ENIGMA Technology and the History of Computers

LaptopEnigmaTom Perera, Ph.D. – EnigmaMuseum.com

December 20, 2016
Copyright © 2016 by Tom Perera, Ph.D.



The German Enigma cipher machine is widely recognized for its important role in world history. Allied deciphering of Enigma-enciphered messages revealed military secrets and helped shorten WW-II. The complex devices developed to decipher Enigma messages are often acknowledged as being early computers and despite the lack of universal agreement on the definition of a computer, they are included in most published histories of computing. However, because of the secrecy, scarcity, and complexity of the Enigma, few people have studied its technology and operation. Consequently, the Enigma itself does not appear to have been considered to be part of the history of computers.

This paper will explain the technology of the Enigma and conclude that it preceded and laid some of the foundations for modern computers. Although there is widespread disagreement about the definition of a computer, (George Ifrah, 1994, The Universal History of Computing) this paper will take the position that the Enigma was a very early form of computer and that the Enigma should take its rightful place in the history of computing in addition to its well known place in world history.

ENIGMA Technology and the History of Computers

It is amusing to note that the Enigma was designed to ‘make’ problems for people. It confounded and confused people by converting plaintext letters into ciphertext letters in one of an unguessably-large 3.28×10^114 different ways.        Most subsequent computers were designed to ‘solve’ problems for people.

The history of the computer has been chronicled in many places. The website http://www.computerhope.com/issues/ch000984.htm and various Wikipedia articles give nice general timelines. There are literally hundreds of books on the history of computing and the book From Abacus to Smartphone: The Evolution of Mobile and Portable Computing by Evan Koblentz (http://abacustosmartphone.com) is particularly relevant because it specifically discusses the history of the development of portable self-contained computers that are similar to the Enigma.

None of these books or articles mentions the Enigma in any of their evolutionary timelines. This is not surprising, because the mere existence of the Enigma was a carefully guarded secret throughout WW-II and for 30 years after the end of the war. In addition, the German High Command ordered the destruction of all Enigmas in the final days of the war, and then Churchill ordered the destruction of Enigmas after the war.  Virtually all of the records of the design and construction of Enigmas were also destroyed during the war. Historians estimate that only a few hundred Enigmas have survived. Those few Enigmas that remain are highly prized by collectors and historians.

The scarcity of Enigmas has made them virtually inaccessible to most historians. Those few Enigmas that survived the war are generally displayed in sealed cases in museums or in private collections where they remain inaccessible to scholars who may want to explore their technology and operation. The author has been hunting for, studying and restoring Enigmas for the past 30 years and has had the unique opportunity to explore their technology in considerable detail.

There are several books and websites that explain the technology of the Enigma. The websites http://cryptomuseum.com/crypto/enigma/index.htm and http://ciphermachines.com/enigma give very fine complete and well-illustrated descriptions of the technology of the Enigma. The book INSIDE ENIGMA: The Secrets of the Enigma Machine and Other Historic Cipher Machines ( http://EnigmaMuseum.com/iead.htm) by the author describes the internal technology but this information is seldom if ever mentioned or referenced by computer historians.

In addition, the terminology that cryptologists use to define and explain the Enigma does not lead a reader to consider it to be a form of computer. For example, cryptologists would describe an Enigma as an enciphering and deciphering device with a cycle-length of 17,576, 9 levels of encryption and a keyspace of 3.28 x 10 ^ 114. Whereas in computer jargon it is a programmable stored-program data processor with 17,576 sequential 9-step programmable operations and 3.28 x 10 ^ 114 possible programs.

The result has been that although many computer historians have recognized the devices used to crack the Enigma code as being, in one form or another, early computers, they have failed to acknowledge that the Enigma itself was an early form of computer with technology that preceded all of these designs.  Many historians are aware of the work of Marian Rejewski and his team of Polish mathematicians as they designed the first Enigma-deciphering computers which they constructed specifically to crack the Enigma code and the subsequent work of Alan Turing to improve these computers. However, no historians have suggested that the very machine these early computers were designed to analyze should be studied as an early form of computer.

Thousands of people have tried to define the characteristics of a computer, but to date there is no universally accepted definition. The best that can be done at present is to explore the history, distill the various definitions and look for common characteristics. The first use of the word computer can be traced back to 1613 and referred to a person who does a calculation. Later, a computer was a mechanical device like an abacus or slide rule that could perform a calculation or more complex computation. The Zuse Z1 designed by Konrad Zuse in 1936 is often called the first electromechanical computer. Today most people consider a computer to be an electronic device that must have at the very least an input device, a programmable stored-program data processing unit, and an output device. 

Because they were shrouded in secrecy from 1933 through 1975, and not widely studied after that, the Enigma deciphering devices that were designed by Marian Rejewski and his team of Polish cryptologists have only recently begun to receive recognition for their place in computer history. In 1933 Rejewski’s “Zyklometer” and later “Bombe” were the first fully functional machines to allow reading of Enigma-encoded messages but they have been overlooked by most computer historians.

The German cipher machine ENIGMA was first patented in 1918 in Germany by Arthur Scherbius. Its design included all of the characteristics that are generally accepted as being necessary in a computer, and by 1926 it had evolved into a totally self-contained, very portable machine that embodied those same characteristics. It was used from 1926 to 1945 with only minor design changes. It had an input device, a programmable stored-program data processing unit, and an output device.  It preceded by 7 years all other such machines and therefore deserves to take its rightful place in computer history. In addition, the technology of the Enigma deserves to be studied as the origin of many of the technologies employed in modern computers.

The history and technology of the Enigma is very clearly presented by Ralph Simpson in his website http://ciphermachines.com/enigma and in the diagram and adapted explanation from his website presented below.

Rather than repeat his detailed explanations here, I will explain the operation of the Enigma in terms that relate to computer history instead of the specialized terms usually used by cryptologists.

The input and output systems require little clarification. The input device is a 26-letter keyboard, and the output device is a simple panel of 26 lightbulbs that illuminate one of the 26 letters of the alphabet.

The data processing unit requires a more detailed explanation. It accepts one of the 26 letters from the keyboard and then performs the first of a programmable sequence of 17,576 9-step programmable operations on the letter before sending the final resultant letter to the output panel, where it illuminates a specific letter.

Each of the 17,576 9-step sequential  operations involves a specific method of converting the incoming letter into a different outgoing letter, and each of these 17,576 9-step sequential operations, or conversions, may be programmed in 3.28 x 10 ^ 114 power different ways.

For example, a keypress initiates the first of the sequence of 9 sequential operations. This first operation is performed by the plugboard, which converts an incoming letter into another letter or itself. Using the analysis presented by Enigma historian Ralph Simpson, it can be seen that there are a total of 532,985,208,200,576 possible ways that the plugboard can be programmed to convert the incoming letter into another letter by using its 26 sockets and the 0 to 13 patch cables.

Each of the remaining 8 sequential operations is also capable of being programmed in a large number of possible ways as shown in Simpson’s analysis. They include operations 2, 3, and 4, performed by the 3 rotors; operation 5, performed by the reflector; operations 6, 7, and 8, performed by the 3 rotors; and operation 9, performed by the plugboard.

The programming of each operation is performed through a manual procedure or setting, or through hard-wiring. The total number of programmable variations of the 17,576 sequential 9-step programmable operations is 3.28 x 10 ^ 114.

The technology is basically quite simple. An internal 4.5-volt battery supplies the voltage that is switched into the circuit by a keyboard key switch.  Each time a key is pressed the voltage undergoes 9 programmable conversions and eventually lights up the bulb that illuminates the output letter.  The total number of key presses that can occur before the program repeats itself is 17,576.

You may follow the sequence of programmed operations quite easily in the following diagram created by Ralph Simpson.

Enigma wiring diagram

This Enigma wiring diagram shows an example that starts with the “A” key being pressed on the keyboard. The path of the electric battery voltage is highlighted in white lines and arrows that continue until the “H” lights up on the lightbulb panel.

The “A” voltage is first converted to an “M” on the plugboard, and then the “M” is sent to the rotor assembly. The “M” is then converted 3 times as it passes through the 3 rotors, once more going through the reflector and 3 more times going through the 3 rotors in reverse. At this point the letter coming into the plugboard is an “O”, which is converted by the plugboard into an “H”. It is sent on through the normally-closed contacts of the “H” key and then to the “H” bulb on the lightbulb panel.

(Adapted from Ralph Simpson: http://ciphermachines.com/enigma, where you can see a more detailed description of the Enigma and a video of these events.)

To summarize this diagram, the Enigma passes alphabetic information through 9 sequential programmable operations that convert the information 9 times and produce an alphabetical output. Each time a key is pressed the rotors rotate one notch and the program goes on to the next 9-step set of operations.  After 17,576 keypresses the program repeats itself.

The above description of how the Enigma works conforms to the generally accepted definition of a computer, which is an electrical device with an input, a programmable stored-program data processing unit and an output device. Since the Enigma preceded Marian Rejewski’s 1933 Zyklometer, his Bombe, the 1936 Zuse Z1, Turing’s Universal Computing machine and all other such electromechanical and electronic devices it clearly deserves a significant place in the history of early computers.

Computer historians may find it worthwhile to trace the various technologies designed into the Enigma.  Some of them were incorporated in the design of later computers and then evolved and changed as new technologies were developed.

For example, the typewriter-style keyboard of the Enigma is similar to those in the most modern computers.  However, unlike today’s computers the keyboard was designed in such a way that each key had a dual function. Notice that when you press the “A” key in the diagram above, the activated (normally-open) contact connects the battery voltage to the rest of the circuit that leads through the non-activated (normally-closed) contact of the “H” key before it illuminates the “H” letter on the lightbulb panel. This allows the Enigma to process the letter “A” into the letter “H”.  Notice also that if you press the “H” key, the activated “H” contact connects the battery voltage to the circuit that eventually leads through the normally-closed contacts of the “A” key before it lights up the “A” lightbulb.  This interesting dual-use of each keyboard key allows fully reciprocal data processing as long as the sequence of the 17,576 9-step operations is returned to the original starting point.

The output system of the Enigma with its individually bulb-illuminated letters in a display panel may be seen in many early data processors.  This type of panel was gradually replaced by multi-segment displays, printers, and most recently by the video display panels found in most modern computers.

The central processing unit of the Enigma will require more extensive study with respect to how its technology may have contributed to the designs of Rejewski’s, Turing’s and then modern computers.

The basic principle of the Enigma data processing unit involved providing a programmable sequence of 17,576 9-step individually-programmable operations. Each of these operations would transform the incoming data and then send it on to the next operation and finally to the output panel. This concept of sequential data processing has been central to the design of most computers and has only recently been expanded to include parallel processing in which several simultaneous sequential processes can occur at the same time.

The technologies used to program a computer have undergone the most dramatic changes over time. Enigmas required manual programming of plugboard connections, rotor ring positions, rotor positions along their shaft, and starting positions of rotors. As computers continued to evolve, this programming was more and more controlled by the keyboard and later by the computer’s internal memory and mass storage devices.

One of the characteristics that some people consider essential in a computer is ‘conditional branching’.  A stored program instructs a hardware-based operation to perform branching dependent on an input.  Branching is under the complete control of the program and perhaps should be called: programmed conditional branching.

In the Enigma, programmed conditional branching is performed by each of the 9 hardware-based steps. For example, each rotor receives one of the 26 letters of the alphabet and its wiring branches to send a voltage out along a different one of the 26 pathways to the next operation. The decision about which output pathway to activate is under the control of the program.

The Enigma was first produced in its portable version in 1926. It was self-contained and had its own internal battery so it is clear that it preceded other known portable computers and may be considered to be an early and original form of laptop, notebook or tablet computer.

The Enigma embodied some historically interesting technological innovations for portable computing.  For example, the on-off switch included a position that decreased the voltage applied to the bulbs when the battery was fresh and as the battery became discharged a different switch position could apply the full remaining battery voltage to the bulbs.  This manual form of voltage regulation has led to the more and more automated battery voltage regulation seen in modern portable computers. The foldable top cover, integrated protective case, keyboard and display located on the same panel, replaceable battery, reflection-reducing filter, spare parts holders in the lid and connectors for the application of external power were also important innovations, some of which persist to this day.

It is hoped that the preceding introduction to Enigma technology will increase appreciation of the role of the Enigma in the history of computing and stimulate research and discussion about how Enigma technology laid some of the foundations for modern computers.