computer science

World War II cryptography

World War II cryptography is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand World War II cryptography rather than just read about it. In short: Cryptography was used extensively during World War II because of the importance of radio communication and the ease of radio interception. The nations involved fielded a plethora of code and cipher systems, many of the latter using rotor machines.

Key takeaways

World War II cryptography belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect World War II cryptography to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of World War II cryptography from memory before moving on to harder problems.

Reference excerpt

Cryptography was used extensively during World War II because of the importance of radio communication and the ease of radio interception. The nations involved fielded a plethora of code and cipher systems, many of the latter using rotor machines. As a result, the theoretical and practical aspects of cryptanalysis, or codebreaking, were much advanced. Most of the codes used in the war were eventually broken by the enemy, with consequences ranging from trivial to crucial. Possibly the most important codebreaking event of the war was the successful decryption by the Allies of the German "Enigma" Cipher. The first break into Enigma was accomplished by Polish Cipher Bureau around 1932; the techniques and insights used were passed to the French and British Allies just before the outbreak of the war in 1939. They were substantially improved by British efforts at Bletchley Park during the war. Decryption of the Enigma Cipher allowed the Allies to read important parts of German radio traffic on important networks and was an invaluable source of military intelligence throughout the war. Intelligence from this source and other high level sources, such as Cryptanalysis of the Lorenz cipher, was eventually called Ultra. A similar break into the most secure Japanese diplomatic cipher, designated Purple by the US Army Signals Intelligence Service, started before the US entered the war. Product from this source was called Magic. On the other side, German code breaking in World War II achieved some notable successes cracking British naval and other ciphers.

Australia Central Bureau FRUMEL: Fleet Radio Unit, Melbourne Secret Intelligence Australia

Finland Finnish Defence Intelligence Agency

France PC Bruno Hans-Thilo Schmidt

Germany Enigma machine Fish (cryptography) British codename for German teleprinter ciphers Lorenz cipher a Fish cipher codenamed Tunny by the British Siemens and Halske T52 Geheimfernschreiber, a Fish cipher codenamed Sturgeon by the British Short Weather Cipher B-Dienst Reservehandverfahren OKW/CHI Gisbert Hasenjaeger

Italy Hagelin machine Enigma machine

Japan Japanese army and diplomatic codes Japanese naval codes PURPLE JN-25

Poland Cryptanalysis of the Enigma Biuro Szyfrów (Cipher Bureau) Marian Rejewski Jerzy Różycki Henryk Zygalski bomba Lacida Machine

Soviet Union 5th Department of NKVD (1941-1943), 5th Department of NKGB (1943-1945) Lieutenant general Ivan Shevelyov, the head of the department 8th Department of Red Army General Staff Lieutenant general Piotr Belyusov, the head of the department

Sweden Arne Beurling

United Kingdom Bletchley Park Cryptanalysis of the Enigma Cryptanalysis of the Lorenz cipher Far East Combined Bureau (FECB) Naval Intelligence Division (NID) Wireless Experimental Centre (WEC) Bombe Colossus computer Typex SYKO Ultra Alan Turing W. T. Tutte John Tiltman Max Newman Tommy Flowers I. J. Good John Herivel Leo Marks Gordon Welchman Poem code

United States Magic (cryptography) Signals Intelligence Service US Army, see also Arlington Hall OP-20-G US Navy Signals Intelligence group Elizebeth Smith Friedman William Friedman Frank Rowlett Abraham Sinkov Genevieve Grotjan Feinstein Leo Rosen Joseph Rochefort, leader of the effort to crack Japanese Naval codes Joseph Mauborgne Agnes Meyer Driscoll SIGABA cipher machine SIGSALY voice encryption SIGTOT one-time tape system M-209 cipher machine Station HYPO cryptanalysis group Station CAST cryptanalysis group Station NEGAT

See also Cryptography History of cryptography World War I cryptography Ultra (cryptography) Magic (cryptography) Cryptanalysis of the Enigma Bombe Enigma (machine) SIGABA TypeX Lorenz cipher Geheimfernschreiber Codetalkers PURPLE SIGSALY JN-25 Bletchley Park Biuro Szyfrów PC Bruno SIS US Army, later moved to Arlington Hall OP-20-G US Navy Marian Rejewski Jerzy Różycki Henryk Zygalski Alan Turing W. T. Tutte John Tiltman Max Newman Tommy Flowers I. J. Good William Friedman Frank Rowlett Abraham Sinkov Joseph Rochefort Agnes Meyer Driscoll Hans-Thilo Schmidt

References

Worked examples

Example 1 — a first encounter with World War II cryptography

Start with the simplest possible case. Write down what World War II cryptography claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to World War II cryptography before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about World War II cryptography ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of World War II cryptography

In research: World War II cryptography appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses World War II cryptography in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: World War II cryptography is common in secondary-school and first-year university syllabi. It links to neighbouring topics History of cryptography, Signals intelligence of World War II, so understanding it makes those chapters shorter.
In everyday life: Look for World War II cryptography outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study World War II cryptography in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what World War II cryptography means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain World War II cryptography out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is World War II cryptography in simple terms?

Why does World War II cryptography matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study World War II cryptography?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on World War II cryptography.