Because of the symmetrical use of the plugboard both before and after the passage of the current through the rotors, it would preserve the self-inverse character of the basic Enigma, and the feature that no letter could be enciphered into itself.
If A were enciphered into Q then, in the same state of the machine, Q would be enciphered into A.
So the plugboard left unaffected this useful—but dangerous—aspect of the basic Enigma.
But it enormously increased the sheer number of states of the Enigma machine.
There would be 1,305,093,289,500 ways of connecting seven pairs of letters on the plugboard, for each of the 6 × 17576 rotor states.
如果一个配线板可以对换7对字母，那就有1, 305, 093, 289, 500种配线状态注，对于每一种配线状态，又有6×17576种转盘状态。
Presumably the German authorities believed that these modifications to the commercial Enigma had brought it 'Very close to practical unsolvability'.
And yet, when Alan joined up at Bletchley on 4 September, he found it humming with the disclosures made by the Polish cryptanalysts.
It was all still fresh and new, for only on 16 August had the technical material reached London.
And this revealed the methods by which, for seven years, the Poles had been deciphering Enigma messages.
The first thing, the sine qua non, was that the Poles had been able to discover the wirings of the three rotors.
It was one thing to know that an Enigma machine was being used; quite another thing—but absolutely essential—to know the specific wirings employed.