雙語暢銷書《艾倫圖靈傳》第5章:解謎接力賽(17)
These could be written as swappings: (AE) (BD) (CG) (FH) in the first case and (AE) (BF) (CG) (DH) in the second.
There was a practical advantage to this Enigma property.
謎機的這種性質，在實踐中有一種優勢，It meant that the deciphering operation was identical with the enciphering operation.
謎機的這種性質，在實踐中有一種優勢，
(In group-theory terms, the cipher was self-inverse).
用羣論的術語來說，密碼是自反的。The receiver of the message had only to set up the machine in exactly the same way as the sender, and feed in the cipher-text, to recover the plain-text.
接收方只要把機器設置成與發送方相同的模式，然後將收到的密文輸入進去，就能解碼得到明文。There was no need to incorporate 'encipher' and 'decipher' modes into the Enigma machine, which made its operation that much less liable to mistakes and confusion.
這樣，謎機就不需要區分編碼和解碼兩種不同模式，這使它變得更加可靠，不容易產生錯誤和混亂。But it was associated with a grave weakness, in that the substitutions thus performed were always of this very special kind, with the particular feature that no letter could ever be enciphered into itself.
但是這也產生了一個嚴重的缺陷：謎機的替換規則總是這樣一種特殊的形式，因此每個字母在編碼之後都不可能是其本身。This was the basic structure of the Enigma.
這是基礎版謎機的構造，But there was much more to the machine actually in military use.
但其軍用版並不只是這樣。For one thing, the three rotors were not fixed in place, but could be removed and replaced in any order.
一方面，那三個轉盤不是固定的，它們可以任意移動或更換。Until late 1938 there was a stock of just three rotors, which therefore allowed a total of six arrangements.
在1938年後期，只製造了三種轉盤，但它們可以按照6種不同的順序安裝。In this way, the machine offered 6 × 17576 = 105456 different alphabetic substitutions.
因此，這個機器就能產生6×17576=105, 456種不同的替換規則。
