Enigma and the Bombe

Lecture by Tony Sale

Tony Sale's
Codes and Ciphers

This page is in the form of Lecture Notes and has not been fully edited for Web publication.

Alan Turing.

The mathematician Alan Turing had been identified, at Cambridge, as a likely candidate for code breaking. He came to the Government Codes & Ciphers School, (GC&CS), in Broadway in London a number of times in early 1938 to be shown what had already been achieved. He was shown the rodding method and some intercepts of German signals enciphered on the German forces Enigma which had the stecker board. Dilly Knox already knew that the German forces Enigma rotors were wired differently to the commercial rotors, did not know the entry rotor order and apparently did not know of the double encipherement of the message key.

Turing began, independently, to discover the same characteristic patterns as Rejewski discovered but Turing's were based on possible German words from which the intercepted cipher text had been produced rather than the repeated message key which Rejewski used.


An early method for decoding Enigma messages was called "Rodding".

It is described in Turing's "Treatise on Enigma" which he wrote, probably in 1940.

Turing's description is not very clearly written, but after a considerable amount of effort it can now be demonstrated.(in a paper still to be completed)

Rodding is basically a paper method involving representing the Enigma wheels by strips of paper or card. Turing called these "comic strips". A colour coding was used to identify the Enigma wheels.

Letter Pairs

The method based on probable words was a far more powerful method than Rejewski's and led later to the development of the Turing Bombe.

GC&CS already had a few intercepts and at least one plain text / ciphertext pair, reputed to have been smuggled to England by a Polish cipher clerk.

Among the characteristics that Turing had found was that occasionally the same cipher/plain text pair of characters occurred at different places in the same message. These were known as "clicks" in Bletchley Park.


Remember that because the Enigma machine is reversible, R->C is the same as C->R and M->E the same as E->M.

Whether such pairings occur is determined by the rotor order and the core rotor start positions. Turing realised that conversely the actual rotor order and core rotor start position could be arrived at by trying all configurations to see if these pairings were satisfied. This would only work for an unsteckered Enigma or for a Steckered Enigma in which C and R were unsteckered. In the early days of Enigma, only six letters were Steckered so this could happen.

Obviously just setting up a single Enigma machine and trying by keying in would take an impossibly long time. The next step was to consider how the tests could carried out simultaneously for a particular Enigma start configuration.

Testing for letter pairs required a method for rapidly determining whether such a configuration was true or false. This led to the concept of electrically connecting together a number of Enigma machines.

This was achieved by using an "opened out" Enigma.

In the actual Enigma electrical current enters and leaves by the fixed entry rotor because of the reflector or Umkerwaltz (U) and this precluded connecting Enigmas together. In Turing's opened out Enigma the reflector had two sides, the exit side being connected to three rotors representing the reverse current paths through the actual Enigma rotors. This gave separate input and output connections and thus allowed a number of Enigmas to be connected in series.

In the Letchworth (so called because the British Tabulating Machine factory which made them was in the town of Letchworth), implementation, the clever thing was to include both forward and backward wiring of an Enigma rotor in one drum. The connections from one drum to the next were by four concentric circles of 26 fixed contacts and four concentric sets of wire brushes on the drum. Three sets of fixed contacts were permanently wired together and to the 26 way input and output connectors. Three drums, representing the original Enigma rotors, could now be placed on shafts over the contacts and this was an opened out Enigma with separate input and output connectors.

To return to the problem of checking whether C enciphers to R, ( written as C->R ), first an offset reference from the start is required. A lower case alphabet written over the cipher text gives this.


This shows that C->R at offset c, e and l from the start and M->E at j, k and n. The opened out Enigma allows an electric voltage to be applied to the input connection "C" and a set of 26 lamps to be connected to the output connector. If the R lamp lights then the drums are in an order and position such that C enciphers to R.

With a single Enigma this can occur at a vast number of drum settings.

However the crib allows an opened out Enigma to be set up for each occurrence of C->R and they can all be tested simultaneously.

The opened out Enigmas are all set up with the same drum order and the drums are then turned to the same settings for the top (left hand) and middle drums but the bottom (right hand) drums are turned to the offset letter along the crib at which the test is to be made. All the inputs are connected in parallel and a voltage applied to the "C" contact. Then a set of relays connected to each of the "R" output contacts tests to see if all the R contacts have a voltage on at the same time. When they do a position of the drums has been found which satisfies the crib at the points chosen for C->R.

If they don't then all the bottom drums are advanced one position and the test is tried again. After 26 positions of the bottom drum the centre drum is advanced one position and this continues until all drum positions have been tested. Then the drums are changed to try a different drum order. A very long process by hand which obviously asks to be automated.

This can be achieved by an electric motor driving all the top drums simultaneously and then "carrying" to the middle drum every 26 positions, with a further carry from the middle to bottom rotor when this has turned through 26 positions. In this way the drums can be driven through all 17,576 possible positions and the occurrence of a correct position for all C->R in the crib can be checked.

But there are still a large number of positions which satisfy the C->R test.
What is needed is a better method for finding the rotor order and rotor setting.

Letter Loops.

An extension of the concept of letter pairs is where letters enciphered from one to another at different places in the crib resulting in loops of letters.


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 fractions of a microsecond. 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 created by Tony Sale
the original curator of the Bletchley Park Museum