Editing Substitution cipher (section)

===Mechanical===
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[[File:HGM Enigma.jpg|thumb|[[Enigma cipher]] machine as used by the German military in World War II]]
Between around [[World War I]] and the widespread availability of [[computer]]s (for some governments this was approximately the 1950s or 1960s; for other organizations it was a decade or more later; for individuals it was no earlier than 1975), mechanical implementations of polyalphabetic substitution ciphers were widely used. Several inventors had similar ideas about the same time, and [[rotor machine|rotor cipher machine]]s were patented four times in 1919. The most important of the resulting machines was the [[Enigma machine|Enigma]], especially in the versions used by the [[Wehrmacht|German military]] from approximately 1930. The [[Allies of World War II|Allies]] also developed and used rotor machines (e.g., [[SIGABA]] and [[Typex]]).

All of these were similar in that the substituted letter was chosen [[electric]]ally from amongst the huge number of possible combinations resulting from the rotation of several letter disks. Since one or more of the disks rotated mechanically with each plaintext letter enciphered, the number of alphabets used was astronomical. Early versions of these machine were, nevertheless, breakable. [[William F. Friedman]] of the US Army's [[Signals Intelligence Service|SIS]] early found vulnerabilities in [[Hebern rotor machine|Hebern's rotor machine]], and the [[Government Code and Cypher School]]'s [[Dillwyn Knox]] solved versions of the Enigma machine (those without the "plugboard") well before [[World War II|WWII]] began. Traffic protected by essentially all of the German military Enigmas was broken by Allied cryptanalysts, most notably those at [[Bletchley Park]], beginning with the German Army variant used in the early 1930s. This version was broken by inspired mathematical insight by [[Marian Rejewski]] in [[Poland]].

As far as is publicly known, no messages protected by the [[SIGABA]] and [[Typex]] machines were ever broken during or near the time when these systems were in service.