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Table 8: Cipher Rotor Stepping Ratios [1]

Now given the cipher rotor stepping counts from Phase 1, we can use Table 8 to determine 

the most likely pairs of control rotor output letters for each cipher rotor. Since these are 

connected to the index permutation, this information combined with the information from 

Table 7 will reduce the number of possible index permutations. 

A valid index permutation must contain five pairs from Table 7 such that two conditions 

are met. The first condition is that each digit is used once and only once. The second 

condition is that the number of letters for the five pairs must sum up to exactly 26. If we 

order all the valid sets of five pairs that have letters that sum to 26 and use each digit only 

once, we find that there are 2148 groupings.

26

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Now that we have the stepping counts from Phase 1, we can calculate the stepping ratios 

for each cipher rotor. The stepping ratio is just the step count divided by the number of 

letters. Once we have the ratios computed, we can attempt to distinguish the number of 

letters connected to each cipher rotor in the index permutation. 

Consider an example where the cipher rotors had the following stepping ratios.

Cipher Rotor Stepping Ratio

C

0

0.15

C

1

0.29

C

2

0.60

C

3

0.74

C

4

0.91