Cryptography Reference
In-Depth Information
Incorrect Correction of a Rotor Machine: The Enigma
The first Enigma models were regular rotor machines with four rotors, as
described above.
But 'good is not good enough for us', said Willi Korn in 1926 and he went
about improving the rotor machine — at least so he thought. His fateful idea was
the so-called
reversing drum
, also called a
reflector
: in place of the sliding
contacts on the far right, his version featured an additional disk, with contact
surfaces on one side only. These contact surfaces were interconnected such that
each surface was connected to exactly one other surface. This corresponds to
a special substitution, namely one where no letter transforms onto itself.
Voltage is applied to a 'letter' on the far left, and propagates through the rotors
toward the right, 'turns around' in the reversing drum, and then traverses the
rotors again from right to left over a totally different path (which explains
why the reversing drum is also called a
reflector
). 'Two are better than one',
cryptologist Korn must have thought. As we will see further below, this was a
fateful mistake. This arrangement guaranteed that, during the ciphering process,
a letter can never transform onto itself, or there would be a short circuit. I
have already mentioned that, in cryptanalysis, even negative statements can
sometimes be very helpful, as we will see in Section 2.5.2.
Since the reversing drum made the Enigma 'more secure', Korn must have
believed that three rotors and the reversing drum would be sufficient. An addi-
tional plugboard was used to swap letter pairs before and after encryption.
The rotors moved roughly like a counter: once a rotor had reached a specific
position, the next rotor was turned forward by one step through a catch. This
specific position was different for each rotor. They probably believed that this
would confuse the adversary. However, it led to the exact opposite: exactly
this difference of all things would later allow the British to find out how the
rotors were arranged. The Germans detected their error only with rotors VI
through VIII and always placed the catches at the same position. (You will find
a more detailed description in the file
enigma-wiring.txt
on our Web site).
Rings with consecutive alphabet sets or numbers were fastened to the rotors
(the photos show both labeling types). These rings were plugged on only when
the rotors were used. Their positions relative to the rotor itself were part of the
so-called
basic key
, which will be discussed further below.
The German army used this machine, called Enigma I, in the described config-
uration. From 1942 onwards, the German navy possessed an Enigma with
