90 | 10 THE ENIGmA mACHINE
The new wheel—which did not move during encryption, and could not be exchanged with the
other Naval Enigma wheels—introduced an additional stage into the encryption process.
The M3, broken by Turing during his epic struggle with the U-boats, is discussed in more
detail later in this chapter.
Sending a message
To prepare the machine—for example the M3—to encrypt a message, three wheels were taken
from the box and placed in the machine in the specified order. The rings of the wheels were then
adjusted to their specified positions. Next the operator connected up ten pairs of letters on the
plugboard (or fewer than ten in the early weeks of the war). His final adjustment was to turn the
wheels to a pre-determined starting position.
The number of possible electrical configurations that can be made in this way is surprisingly
large. It is calculated by multiplying the number of plugboard configurations (150.7 million
million) by the number of ways of selecting the wheels (336), and then multiplying the result by
the number of possible starting positions of the wheels (17,576).
Once the machine was fully set up, encrypting a message by Enigma was a relatively simple
process. The operator typed the message—in plain German—at the keyboard, and as he typed,
the letters of the encrypted form of the message lit up, one by one, at the lampboard. Decryption
was equally straightforward. Once the encrypted version of the message had been received, and
the recipient’s Enigma had been set up, the recipient simply typed out the encrypted version at
the keyboard, and the letters of the plain German text lit up at the lampboard (see Fig. 12.5). All
the complexity lay in ensuring that the sender and recipient set up their Enigma machines iden-
tically. A widely used procedure for doing this involved what Bletchley Park called the ‘daily
key’. This procedure, as employed with the principal M3 Naval Enigma ciphers, is described in
the next section.
Once encrypted, messages were normally broadcast via radio. A complicated programme
of changing frequencies was used during transmissions to and from the U-boats. Naval mes-
sages were usually transmitted in four-letter groups, as in the example set out here (although
a few naval networks followed the army and air force practice of using five-letter groups). Two
additional four-letter groups were placed at the beginning and again at the end of the message.
These were called the ‘indicator’ groups, and their function is described later in the chapter.
An important German naval signal was intercepted on 27 May 1941. The intercepted mes-
sage would have looked like this (using, for illustration, the indicator groups constructed later
in the chapter)^7 :
MMÄ 1416/27/989 38
IJTV USYX DERH RFRS OQRV DTYH QWBV HILS CXHR OPOD
GTQL DDHI KFTG EDZS WXQS EDFR HGYG EDZZ UYQV DTYY
EDGH KIRM SYBK PANX JSTP QXDT ERGP JMSX VFWI FTPZ
ADHK WDLE QPAL ALDH XNDH RYFH IJTV USYX
1231 7640
MMÄ was the call sign identifying the transmitting station, 1416 was the time at which the
transmission began, 27 was the day of the month, 989 was the message serial number, and
38 was the total number of four-letter groups (a check that the complete message had been