Enigma Machine

Enigma Machine


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NEW YORK, NY -- After Germany’s defeat in the First World War, the provisions of the Treaty of Versailles of 28 June 1919, which included disarmament and the payment of major reparations, provoked a great deal of discontent in Germany. Though Hitler only announced that Germany would rearm (in violation of the Treaty) in March of 1935, in reality rearmament had been covertly in progress almost before the ink was dry. The push towards reestablishing military power gained considerable momentum after Hitler and the NSDAP (Nazi) Party seized power in January of 1933.

As early as 1926, the Reichswehr, the predecessor to the Wehrmacht, had been interested in advanced means of covert communication. They (together with several other European militaries) were especially interested in the Enigma brand of cipher machines manufactured by Chiffriermaschinen Aktiengesellschaft (ChiMaAG). These products were based on the work of Arthur Scherbius, then on the board of the company.

His firm, Scherbius and Ritter, built the first model of the Enigma machine, the Enigma A (of which, apparently, no specimen now exists) in 1923. A hulking unit, which included a printing engine, it lacked the necessary level of encryption for military use. The first Enigma machine to incorporate removable rotors, a key development, was the Enigma D of 1926. The Enigma brand went through various recensions and improvements, culminating in the Enigma I (developed 1927); this was only available to the Reichswehr, who purchased exclusive rights to the machine. The manufacturer’s name changed again in 1934, to Chiffriermaschinengesellschaft Heimsoeth und Rinke, and large quantities of Enigma I machines were manufactured and purchased by the Reichswehr and subsequently the Wehrmacht (many of these machines were produced by other firms under license).

It is no coincidence that the interest on the part of the German military establishment in the Enigma machine parallels the rearmament of Germany, which grew apace in the early 1930s. A prerequisite of modern warfare is secure communication; indeed, communications must be not only secure, but also massively scalable, across multiple theaters and services. While there are theoretically perfect ciphers, impervious to decryption (the best example of which is the one-time pad) these are cumbersome in practice: hard to distribute, ill suited to field operations, and slow in use. The German military machine felt that the Enigma I machine was, practically speaking, impervious to decryption, and best suited its needs: this assumption ultimately proved to be fatally flawed.  

The Anatomy of the Enigma Machine

The Enigma machine is an electromechanical unit for converting plaintext into ciphertext, or (in the process of decrypting a message prepared with another Enigma machine) ciphertext to plaintext. It is used by a trained operator to prepare a message, which is then sent by telegraphy via radio or similar means. The Enigma itself just enciphers: it does not transmit. It is powered by a battery in a compartment in the rear of the machine, or by using an external power source, and it operates at about 4.5 volts with low amperage.

The core of the machine is the rotor assembly. The Enigma I had three wheels operational at any given time (usually, of five issued). The Wehrmacht issued these machines to the Heer, or land forces, and the Luftwaffe, the air force. The German Navy also used Enigma machines, first the unit designated the M3, then in 1942 the four-wheel M4 (which used three wheels from an eight-wheel set, together with an extra wheel, the Zusatzwalze).

Each wheel or rotor has a set of contacts and pins, together with a ring numbered 01-26, which corresponds to the letters a-z (the Naval machines used the alphabet, but the Wehrmacht operators used simple number-letter transposition). Each of these rings has 26 alterable ring settings (Ringstellung), adjustment of which reconfigures the passage of electricity through the ring. A thumbwheel for altering relative settings is affixed to the right of the wheel; in the earliest machines, these were metal, but later Bakelite plastic was used. The wheels are rotated by a ratchet and pawl mechanism, so that each letter pressed on the keyboard advances the rightmost wheel by one increment, or step, resetting the electrical path; every 26 presses the middle wheel advances one step; and finally the leftmost wheel will advance one step every 26 increments of the middle wheel, though given that messages were generally kept to 200 characters or less, this infrequently occurred, and this limitation is one of several weaknesses that were exploited by the teams involved in decrypting German military traffic at Bletchley Park and elsewhere.

Flanking the rings are the entry wheel, the stator, or Eintrittswalze (on the right, viewed from the keyboard) and the reflector. on the left. These assemblies do not rotate, and are not field-programmable in the Enigma I machine. The three variables for encoding are the selection of wheels used, and their order; the alterable ring setting, with its 26 settings; and the relative positions of the numbers (or letters) on each wheel, referred to by Enigma operators as the Grundstellung. On the front of the machine is a plugboard (the Steckerbrett), into which patch cables (Stecker), with dual pins at either end, are inserted. This causes further combinatorial complexity, switching letters, so that an A might be routed to the “z” position, for example, if a cable was placed from the “a” to the “z ” position. Up to thirteen cables can be used, though the typical German practice was to use ten. Completing the machine is the input device, a keyboard (with the German QWERTZY layout). The output device is the lampboard, an array of 26 bulbs, each of which, when lit, illuminates a letter of the alphabet.

The Enigma Machine in Use

Over the period in which Enigma machines were in field use, the instructions to the personnel in charge of varied to some degree. In addition, the different branches of the German military services had a variety of procedures, enumerated in manuals that (because of their secret nature) have rarely survived, so it is difficult to speak in absolutely general terms of all the protocols which governed the use of the machines. One of the most comprehensive available manuals is the 1940 Naval example at Bletchley Park that was translated by the late Anthony Sale. See Manual

These comprehensive procedures are outside the scope of this article, so I have provided the link for those who wish to see the various provisions under which an Enigma operator worked. I provide a (relatively) simple guide to basic operation below.

All Enigma operators were provided with a codebook, which enumerated the various settings for the day (which began at midnight). These settings were, practically speaking, five in number.

1: Date: this governed which settings would be applied to the machine for the day, midnight to midnight.

2: Wheel order (Walzenlage): this dictated the rotor order for the given day. The three wheels sit in a metal spindle. Once tension is released with a spring arm on the left, by the reflector, the assembly of three wheels on their spindle can be lifted out. Each wheel has a Roman number on one surface, and they can be slid from the spindle and replaced in the specified order, e.g. III, V, I, though practically speaking, first:

3: Ring setting: the operator would turn the ring on each rotor to the specified number for the day e.g. 11  05 25. This setting is the Ringstellung or ring setting.

Once the rotors had their ring number set, and were placed in the correct order on the spindle and the unit replaced and locked back into place between reflector and entry wheel, after closing the case the next step would be to…

4. Set the plug order, or Steckerverbindungen, for the date with the patch cables. For brevity, I will assume five cables are in use, although as previously noted, up to thirteen could be used, and more typically, ten. These sequence would read (for example) BM  KL AR ZN PI.

5. Finally, the Kengruppen, or discriminant for the day is looked up.

Finally, the operator would enter three random letters, and the thumbwheels would be turned until the numbers for those letters showed uppermost on the rotors (in the three small viewing windows of the case lid).

There were a few more procedures unrelated to the functioning of the machine before a message could be enciphered. Typically a message would be 250 characters or fewer, and would include a variety of details including time, how many parts were in the message, the part-number, the number of characters in the message part, the random rotor start position and an encrypted message key. Some of this information (depending upon procedure) would be sent as unenciphered plaintext, transmitted in clear.

For the encrypted portion of the message, the operator would key in the plaintext letter by letter, noting which lamp was lit with each key press. It is this that sequence that constitutes the ciphertext to be transmitted.

It is worthy of note that part of standard operating procedure was the destruction of the machine to the extent possible if there was a risk of its capture. This is why few comparatively few military Enigma machines survive intact today and why, of those that do, relatively few are in fully functioning order.

What Went Wrong?

Famously, Admiral Kurt Fricke, Chief of Staff of the German Seekriegsleitung, the Maritime War Command stated, “All specialists unanimously agreed that a reading [of Enigma coded traffic] is impossible,” this following a singular naval debacle. As we now know, he was wrong. With a few breaks when machine procedures were changed, (the most notable and prolonged when the four-rotor naval machine was introduced in 1942) the Allies were able to read the bulk of the Enigma-encoded traffic of all the services, from early in the war.

The Enigma machine is an extraordinarily ingenious device that should be possible of providing a level of security that has been calculated as being equivalent (in digital terms) to a 76-bit key, under the best assumptions. There were, however, both mechanical and operational weaknesses. Before May 1940, for example, the encipherement of the message setting was repeated twice at the beginning of the message. This simple act of repetition was sufficient to enable a group of Polish cryptographers to break the Enigma cipher, using a “cryptologic bomb,” a specialized calculating machine that enabled them to establish the daily keys. They communicated this information to the French and British in July of 1939. Though after the change in procedure to a single repetition this was no longer possible, this preliminary foray into the cryptology of the Enigma on the part of the Poles gave the British team a head start.

Beyond bad cryptologic practice, there were some weaknesses in the machine itself. Though a full discussion of these exceeds the scope of this article, the following weaknesses should be noted.

1.    Because of the nature of the wiring of the machine, no letter is ever mapped to itself. In other words, while the letter “e” may be changed to any other letter by use of the Enigma machine, it will never remain unchanged in the ciphertext.

2.    The fact that the rotors advanced always in the same direction, and did not reverse, and that the third rotor rolled over very infrequently within a message both conspired to reduce the security substantially.

3.    “The Double Stepping Anomaly.” This refers to a mechanical issue inherent to the design that occurs with certain wheel positions when the middle and leftmost wheel step. This serves to reduce the cryptographic periodicity of the system, effectively providing a marker for such transitions.

Though these and other weaknesses exist, and were successfully exploited by the Allies (by some estimates, reducing the length of the war by two years), the fact remains that the Enigma machine was a phenomenally powerful and profoundly ingenious system of encipherement, one that demanded great ingenuity by an exceptionally gifted group of mathematicians to solve, and that this was done (under extremely difficult conditions) in an era before digital computing. Both the Enigma machine and its decipherment were extraordinary endeavors.

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