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

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chapter 4
and
1 and 2 are 9 places apart
2 and 3 are 2 places apart
3 and 4 are 17 places apart
4 and 5 are 25 places apart
and the digraph attack that was used before will no longer work.
The security of the double transposition cipher would be further
increased if we used two different keywords rather than using the same one
twice, particularly if the keywords were of different lengths. In such a
system, however, there is the increased risk that the sender will make a
mistake by using the two keywords in the wrong order. This will, in
general, produce a different cipher text which the receiver won’t be able
to decipher and the message will have to be sent again in its correct form.
This would provide the cryptanalyst with two versions of the same text,
enciphered with the same keys, but in opposite orders; a situation he may
be able to exploit. This type of error is a hazard of any system involving
double encryption. Some Enigma messages were enciphered twice, on
different settings, for extra security and there was at least one occasion
when the encipherments were carried out in the wrong order [4.1].
Example 4.2
Apply the double transposition method to the text
A B C D E F G H I J K L M N O
using the two keys
3-1-5-2-4 and 3-1-2
(i) in the order given, (ii) in the opposite order. Hence verify that the two
cipher texts are different.
Verification
(i) We apply 3-1-5-2-4 (see Table 4.6),
Table 4.6
3 1 5 2 4
A B C D E
F G H I J
K L M N O