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

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chapter 7
Table 7.7
6 1 10 4 8 11 3 7 12 2 9 5
D O L Q Q P
T C E T A R T D C X V D
C A W F M X I X Y G D O
Q Y Y X T W N N O I A G
I W Q O R C G E
We now use the transposition box again. Since the day of transmission is
Tuesday (the date is irrelevant in this simplified form of garbo’s cipher)
we begin writing the text under the column headed 3: see Table 7.7.
Finally, we take the text out column by column, starting with the column
numbered 1, and write it out in five-letter groups ready for transmission:
CAYWQ XGIDT INGTF XOPDO GTCQI ODXNE AMTRQ
VDAEW YQRXW CLCYO.
Decipherment in this system is a tedious process in which it is easy to
make mistakes. To begin the decipherment the receiver has to work out,
from the day of the week and the length of the message, which columns of
the transposition box will have an extra letter and which they are. In the
example above since the message contains 50 characters there will be 10
columns of 4 letters and 2 columns of 5 letters. Since the day of the week is
Tuesday (�3) the columns headed 3 and 7 will be the ‘long’ columns and
the others will be ‘short’. The same analysis will have to be used when the
transposition is used again; in this case the month is May (�5) so the long
columns will be those headed 5 and 6 (since the column headed 6 happens
to follow the column headed ‘5’).
For more details of garbo’s cipher systems see [7.1].
One-time pad
The basic weakness of the book cipher as used above is that both the
message and the key were in English and by dragging cribs based upon
common English words which might occur in either we were able to
recover both. Had the key not been in English, decryption would certainly
have been more difficult for the cryptanalyst but the messages
would nevertheless be read eventually once he had discovered that this
was the case, since cribs from the other language could also be used. If, on
the other hand, the key was not based upon a natural language but was