Code and ciphers: Julius Caesar, the Enigma and the internet

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chapter 2
15 16 17 18 19 20 21 22 23
CWSACPZ XGL ZHSWBNZ XZT TGRNV MJCPO MJSM CX MJZLZ
IMA.INE .OR E.AM..E .E. WO... THIN. THAT I. THERE
24 25 26 27 28 29 30 31 32 33
SLZ MTZPMQ MJLZZ BZGBNZ CP S LGGW MJZP MJZLZ CK
ARE TWENTY THREE .EO..E IN A ROOM THEN THERE IS
34 35 36 37 38 39 40 41 42 43
SP ZDZPK IJSPIZ MJSM MJZLZ SLZ MTG GX MJZW TCMJ
AN E.ENS ..AN.E THAT THERE ARE TWO O. THEM .ITH
44 45 46 47 48 49 50
MJZ KSWZ ECLMJVSQ ERM MJCK CK KG
THE SAME .IRTH.A. ..T THIS IS SO
The remaining letters are now easily identified and the entire decryption
substitution alphabet, denoting ‘space’ by ^ , is
ABCDEFGHIJKLMNOPQRSTUVWXYZ
GPIVBQOXCHSRTLKNYUAW.DMF^E
The encryption alphabet, which the sender would have used to produce the
cipher text from the plain, is of course the inverse of this viz:
ABCDEFGHIJKLMNOPQRSTUVWXYZ
SEIVZXAJC.ONWPGBFLKMRDTHQY
In general the encryption and decryption alphabets will be different in
a simple substitution or Julius Caesar system; in the latter case they are
the same only when the shift is 13; in the former case they can be made the
same by arranging most, if not all, of the letters in pairs so that the letters
of a pair encipher to each other, and leaving the remaining letters
unchanged. Some cipher machines including both the Enigma and
Hagelin machines automatically produce such reciprocal alphabets, making
the processes of encryption and decryption the same, which is a convenience
for the user but also weakens the security. In a simple substitution
system based on a 26-letter alphabet the number of possible substitution
alphabets is reduced from more than 10 26 to less than 10 13 . (For details of
this calculation see M2.) Whilst this is still a large number it is significantly
less formidable from a cryptanalytic point of view. Such reciprocal
simple substitution ciphers have, nevertheless, been used occasionally,