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How the Enigma Works
by Alan Stripp
The Enigma machine, first patented in 1919, was after various
improvements adopted by the German Navy in 1926, the Army in
1928, and the Air Force in 1935. It was also used by the
Abwehr, the Sicherheitsdienst, the railways, and other
government departments. From then until 1939, and indeed
throughout the war, successive refinements were introduced,
varying from service to service, and there were detailed
changes in operating procedure until 1945. The following short
description can therefore summarize only its main features and
mention only a few of the Enigma variations.
The Enigma (see diagram below) was used solely to encipher and
decipher messages. In its standard form it could not type a
message out, let alone transmit or receive it. From the cipher
operator's point of view, it consisted of first a keyboard of
26 letters in the pattern of the normal German typewriter:
Q W E R T Z U I O
A S D F G H J K
P Y X C V B N M L
with no keys for numerals or punctuation. Behind this keyboard
was a "lampboard" of 26 small circular windows, each bearing a
letter in the same QWERTZU pattern, which could light up, one
at a time, from bulbs underneath. (The model with an A-Z
keyboard, shown in several books on the Enigma, is a
Polish-French replica, not an actual Enigma machine.) It
measured about 13.5" x 11" x 6", and weighed about 26 lbs.
Behind the lampboard is the scrambler unit, consisting of a
fixed wheel at each end, and a central space for three
rotating wheels. The wheel to the right of this space is the
fixed entry or plate (Eintrittwalze) carrying 26
contacts round its left side, ultimately connected to the keys
of the keyboard in ordinary alphabetical order. To the left of
the space is the reversing wheel (Umkehrwalze), which
scrambles the current it receives and sends it back by a
different route from that by which it came. This wheel too has
a circle of 26 contacts.
The three central wheels were selected from a box of five.
Monthly orders specified a new choice every day, as well as
their relative order in the machine, e.g., V-I-III or II-IV-I,
etc. Each of these rotating wheels has a circle of 26
spring-loaded terminals on its right side and 26 flat circular
terminals on its left, so as to provide an ever-changing
series of connections as it revolves. Each contains a
different internal wiring and carries the letters A-Z or the
numbers 01-26 round its inner ring, which can be turned and
locked in any setting before the wheels specified for a given
day are inserted into the machine in the prescribed order,
though they can still be turned bodily through slits in the
inner lid, and the letters A-Z can be read one at a time in
the window beside each slit. The specified setting of ring
against wheel was called the Ringstellung.
Each time a letter key is pressed, the right wheel moves on
one of its 26 places. Once during every 26 moves, at the
"turnover position" on the right wheel, the middle wheel will
also move on one place; and when the middle wheel reaches its
own turnover position it moves on again when the next letter
is keyed, together with the left wheel.
Finally, the vertical front of the Enigmas used by the Armed
Services contained a "plugboard" with 26 pairs of sockets,
again in the QWERTZU pattern. These could be connected by
twin-cable leads—for example, coupling C to P, M to Z, J
to S, and so on; but some sockets, usually six, were left
unconnected. They were said to be "self-steckered."
Stecker is a plug; Steckerbrett (usually called
"steckerboard" at Bletchley) is a plugboard.
Each time the cipher clerk keyed a letter, the right wheel
moved on mechanically one place and, as explained above, from
time to time the center and left wheels also moved. As each
new letter (e.g., P) was keyed, the current, normally provided
by an internal 4.5 volt battery—although an outside
power source could be used—flowed from a terminal under
that key to a socket (e.g., P) on the plugboard. From there it
travelled via a lead to another socket (e.g., L), or, if the
first socket was self-steckered, it stayed as P. Either way,
it ran to the entry wheel, which did not alter it, through the
pairs of terminals on all central wheels—each of which
normally altered it again—to the Umkehrwalze or
reversing wheel (with another alteration) and back through
different circuits in all three wheels (hence still further
alterations), out unaltered through the entry wheel, and back
to the plugboard. Here its course again depended on whether
that socket was self-steckered or cross-steckered; either way,
it finally reached the lampboard and lit a bulb (e.g., W).
Although the process, involving up to nine changes on the
standard three-wheel machine, has taken some time to describe,
it naturally took place virtually instantaneously. And it must
be remembered that the moving on of at least one wheel, for
every new letter keyed, introduced a new set of circuits for
each new letter.
It is important to note that, if you press any key (e.g., B),
any other letter may light up (e.g., T); but if you continue
to key letter B, the lampboard may give, say, P, F, O, J,
C..., but never B. The sequence will repeat only after 16,900
(26 x 25 x 26) keyings, when the inner mechanism returns to
the same position. Messages were limited to a maximum of 250
letters to avoid this recurrence, which might have otherwise
helped us.
Continue: Basic set-up and possible keys
Crack the Ciphers
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Send a Coded Message
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Mind of a Codebreaker
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How the Enigma Works
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| Updated November 2000
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