Once the signal had been transmitted in this form, and the text handed to the receiving cipher clerk—whose wheels would already comply with the same daily instructions Nos. 1-3—he would duly move his wheels to JCM (No. 4 above), key TNUFDQ (No. 6), and read the reciprocally enciphered result BGZBGZ (No. 5.)
He then turned his wheels to BGZ and deciphered the text by keying it out, with his Number Two noting each letter in turn.
Envision yourself as a British tactician and you have to deocode the details wiretapped from German spies in Britain.
Match the settings given below to the Enigma simulator to decode message from the German Officials and save millions of people!
Rotors: IV VI III
Reflector: B
Plugboard: CH EJ NV OU TY LG SZ PK DI QB
Message: LEFBLRMGYAVZPUIUTYXV
After 1 May 1940 this procedure was changed. Presumably the German cryptographic authorities had belatedly recognized that the double encipherment of the text-setting represented a security risk which far outweighed the advantage of the double-check it provided.
From that date the random choice of text-setting (e.g., BGZ as in No. 5) was keyed only once, giving TNU instead of TNUFDQ.
All these messages (Enigma Nachrichten) were recovered from U534 in 1993. The original ENIGMA M4 key sheets didn´t survive the time under water, because they were printed on water soluble paper.
April 1945 (Key Potsdam - codenamed “Plaice” by the Allies):
Reflector (Umkehrwalze): C
Greek (Griechenwalze): B
Wheels (Walzenlage): 568
Wheel positions (Walzenstellung): individual for every message
Rings (Ringe / Ringstellung): AAEL / EPEL
Plugs (Stecker / Steckerbrett): AE BF CM DQ HU JN LX PR SZ VW
(Found by Dan Girard)