86 | 10 THE ENIGmA mACHINE
The version deployed by the navy, called the M1, was functionally compatible with Model I,
but there were some minor differences. For example, the rings around the three wheels were
inscribed with letters (A–Z), rather than numbers (01–26), and the M1 had a 4-volt power
socket, making it suitable for use on board a ship. Approximately 611 M1 machines were built.
In 1938, the M1 was followed by the M2, of which 890 units were delivered. Then in 1940,
the M2 was replaced by the M3, some 800 of which were built. All three machines, M1, M2,
and M3, had the same internal marking ‘Ch. 11g’. ‘Ch.’ stood for Chiffrienmaschinen AG, and
the marking identified all three models as variants of the same design, which was just one of
the family of Enigma designs manufactured by the company.^4 This chapter focuses mainly on
the naval machines—as did Turing.
Enigma’s components
In a nutshell, the Enigma carried out varying letter-for-letter substitutions on plain German
text, producing what was called the ‘ciphertext’ of the German message. The ciphertext looked
like a random jumble of letters—although in fact it was far from being truly random.
Enigma was an ‘offline’ device, in the sense that it was used only to encrypt and decrypt the
message; the actual transmission of the message was a separate process, performed with other
equipment. The Enigma was ideal for mobile communications. Weighing about 25 pounds, it
sat in a wooden box that could be easily carried using a fitted leather strap. The machine ran
from either the mains supply or a battery.
Enigma had a keyboard with the twenty-six letters of the alphabet laid out in three rows
(Fig. 10.1). The top row, from left to right, read QWERTZUIO, as was standard in Germany (the
top row of the British and American keyboard read QWERTYUIOP, as on a modern ‘Qwerty’
keyboard). The Enigma keyboard had no punctuation or number keys, so if an operator wanted
to enter the number 1, for example, he would have to type eins, the German word for 1. The
three German umlauted letters ä, ö, and ü had to be entered as ‘ae’, ‘oe’, and ‘ue’.
Adjacent to the keyboard were twenty-six small circles of glass, each stencilled with a letter of
the alphabet. (These were probably made of acrylic ‘safety glass’, known as Plexiglas in Germany
and Perspex in the UK.) The circles were again laid out in three rows of letters, in the same
order as the keyboard. This complete component was known as the ‘lampboard’. Underneath
each circle of glass was a small bulb, much like one from a torch of the pre-LED era. Every time
a key was pressed, one of the bulbs lit up. The stencilled letter that the bulb illuminated was the
encryption of the letter on the key that had been pressed. For example, if the O key was pressed
and the bulb under Q lit up, then Q was the encryption of O at that point in the message (see
Fig. 12.4).
In the middle of the machine were three wheels. These were the heart of the encryption
mechanism. They rotated on a removable shaft that passed through a hole in the centre of each
wheel (Fig. 10.2). Every time a key was pressed, some or all of the wheels rotated. Due to their
relative rates of rotation, the left, middle, and right wheels were referred to as the slow, middle,
and fast wheels, respectively.
The Enigma was designed in such a way that as many as about 17,000 key presses could be
required in order to return all three wheels to their initial starting positions, although if several
of the three special navy wheels—described below—were in use, this figure could drop to as few
as 4056.^5 In practice, however, even this smaller number of key presses would never occur in