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I have seen in the Imitation Game that the Germans reset the Enigma at midnight sharp, and then they start messaging by sending the weather report in the morning.

But the device made by Alan Turing, the Christopher, sometimes only took a few minutes to decode Enigma.

How did the German receiver know the settings of the machine? I thought that they may be sending the next day's setting at midnight, but if they were compromised once, the enemy would know all the next day's settings too.

So, how did they do it?

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    $\begingroup$ Anything not answered there? $\endgroup$ – fgrieu Dec 2 '18 at 17:35
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The simple answer is also part of one weakness with all encryptions that use symmetric key, AKA pre-shared key. Key sheets that lasted 31 days where generated and sent out by courier or so. The weakness is ofcourse that if the enemies get their hands on it then they can read all the messages that used that key. And yes - sometimes the allied did get their hands on a key sheet and then they had the keys for one network for up to a month.

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There was more than one procedure for using the Enigma, and this turned out to be a critical issue which aided Allied cryptanalysis.

Germany Army Enigma Procedure Prior to 1940

The parameters were variable, usually set for one 24-hour period, and they had to be synchronized. A random three-letter value was chosen for wheel settings. Let's use GFB.

Everyone on a radio net had a ground setting for each particular period of time, and it was printed on a key sheet--examples of which can be seen here. Our example will be: ZMF.

The wheels were set to the ground setting (ZMF), and then the operator encrypted a random three-letter value (GFB) two times over the ground setting. Let's say the result is LBEQHG. Now the operator is ready to send traffic. Rotate the wheels to the chosen random setting (GFB) and encrypt a message. Concatenate the indicator (LBEQHG) with the ciphertext of the message. Send.

The receiver sets Enigma to the ground (ZMF) and enters in the indicator--which is, fatally, a double indicator of the same thing--LBEQHG. Ironically, this precaution against errors eventually helped lead to Enigma's demise. To quote Dirk Rijmenants:

However, this procedure was actually a security flaw. The message key is encoded twice, resulting in a relation between first and fourth, second and fifth, and third and sixth character. Moreover, many message keys on a particular day would have the same setup and startpositions. This security problem enabled the Polish Cipher Bureau to break the pre-war Enigma messages.

Or, here, in Code and ciphers: Julius Caesar, the Enigma and the Internet, p.122, by Robert Churchhouse:

The fundamental flaw which led to the decryption of Enigma messages was not due to the design of the machine itself, but to the method which was used by the Germans to send messages.

The receiver gets the ciphertext message. Deciphering the indicator should result in a pair of trigraphs such as ZMFZMF. If not, something is wrong. The receiver then turns the wheels to ZMF and decrypts the ciphertext.

Dirk Rijmenants goes into incredible detail about Enigma on on his website. German Army procedures became more complex after 1940 and can be seen on Dirk's website.

Enigma Procedures of the German Wartime Navy

These were much more complex than German Army or German Air Force Enigma procedures, and they can be read about in full detail on the website listed above.

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The point is that the first line of the encrypted message always included a formatted date (or weather with a date). When this structure became known, it only took a few characters of ciphertext to begin decryption.

Using 26 characters to ~ 20 degrees with spaces, you would know that the first is a day, then a month, then a year, and it all became easy to decrypt.

The first sentence was comprised of pre-established code words, where for each word there were certain meanings (day, month, etc.), and this helped in the decoding of the messages.

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