All specialists unanimously agreed that a reading [of the Enigma] is impossible.
Admiral Kurt Fricke, Chief of Naval War Command
The operator was provided with a code book which he consulted at midnight which was when the new key for that day came into effect.
The code book listed the five parameters for setting up the Enigma:
1. The Datum or date.
2. The Walzenlage or rotor order for that date. For example: IV, I, V.
3. The Ringstellung, or ring setting for that date. For example: 23 02 17.
4. The Steckerverbindungen or plugging for that date. For example: AR KT MW LC XD EJ ZB UY PS HN.
5. The Kenngruppen, or discriminant for that date. For example: TXM.
According to this example, the operator would take rotor IV and turn its ring until the 23 (W) position was next to a zero mark on the rotor and then clip the ring into position. He would repeat this process with rotor I setting its ring in the 02 (B) position and with rotor V setting its ring in the 17 (Q) position.
The operator would then put rotor IV on the spindle in the left position, followed by rotor I in the middle position and then rotor V in the right position, and then slot the spindle and rotors into the Enigma and secure them in place with a lever.
Next, the operator would set the steckerboard by plugging A to R, then K to T, then M to W, and so on.
The operator would then think of three letters at random, say RNF, for the Grundstellung or indicator-setting. He would then manually rotate the left rotor until it had R uppermost, the middle rotor until it had N uppermost and the right rotor until it had F uppermost.
Next, the operator would think of another three letters at random, say JRM, for the message-setting. He would then press the J key, and B, say, would light up, he would then press R, and K, say, would light up, next he would press M, and T, say, would light up. The operator's assistant would make a note of the enciphered message-setting (BKT in this example). The operator then set the rotors to JRM.
The Enigma is now set for enciphering or deciphering.
To encipher a message the operator would then key-in the plaintext of the message and his assistant would write down the ciphertext as each letter illuminated.
Typically, a complete transmision would include the sender's and receiver's call-signs, frequency, signal strength, readability, intercept station number, time of origin, urgency, number of parts in the signal, number of current part, number of letters in current part, Kenngruppe discriminant (to state which key was being used - TXM in this example) and Grundstellung (RNF in this example). This was all transmitted en clair and followed by the enciphered message-setting (BKT in this example), the enciphered message and finally the end of message signal.
Prior to 1st May 1940, in order to reduce errors, the procedure was to repeat the encipherment of the message setting, so in this example, JRMJRM yields, say, BKTRFQ. This repeated encipherment of the message-setting was both foolish and unnecessary. The Poles' exploitation of it by analytically comparing the first triplet BKT to the second triplet RFQ, enabled them to break Enigma. On 1st May 1940 the Germans abandoned repeated message-settings after which the Poles' techniques were useless.
To improve security it was an operational rule that no message should exceed 250 letters.
To decipher a message, the operator would set up his Enigma in accordance with the key for the day, turn his rotors to the Grundstellung (RNF in this example) and key-in the enciphered message-setting (BKT in this example). Keying-in BKT would yield JRM so the operator would reset the rotors on his Enigma to JRM and then key in the ciphertext, his assistant would read off and record the plaintext as each letter was illuminated.
Prior to 1st May 1940 the operator would key-in the first six letters of the ciphertext, to yield, in this example, JRMJRM. He would then reset the rotors to JRM and key in the remainder of the ciphertext as above.
Copyright © Graham Ellsbury 1998, 2003
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