed in one end of Hut 1. Now the work began on finding out how to use this new device. Results at first were not very encouraging. The difficulties in finding cribs meant that when a menu was constructed between intercepted enciphered text and a crib, it usually did not have enough loops to provide good rejection and therefore a large number of incorrect stops resulted.

The Diagonal Board.

Then Gordon Welchman came up with the idea of the diagonal board. This was an implementation of the simple fact that if B is Steckered to G then G is also Steckered to B. If 26 rows of 26 way connectors are stacked up, then any connection point can be referenced by its row letter and column letter. A physical piece of wire can now connect row B element G to row G element B. The device was called a Diagonal Board because such a piece of wire is diagonally across the matrix of connections.

Now the double ended Enigma configuration knows nothing about Steckers. It can only deduce rotor core wiring positions which satisfy the menu. However the possible Steckers such as R<->S1, can by exploited by the Diagonal Board. If the joins between double ended Enigmas are also connected into the Diagonal Board at the position corresponding to the original cipher / plain text pair on the menu, say R, then this can significantly increase the rejection of incorrect double ended Enigma drum positions.

It has already been shown that if a set of drum positions has been found where S1->S2->S3->S1 then a physical wired connection has been made through the joins between opened out Enigmas at S1, S2 & S3. The deduction from this is that R is Steckered to S1 etc. Now if the join representing R on the menu is plugged to the R row of the Diagonal Board, a physical piece of wire will connect through the Diagonal Board from row R at position S1 to row S1 at position R. Since S1 is not plugged to anything the voltage on this wire goes nowhere else. Similarly for the other joining positions between opened out Enigmas. Thus the Diagonal Board does not affect the finding of the correct drum positions.

But if the drums are not in the correct position to make the connection S1, S2 & S3, then a voltage travelling around the network and finally arriving at say row N position S will be passed via the Diagonal Board wire to row S position N and will thus continue through the wiring in the opened out Enigmas on both sides of the join S. The Diagonal Board thus greatly contributes to the voltage flow around the network of wires in the opened out Enigmas due to the extra connectivity that it provides. This increases the rejection of drum positions which do not satisfy the menu.

This page was originally created by the late Tony Sale the original curator of the Bletchley Park Museum and is currently being hosted by Rich Sale Limited.