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The German Enigma machines are the key for German victories during the early years of World War II. When the Allies managed to capture few Enigma machines and decode then the face of war began to change in favor of Allies.

My question is, is it possible to have multiple encryption/decryption keys in a same enigma machines? So that it enables the users to send/receive the 1st message using a different encryption/decryption key and the next message using a different key.

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David O'Keefe's book One Day in August details much background on the British attempts (and occasional successes) to obtain the keys and plug settings in pinch operations. His research describes how he came to discover that the Dieppe Raid in August 1942 was such a pinch attempt, kept secret for 70 years. –  Pieter Geerkens Apr 21 at 5:42
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Capturing the Enigma machines had very little to do with it. The machine was patented (its manufacturers originally intended it for business use) so its basic design was public knowledge. The German military adapted the design and changed it a few times during the war but figuring out the changes that had been made wasn't a significant difficulty. –  David Richerby Apr 22 at 9:22

5 Answers 5

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The Enigma machines and the breaking of the Enigma code were not the main determinants of the outcome of World War II, but did contribute to the outcome.

There were only a few types of Enigma machines, so they had to be capable of using different encryption keys. If machines used the same encryption key for message after message, the encryption would be broken before too long, and then all successive (and preceding recorded) messages could be decrypted by the opponents.

The machines had several physical settings (as described by the Wikipedia article), including selection of wheel order and wiring of some movable plugs. The usual procedure was to have prearranged settings for each day of use, with the machine changed at midnight to the new day's settings. In addition, the operator would rotate the wheels arbitrarily (for randomness) before starting encryption of a new message. His arbitrary selection would be communicated at the front of the sent message. The procedures are described in more detail on this page and this page.

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+1 Nice answer :-) –  andy256 Apr 21 at 5:43

Yes, they do allow multiple keys. Typically they had a new key every day.

See this Wikipedia article:

Though Enigma had some cryptographic weaknesses, in practice it was German procedural flaws, operator mistakes, laziness, failure to systematically introduce changes in encipherment procedures, and Allied capture of key tables and hardware that, during the war, enabled Allied cryptologists to succeed.

It was not capturing the hardware alone that enabled the decryption. The key tables were books of keys to be used on a given day.

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The coded messages could also be broken and the message decrypted, (though every day when the code changed and often long process) but various little flaws in the German system.(thuogh a complex semi lucky process which was often not achieved for months) –  pugsville Apr 21 at 15:01
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According to The Secret War (described below), a major cryptographic weakness was that a letter was never encoded as itself. This led to some shortcuts in solving the key when there was an expected word or phrase in the plaintext. The Allies did not need the key tables to decipher Enigma dispatches, but they were probably helpful in speeding up things when one was available. Note that the German military used dozens of key tables, one for each operational division (i.e., not an entire service on one key) and they changed daily. –  Phil Perry Apr 21 at 22:09

"Allies" capturing Engima machines (what you really mean was British navy, who then in Hollywood were magically transformed into US navy) was really of no importance. What was important was capturing code books. The wiring of the Enigma machine was known since the 1930s, when Polish mathematicians managed to reconstruct it from very limited information.

The latest Enigma machines came with a set of eight wheels, of which three were plugged into the machine, plus a fourth extra wheel for submarines. There was a "day code" which told the personell which three wheels to pick, how to rotate them, and which nine cables to use on the plugboard. When a message was sent, sender and receiver put their enigmas into the day position. Then the sender would send a random three digit key, and then both sender and receiver would switch the wheels to that key.

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The reason that capturing code books was important is also interesting. It wasn't about capturing day codes (although there were some important day code captures). Aside from any encipherment there were "codes" in use for describing certain routine things concisely (weather and ship movements). Capturing these codes provided so-called "cribs" -- a section of message text that could confirm when the correct day code had been guessed. The "guessing" process is what was automated by the bombes. –  Steve Jessop Apr 21 at 16:00

The first Enigma machine to come to the notice of Germany's foes was in Poland, around 1928. Polish customs (and their intelligence service) were suspicious of the German embassy's unseemly desire to get a certain package out of Customs on a Saturday. The Poles spent the weekend copying the manuals and examining the mechanism, and then delivered it in the Monday morning post. In August 1939, facing invasion, the Poles gave all their knowledge (including 10 years of mathematical advances in code breaking) to the French and British.

You should watch the BBC series "The Secret War" (circa 1975), especially the episode "Still Secret", which covers Enigma/Ultra/Fish and Bletchley Park very thoroughly. The Allies were NOT dependent on capturing Enigma machines -- they made tremendous advancements in code breaking without having the physical machine or both plain and encrypted messages.

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There were multiple Cypher Keys for example Supply Ships sheltering in neutral ports used the Tibet Cypher. merchant Raiders used Special Cypher 100. Cypher Bertok was used to communicate with the Naval attache Tokyo. Cypher Freya was used between OKM and naval shore installations. Triton Cypher was used by U-boats in the Indian Ocean.

There were even special Enigma keys created for co-operation between the German and Japanese navies operated on three rotor machines known as T-Enigma under the Tirpitz agreement of 1942 (Tirupitsu to Japanese/JN-18 to USN) The code key for this was originally known as Sumatra and later as Gartenzaun. This was mostly used by German I-class U-boats which sailed to Europe such as I-29 and I-52 for example.

The SS had their own Cypher keys such as Kliest many of which were rarely broken. German Railways had their own Cypher Keys as did various Luftwaffe units. The Kreigsmarine used four rotor Enigma machines as opposed to the three rotor machines of the Luftwaffe.

These were books of Cypher keys relevant for specific days and the way they were used changed as the war progressed. Generally a vessel was only issued with a book of Cypher keys lasting three months. Later in the war Keys would change up to twice per day. They were issued on pink water soluble paper.

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