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.
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.
Central Bureau
FRUMEL: Fleet Radio Unit, Melbourne
Secret Intelligence Australia
Finnish Defence Intelligence Agency
PC Bruno
Hans-Thilo Schmidt
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
Hagelin machine
Enigma machine
Japanese army and diplomatic codes
Japanese naval codes
PURPLE
JN-25
Cryptanalysis of the Enigma
Biuro Szyfrów (Cipher Bureau)
Marian Rejewski
Jerzy Różycki
Henryk Zygalski
bomba
Lacida Machine
Arne Beurling
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.
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