As the German military grew in the late 1920s, it began looking for
a better way to secure its communications. It found the answer in a
new cryptographic machine called "Enigma." The Germans believed the
encryption generated by the machine to be unbreakable. With a
theoretical number of ciphering possibilities of 3 X 10114, their
belief was not unjustified.1 However, they never reached that
theoretical level of security. Nor did they count on the
cryptanalytic abilities of their adversaries. The Enigma machine
based its cipher capabilities on a series of wired rotor wheels and
a plugboard. Through a web of internal wiring, each of the 26 input
contacts on the rotor were connected to a different output contact.
The wiring connections of one rotor differed from the connections
on any other rotor. Additionally, each rotor had a moveable
placement notch found on an outer ring. The notch forced the rotor
to its left to step one place forward. This notch could be moved to
a different point on the rotor by rotating the outer ring. The
Germans followed a daily list, known as a key list, to indicate
where the notch should be placed each day. Another complication to
the machine involved the plugboard, which the Germans called a
"Stecker." The plugboard simply connected one letter to a different
letter. That also meant that the second letter automatically
connected back to the first. Again, the key list indicated which
letters should be connected for that day. Each day, the Germans
followed the key list to plug the plugboard connections, select the
rotors to be placed in the machine, change the rotor notch
placement, and place the rotors in the left, center, or right
position within the machine. Finally, the code clerk chose which
three letters were to appear through three small windows next to
the rotors. These letters indicated the initial rotor settings for
any given message, and the code clerk changed those settings with
every message he sent. The path the electrical current took
initiated with the keystroke. The current passed through the
plugboard, changing its path if that letter was plugged to a
different letter. From there it entered the first, or rightmost,
rotor at the input contact. The rotor wiring redirected it to a
different output that went directly into the next rotor's input.
After passing through, and changing directions in each rotor, the
current entered a reflecting plate. This plate not only changed the
"letter," but also sent the current back through the rotors, again
resulting in three more changes. The current made one last pass
through the stecker and finally on to the light panel where the
cipher letter lit up. To decipher an Enigma message, the recipient
had to have an Enigma with the same plugboard connections, rotors,
notch placement, left/center/right positions, and initial settings.
This enabled the current to follow the same pathway in reverse and
resulted in the plaintext letter lighting up on the light panel.
The Germans, with their published key lists, had the necessary
information. The Allies did not. The Enigma eliminated whatever
intricacies a language may possess that previous methods of
cryptanalysis exploited. One such practice was frequency counts.
Certain letters in any language are used more often than others. By
counting which cipher letters appeared most often, cryptanalysts
could make an assumption about which plaintext letter they
represented. Machine encryption like the Enigma destroyed the
frequency counts. Cipher letters tended to appear equally often.
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