Recovery of the C-steppers is considerably more complicated, because the consonant substitution has the very large period of 253. Rowlett [1998, p. 151] describes the excitement when Genevieve Feinstein (née Grotjan) discovered the characteristic property of the C-stepper alphabets that was crucial in the success of Magic, the United States codename for intelligence derived from the cryptanalysis of PURPLE.
Table 7.52 lists the positions of the three banks of C-steppers for 0 ≤ i < 78.
the ciphertext alphabets in positions [0, 23] and [26, 49] are related as follows:
Table 7.52 shows
If x2, x2 ∈ CON and
for 0 ≤ i ≤ 23. That is, when