Difference between revisions of "Enigma machine"

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[[Image:Enigma.jpg|thumb|right|Enigma German Cipher Machine (Stripp). ]]
 
[[Image:Enigma.jpg|thumb|right|Enigma German Cipher Machine (Stripp). ]]
  
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[[Image:Bletchley.jpg|thumb|left|Bletchley Park, England, home to Britain's codebreakers during World War II (Pincock 98). ]]
 
[[Image:Bletchley.jpg|thumb|left|Bletchley Park, England, home to Britain's codebreakers during World War II (Pincock 98). ]]
  
The Enigma machine was the German’s secret force during World War II; however, it was the research performed at Bletchley Park that put an end to the technologies use. Bletchley Park, “a country estate used by the British Government Code &amp; Cypher School during World War II,” was home to many brilliant minds during the 1930s and late 1940s. Project ULTRA was developed with the mission of decoding the impossible. This project was successful largely because of Alan Turing with the result being that by 1943 the codebreakers at the British Government Code &amp; Cypher School could intercept and decipher German messages at about speed of the intended recipients (Newton 34). However, one cannot thank Turing without first acknowledging Hans-Thilo Schmidt first. For a long time Schmidt, a German native, had been a spy for the French although his most fruitful betrayal was the information he passed on to the French about the Enigma machine (Newton 251). After the Polish cryptanalysts got hold of the information, which later ended up at the British Government Code &amp; Cypher School, the mystery of the enigma came to an end by as early as 1940.   
+
The Enigma machine was the German’s secret force during World War II; however, it was the research performed at Bletchley Park that put an end to the technologies use. Bletchley Park, “a country estate used by the British Government Code & Cypher School during World War II,” was home to many brilliant minds during the 1930s and late 1940s. Project ULTRA was developed with the mission of decoding the impossible. This project was successful largely because of Alan Turing with the result being that by 1943 the codebreakers at the British Government Code & Cypher School could intercept and decipher German messages at about speed of the intended recipients (Newton 34). However, one cannot thank Turing without first acknowledging Hans-Thilo Schmidt first. For a long time Schmidt, a German native, had been a spy for the French although his most fruitful betrayal was the information he passed on to the French about the Enigma machine (Newton 251). After the Polish cryptanalysts got hold of the information, which later ended up at the British Government Code & Cypher School, the mystery of the enigma came to an end by as early as 1940.   
  
  
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&lt;i&gt;Radio operator: We have often cracked the British code, during the Norwegian campaign for example, but they will never crack the code we had in the Navy.  It’s absolutely impossible to crack.
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<i>Radio operator: We have often cracked the British code, during the Norwegian campaign for example, but they will never crack the code we had in the Navy.  It’s absolutely impossible to crack.
  
 
Abwehr commando: Everyone says that of their own code.
 
Abwehr commando: Everyone says that of their own code.
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Abwehr commando: Oh, that’s just one of those silly ideas people have.
 
Abwehr commando: Oh, that’s just one of those silly ideas people have.
  
Radio operator: No. &lt;/i&gt;
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Radio operator: No. </i>
  
 
(Mulligan 75)
 
(Mulligan 75)
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==References==
 
==References==
  
*Burchard, Hank. &quot;Reading Hitler's Mail.&quot; The Washington Post 27 Mar. 1981: W5.
+
*Burchard, Hank. "Reading Hitler's Mail." The Washington Post 27 Mar. 1981: W5.
  
*&quot;Codemaking and Breaking: The Enigma Machine.&quot; Planet Science / Out There. Planet  
+
*"Codemaking and Breaking: The Enigma Machine." Planet Science / Out There. Planet  
Science. 23 Sept. 2008 &lt;http://www.scienceyear.com/outthere/index.html?page=/outthere/spy/index.html&gt;.
+
Science. 23 Sept. 2008 <http://www.scienceyear.com/outthere/index.html?page=/outthere/spy/index.html>.
  
*&quot;Enigma Exhibit.&quot; The Enigma Exhibit. National Security Agency. 23 Sept. 2008 &lt;http://www.nsa.gov/museum/museu00007.cfm&gt;.
+
*"Enigma Exhibit." The Enigma Exhibit. National Security Agency. 23 Sept. 2008 <http://www.nsa.gov/museum/museu00007.cfm>.
  
*Hofstadter, Douglas R. &quot;Mind, Body and Machine.&quot; The New York Times 13 Nov. 1983.
+
*Hofstadter, Douglas R. "Mind, Body and Machine." The New York Times 13 Nov. 1983.
  
*Kahn, David. &quot;Codebreaking in World Wars I and II: The Major Successes and Failures, Their Causes and Their Effects.&quot; The Historical Journal 23 (1980): 617-39.
+
*Kahn, David. "Codebreaking in World Wars I and II: The Major Successes and Failures, Their Causes and Their Effects." The Historical Journal 23 (1980): 617-39.
  
 
*Kittler, Friedrich A. Gramophone, Film, Typewriter. Stanford, CA: Stanford UP, 1999.
 
*Kittler, Friedrich A. Gramophone, Film, Typewriter. Stanford, CA: Stanford UP, 1999.
Line 120: Line 112:
 
History, NSA, 2001.
 
History, NSA, 2001.
  
*Mulligan, Timothy. &quot;The German Navy Evaluates Its Cryptographic Security, October 1941.&quot; Military Affairs 49 (1985): 75-79.
+
*Mulligan, Timothy. "The German Navy Evaluates Its Cryptographic Security, October 1941." Military Affairs 49 (1985): 75-79.
  
*Newton, David E. &quot;Bletchley Park.&quot; Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 34-35.
+
*Newton, David E. "Bletchley Park." Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 34-35.
  
*Newton, David E. &quot;Enigma.&quot; Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 99-100.
+
*Newton, David E. "Enigma." Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 99-100.
  
*Newton, David E. &quot;Scherbius, Arthur.&quot; Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 250-251.
+
*Newton, David E. "Scherbius, Arthur." Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 250-251.
  
*Newton, David E. &quot;Schmidt, Hans-Thilo.&quot; Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 251.
+
*Newton, David E. "Schmidt, Hans-Thilo." Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 251.
  
*Pincock, Stephen. Codebreaker : The History of Codes and Ciphers, from the Ancient Pharaohs to Quantum Cryptography. Boston: Walker &amp; Company, 2006. 98-99.
+
*Pincock, Stephen. Codebreaker : The History of Codes and Ciphers, from the Ancient Pharaohs to Quantum Cryptography. Boston: Walker & Company, 2006. 98-99.
  
 
*Ratcliff, R. A. Delusions of Intelligence: Enigma, Ultra, and the End of Secure Ciphers. New York, NY: Cambridge UP, 2006. 11-32.  
 
*Ratcliff, R. A. Delusions of Intelligence: Enigma, Ultra, and the End of Secure Ciphers. New York, NY: Cambridge UP, 2006. 11-32.  
  
*Rohwer, Jurgen. &quot;Signal Intelligence and World War II: The Unfolding Story.&quot; The Journal of Military History 63 (1999): 939-51.
+
*Rohwer, Jurgen. "Signal Intelligence and World War II: The Unfolding Story." The Journal of Military History 63 (1999): 939-51.
  
*Sale, Tony. &quot;The translated 1940 Enigma General Procedure.&quot; The Enigma General  
+
*Sale, Tony. "The translated 1940 Enigma General Procedure." The Enigma General  
 
Procedure. 1940. Bletchley Park. 26 Sept. 2008  
 
Procedure. 1940. Bletchley Park. 26 Sept. 2008  
&lt;http://www.codesandciphers.org.uk/documents/egenproc/eniggnix.htm&gt;.
+
<http://www.codesandciphers.org.uk/documents/egenproc/eniggnix.htm>.
  
 
*Scherbius, Arthur. Ciphering Machine. Mesne Assignments, assignee. Patent 1657411. 1923.
 
*Scherbius, Arthur. Ciphering Machine. Mesne Assignments, assignee. Patent 1657411. 1923.
  
*Stripp, Alan. &quot;The Enigma Machine: Its mechanism and use.&quot; Codebreakers: The Inside Story of Bletchley Park. Ed. F. H. Hinsley and Alan Stripp. Oxford, England: Oxford UP, 1993. 83-88.
+
*Stripp, Alan. "The Enigma Machine: Its mechanism and use." Codebreakers: The Inside Story of Bletchley Park. Ed. F. H. Hinsley and Alan Stripp. Oxford, England: Oxford UP, 1993. 83-88.
  
 
*Winkel, Brian J., Cipher Deavors, and David Kahn, eds. The German Enigma Cipher Machine. New York: Artech House, Incorporated, 2005.
 
*Winkel, Brian J., Cipher Deavors, and David Kahn, eds. The German Enigma Cipher Machine. New York: Artech House, Incorporated, 2005.
  
 
[[Category:Dossier]]
 
[[Category:Dossier]]

Latest revision as of 14:54, 24 November 2010

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Enigma German Cipher Machine (Stripp).

History of the Enigma

The Enigma machine was invented by German electrical engineer and inventor Arthur Scherbius in 1918 in the wake of World War I and the newfound need for cipher machines that made encryption faster and more efficient (Newton 99). Originally intended to be sold commercially, Scherbius’s machine found no market for buyers. He then died in a fatal accident in the late 1920s (Winkel, Deavours, Kan, and Kruh 2-3). It was not until 1926, after several remodels by Scherbius and other German inventors, and the recent awareness that “British intelligence had been reading its code and cipher messages for many years” that the German navy aggressively pursued the machine for its enciphering and deciphering use (Newton 250). By 1928 the German army had adopted the Enigma machine and then in 1935 the German Air Force did so as well (Stripp 83).


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Bletchley Park, England, home to Britain's codebreakers during World War II (Pincock 98).

The Enigma machine was the German’s secret force during World War II; however, it was the research performed at Bletchley Park that put an end to the technologies use. Bletchley Park, “a country estate used by the British Government Code & Cypher School during World War II,” was home to many brilliant minds during the 1930s and late 1940s. Project ULTRA was developed with the mission of decoding the impossible. This project was successful largely because of Alan Turing with the result being that by 1943 the codebreakers at the British Government Code & Cypher School could intercept and decipher German messages at about speed of the intended recipients (Newton 34). However, one cannot thank Turing without first acknowledging Hans-Thilo Schmidt first. For a long time Schmidt, a German native, had been a spy for the French although his most fruitful betrayal was the information he passed on to the French about the Enigma machine (Newton 251). After the Polish cryptanalysts got hold of the information, which later ended up at the British Government Code & Cypher School, the mystery of the enigma came to an end by as early as 1940.


It was not until 1974 that any of the government knowledge surrounding the enigma machine was released to the general public (Newton 34).

Enigma Basics

The Enigma machine, patented in 1919, displays a keyboard of the twenty-six letters in the pattern of the normal German typewriter, although without numeral or punctuation keys (Stripp 83). It contains three basic parts: “a typewriterlike keyboard on which the plaintext is typed, an internal electromechanical system that converts plaintext to ciphertext, and a display system in which the ciphertext is displayed (Newton 99). The original 1918 Enigma machine contained three rotors, which is the area in charge of transcribing one letter to another and weighed more than one hundred pounds at fifteen inches high. Later editions adopted a more streamline appearance for simpler use and transportation, only weighing fifteen pounds at four inches in height (Newton 100).

Military Necessity

Codebreaking and encryption had not been an essential wartime tactic prior to 1914. However, World War I introduced a necessity for cryptology within the military, especially in regards to commands from high generals and other commanders (Kahn 621).

Pre-WWI Cryptology

During the days of the telegraph and radio, an enemy was capable of intercepting messages sent through the wires or airwaves. When messages were encrypted, breaking the code tended to involve a few brilliant men sitting around a table playing with the cryptogram until a message was configured. The most famous of which occurred during the very beginning of World War I with the Zimmerman Telegraph. The message, from Arthur Zimmermann, the German foreign minister, “proposed that Mexico declare war upon the United States, and that, upon victory, she regain the territories of Texas, New Mexico, and Arizona that she had lost in the Mexican-American War of 1846”. Delivered and published to America, the United States government was able to handle the situation before the Axis Power gained another Ally (Kahn 620). However, this system for cryptanalysis was not the best way to decrypt messages. There was no system for breaking codes, no rulebook or textbook to study in order to simply crack a code. As a result, not all codes could be broken in a timely fashion to be effective.

Post-WWI Cryptology

Most codebreaking occurred in such a fashion throughout the Great War. Battles were won and lost at times based solely on intelligence intercepted and decoded in time to be put to use. World War I demonstrated the importance that cryptology would soon serve in times of battle from then on, including present-day warfare. Even during the time of peace that followed the first World War, “many nations…set up permanent agencies for” cryptanalysis. Many nations had some form of an agency, however, not nearly as large as countries, such as Russia, France, and Italy. Germany, Britain, and America, on the other hand, were the only three major countries to not have such an agency before World War I that developed one soon afterwards. World War I also called for fixing the problem of “error-prone” and “time-consuming” cryptology systems done by hand. When the war was over, many cipher machines were invented and came out onto the market. Most were simple, involving pressing letters on a “typewriter-like keyboard, and the machine would automatically encipher the message.” (Winkel, Deavours, Kahn, Kruh 2). Mechanizing the encryption process made encoding messages, as well as decoding them, faster, more accurately, and more efficiently. Going into World War II, a more complicated cipher machine would be found in Arthur Scherbuius’s Enigma machine.

Enigma Machine: How it Works

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Basic Enigma wiring diagram (Ratcliff 15).

What made the Enigma machine so ingenious was, perhaps, its ability to deeply encode messages, almost instantaneously. Encoding a message was no more complicated than simply typing a message on a typewriter (Ratcliff 24). The machine enciphered messages electromagnetically (Ratcliff), and was usually powered by a 4.5 volt battery (Stripp 85). The parts of the machine that were used in the encoding process were the rotors, and a plug board on the front of the machine, called the steckerboard. The steckerboard was a simple plug board which could either be used or unused, or unsteckered. The steckerboard had 26 plugs for each letter, which could be connected by cables to couple certain letters (Stripp 85). Anywhere from 0-13 cables could be used for any given setting, however, usually only 10 cables were used, allowing for the maximum number of permutations for the key (Miller). Additionally, there were three out of five different rotors chosen each day, in a specific order. Each of these five different rotors had a different internal wiring that would move the current in a different manner. Each rotor also had 26 different positions from which the encoded message could begin (Stripp 85).


As each of the different letters of a message was pressed, a current would flow first through the steckerboard. Thus for example if ‘a’ was pressed the current would go to ‘a’ on the steckerboard, where, if the letter was not plugged to another, it would remain the same, or if the letter was plugged to another, the current would flow through, to the new letter (Stripp 85). Next, the current would move through the first rotor, where through the rotor’s internal wiring, it would exit at a different point, or letter, from which it entered. The current would do the same six more times, through the additional two rotors, through the reflector rotor (which was differently wired to ensure the letter would always be encoded and decoded the same way), and back through the three rotors again, lighting up a corresponding, coded, letter on the lamp board. Additionally, after any letter was pressed, the first rotor would turn one step to change the path of the current, so that if ‘a’ were pressed multiple times, it would not yield the same encoded letter. After the first rotor made the full trip around, the second rotor would make one turn, and so on (Ratcliff 15).

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Block diagram of the Enigma machine (Kittler 252).

Due to this vastly advanced encoding system, the numerical possibilities for the different codes was “astronomical.” Although it was not completely random, the near-randomness of the machine ensured that the usual manners for deciphering codes would be useless against the Enigma (Ratcliff 14-16). It is said that the number of possible combinations was 3 x 10^114 (Ratcliff 18). This vast number of various combinations had three different components. First, the steckerboard, which had 0-13 possible connections at a time, to connect different sockets, and connect different letters, each time the machine was set up. Second, was the three rotors, which were chosen and in what order. Finally, was the initial rotational position in which each rotor was set. In each of these combinations, with the rotors turning each time the key was pressed, a key must be pressed 16,900 times before the rotors returned to their original position (Stripp 86). For more on the numerical possibilities in code deviation, see Dr. A. Ray Miller’s “Cryptographic Mathematics of Enigma.”


Each day, the machine would be set up in a different way, which would change the code. The operator would receive orders containing three different essential pieces of information: Which wheels were used and in which order, the ring setting of each of the three wheels, and the steckering. He would then turn the three wheels randomly to a start point. Next, the operator would type out a randomly selected key of three letters, inputted twice, which would give an ‘indicator’ (the letters, as they appeared on the lamp board). He would then set his wheels to the letters typed to yield the lamp board. Each message transmitted would contain the indicator in the beginning to instruct the operator receiving the message in how to set his machine to decipher the message (Stripp 86-87). The receiving operator had his machine set up the same way, as per the daily orders, but he only knew where to start each rotor as per the indicator (Ratcliff 18).


Although the Enigma machine did provide an astronomical number of combinations of codes, there were some limitations. The steckerboard could only connect letters in pairs that denoted the other, thus ‘a’ would be substituted by ‘c’ and vice versa – ‘c’ could not be substituted by anything else in the steckerboard. Additionally, as used by the Nazis, the steckerboard always utilized 10 different connections, not more, and not less. And lastly, the steckers could not link sequential letters, thus ‘a’ could not be linked to ‘b’ and so on. These limitations diminished the number of combinations from 3 x 10^114 to 1 x 10^23.

Enigma Machine: How it was Used

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A simple encoding of a message under the standard Kriegsmarine cipher (Mulligan 77).

The Enigma machine was used by the German Navy to disguise their communications as gibberish to avoid allied spies from retrieving sensitive information. The device required an intensely detailed set of actions to be operated properly. This placed heavy importance on the Enigma machine’s instruction manual (which went through several different editions). Without it, the machine was practically useless. The machine was to be kept locked up and separated from the cipher wheels when not in use. The key to the enigma and wheel boxes were to be held by an officer. The uses of the Enigma Machine can be divided into two main categories: Enciphering and Deciphering.

Enciphering

Enciphering involves the actions related to scramble the message. The 1940 Instruction manual, Titled: “The Enigma General Procedure”, explained in detail the intricacies involved in properly enciphering a message. The Germans employed a number of strategies to make it harder to figure out what the messages said. These strategies included: using abbreviations and short hand for standard information such as position, course and speed; repeating important words such as “proper names, unusual geographical terms, words from foreign languages, etc;” (“The Enigma General Procedure” 4) using ‘clarifying tag’ words to distinguish between words that possess multiple meanings (“The Enigma General Procedure” 4); repeating letters to distinguish between service stations with similar names; inserting useless ‘dummy’ letters and irrelevant words throughout the message in order to confuse the enemy; dividing long messages into at least two parts and having the word FORT (short for Fortsetzung= continuation) at the end of each part besides the final installment (“The Enigma General Procedure” 8), as well as other techniques. Messages sent by high officials could only be read and deciphered by officers of equal or higher rank. The Enigma machine would handle the rest of the ciphering process. The cipher wheels would systematically scramble each letter of the message through a three-step process.


Deciphering

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General Guderian on the Enigma in his general's tank (Kittler 254).

Deciphering involves the actions related to unscramble the message. The process was essentially the inverse of enciphering. A team of usually two people were involved: a cipherer and a copyist. Both the cipherer and the copyist had to read the message. The cipherer keys, the message into the enigma machine, and the copyist writes down the deciphered message on a piece of paper. It was forbidden to dictate the message aloud (“The Enigma General Procedure” 20). Those who are reading the message would have to take into account all of the tricks that were involved in the process of enciphering the message (see Enciphering). The deciphering team will have to consult a cipher table in order to identify “the cipher of the day” which can be identified by the indicator group (which is represented by a three-letter group within the message). Each day, a different set of indicator groups become available so as to make it difficult to decipher. Once the message has been deciphered, it can then be delivered to its proper destination.

Aftermath

The Enigma machine, was, at once the greatest strength as well as the greatest weakness for the Nazis. For a long time during the war, they were able to completely conceal their communications from the Allies. Once the Allies cracked the code, however, the machine became a liability, as due to their hubris, the Nazis were unwilling to face the fact that their code was fallible. This can perhaps be seen by this exchange between two German POW’s, recorded by British interrogators:


Radio operator: We have often cracked the British code, during the Norwegian campaign for example, but they will never crack the code we had in the Navy. It’s absolutely impossible to crack.

Abwehr commando: Everyone says that of their own code.

Radio operator: What! They can’t crack it.

Abwehr commando: There’s only one method that can’t be deciphered and even that can be deciphered by expert mathematicians; I think they can break a code in the course of two years….

Radio operator: No, they can’t crack it.

Abwehr commando: Oh, that’s just one of those silly ideas people have.

Radio operator: No.

(Mulligan 75)

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Codebreakers at work in the machine room of hut 6, Bletchley Park, 1943 (Pincock 99).

After the code was broken, the Nazis were unwilling to see the evidence of it as a breaking of the code rather, they assumed that the British simply had exceptional spies who were leaking information. Thus they concentrated their efforts on finding these spies, instead of adapting their code further (Hofstadter 2).


Because of the Enigma’s vast number of possible code combinations, the breaking of the code required collaboration between the allies. Most of the efforts were by British and Polish mathematicians at Bletchley Park in England, under the codename Project ULTRA. These mathematicians used both high-speed machine technology as well as hand testing, to crack the code. By the early 1940’s, these mathematicians could already crack the code, however, at first it would take weeks to crack a single message, far too late to be of any help strategically. By 1943, they would be able to decode a message in minutes, and would finally be able to use the information therein. This is essentially the moment that the Enigma machine’s effectiveness was terminated. And in the end, the Nazis insistence on maintaining their use of Enigma may have been what cost them the war.


As Ratcliff posits, “Enigma…demonstrates how a new technology can quickly move from startlingly revolutionary to so familiar that its operators fall into complacency” (12).

References

  • Burchard, Hank. "Reading Hitler's Mail." The Washington Post 27 Mar. 1981: W5.
  • "Codemaking and Breaking: The Enigma Machine." Planet Science / Out There. Planet

Science. 23 Sept. 2008 <http://www.scienceyear.com/outthere/index.html?page=/outthere/spy/index.html>.

  • Hofstadter, Douglas R. "Mind, Body and Machine." The New York Times 13 Nov. 1983.
  • Kahn, David. "Codebreaking in World Wars I and II: The Major Successes and Failures, Their Causes and Their Effects." The Historical Journal 23 (1980): 617-39.
  • Kittler, Friedrich A. Gramophone, Film, Typewriter. Stanford, CA: Stanford UP, 1999.

252-54.

  • Miller, A. Ray. The Cryptographic Mathematics of Enigma. Center for Cryptologic

History, NSA, 2001.

  • Mulligan, Timothy. "The German Navy Evaluates Its Cryptographic Security, October 1941." Military Affairs 49 (1985): 75-79.
  • Newton, David E. "Bletchley Park." Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 34-35.
  • Newton, David E. "Enigma." Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 99-100.
  • Newton, David E. "Scherbius, Arthur." Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 250-251.
  • Newton, David E. "Schmidt, Hans-Thilo." Encyclopedia of Cryptology. Santa Barbara, CA: ABC-Clio Inc, 1997. 251.
  • Pincock, Stephen. Codebreaker : The History of Codes and Ciphers, from the Ancient Pharaohs to Quantum Cryptography. Boston: Walker & Company, 2006. 98-99.
  • Ratcliff, R. A. Delusions of Intelligence: Enigma, Ultra, and the End of Secure Ciphers. New York, NY: Cambridge UP, 2006. 11-32.
  • Rohwer, Jurgen. "Signal Intelligence and World War II: The Unfolding Story." The Journal of Military History 63 (1999): 939-51.
  • Sale, Tony. "The translated 1940 Enigma General Procedure." The Enigma General

Procedure. 1940. Bletchley Park. 26 Sept. 2008 <http://www.codesandciphers.org.uk/documents/egenproc/eniggnix.htm>.

  • Scherbius, Arthur. Ciphering Machine. Mesne Assignments, assignee. Patent 1657411. 1923.
  • Stripp, Alan. "The Enigma Machine: Its mechanism and use." Codebreakers: The Inside Story of Bletchley Park. Ed. F. H. Hinsley and Alan Stripp. Oxford, England: Oxford UP, 1993. 83-88.
  • Winkel, Brian J., Cipher Deavors, and David Kahn, eds. The German Enigma Cipher Machine. New York: Artech House, Incorporated, 2005.