When they had been studying the double enciphered message settings in order to construct
Rejewski's characteristics, cases had been noticed where the same enciphered
letter occurred in either the 1st and 4th, or 2nd and 5th, or 3rd and 6th
positions in the enciphered message settings. These positions, which were called females,
corresponded to positions at which the same letter had been keyed by the
German operator into the Enigma machine because of the repeat of the
three letter message key.
Zygalski realised that whether this occurred or not depended on the wheel
order and the start position, therefore inversely if it did occur it
implied one of a set of possible Enigma configurations. If enough of these
doubles occurred during a days traffic it might be possible to find a
unique configuration for which all of these doubles could occur.
Zygalski realised that the analysis of the vast amount of information required
could be achieved by a grill method using perforated sheets.
The sheet procedure involves working through each of the six possible wheel
orderings for three wheels and for each wheel order working through the 26
possible left hand wheel ring letters, 156 tries in all, but on average
only half before the answer is found.
There were sheets prepared for each left wheel letter for each wheel order.
Each sheet contained four squares of 26 by 26 ie two alphabets along the
top and down the side.
The first decision was which wheel order to try, (in our case 132) and
which ring first letter, in this case Q.
Next take the first letter of the first indicator that gives a 1,4 female in
this case P and get the sheet 123 Q P. Place this on the table offset by
the next two letters of the indicator, TJ. Now get sheets for succeeding
indicators overlaying them on the table. If the wheel order and the first
ring letter are what was used to encipher the message key then one square
will line up and let light through from below the table. The original ring
setting can now be deduced from the co-ordinates of this lit square.
This emulation of the Zygalski sheets calculates the offset of a sheet and
recreates the overlaid portion of each sheet at run time. This minimises
the downloaded information but takes a little time to recalculate
each sheet. Click here for a
description of the calculations required to do this.
In order to deduce the steckers, the set of indicators and enciphered message
keys which have repeated letters, need to be extracted and listed. Then
by setting up an Enigma machine to the wheel order and ring setting just found, but no
steckers, the wheels can be turned to one of the indicators from the list. Now successive
keyboard letters can be input looking for a letter which repeats in the
same positions as in the enciphered message keys. At least one letter will
do this, sometimes two will. This letter or one of these letters is the stecker
of the original repeated letter. By going through this for all the 1,4
2,5 3,6 sets the steckers can be deduced.
This page was originally created by the late Tony Sale the
original curator of the Bletchley Park Museum