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

Example 5.1
HAPPY BIRTHDAY
enciphers as
BCAAC ECEED ABBDD BDDBC ADAAE D
As it stands this is a weak cipher but its security can be improved by two
modifications
(i) using a shuffled alphabet inside the 5� 5 box;
(ii) applying a transposition to the enciphered text.
The first of these is sometimes partially achieved by choosing a keyword,
writing this into the box and then filling the rest of the box with the
unused letters of the alphabet, excluding J. If the keyword has any
repeated letters they are ignored.
Example 5.2
Repeat the example above using a box with the keyword THURSDAY and
apply the transposition 5-1-4-2-3 to the cipher text. The box is as shown
in Table 5.2
Table 5.2
A B C D E
A T H U R S
B D A Y B C
C E F G I K
D L M N O P
E Q V W X Z
and HAPPY BIRTHDAY enciphers to
ABBBD EDEBC BDCDA DAAAB BABBB C
Two-letter ciphers 55
Since the transposition involves 5 numbers we write the cipher text into a
box which has 5 columns. Note that since there are 26 letters in the cipher
one column in the box needs to have 6 letters in it.The recipient of the
message will need to know which, if any, are the ‘long’ columns so this
will have been arranged beforehand. We will make the arbitrary assumption
that the ‘long’ columns are formed in the order of the transposition.
In the case of this example therefore the solitary ‘long’ column is the one