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

76
chapter 7
We first convert the texts to numbers by using the table:
Text 1 T H E X C U R F E W X T O L L S X
19 7 4 23 2 2017 5 4 222319141111 1823
Text 2 O N C E X U P O N X A X T I M E X
14 13 2 4 23 20 15 14 13 23 0 23 19 8 12 4 23
Now we add them and then reduce them (mod 26):
19 7 4 232 20175 4 2223191411111823
14 13 2 4 23 20 15 14 13 23 0 23 19 8 12 4 23
Sum: 33206 2725403219174523423319232246 (mod 26): 7 20 6 1 25 14 6 19 17 19 23 16 7 19 23 22 20
Finally we convert the numbers back into letters, using the table:
H U G B Z O G T R T X Q H T X W U
and this is the cipher text that would be sent. The recipient would, of
course, need to subtract Text 2 from the cipher (mod 26) in order to recover
Text 1, viz:
Cipher H U G B Z O G T R T X Q H T X W U
Text 2 O N C E X U P O N X A X T I M E X
Convert 7 20 6 1 25 14 6 19 17 19 23 16 7 19 23 22 20
14 13 2 4 23 20 15 14 13 23 0 23 19 8 12 4 23
Subtract (mod 26), i.e if the result is negative, add 26:
Re-convert to letters:
19 7 4 23 2 2017 5 4 2223191411111823
T H E X C U R F E W X T O L L S X
which is Text 1, the original ‘message’.
Obviously it would be a very tedious and error-prone process to have to
convert the texts to numbers, add them, subtract 26 where necessary, and
re-convert to letters every time a message was to be enciphered so it is very
worthwhile having two tables, one for enciphering and one for deciphering,
from which the result of applying these processes can be read off
immediately. Experienced users would not need such tables since they
would soon learn to ‘add the letters’ at sight but for others the tables save
a lot of time and effort. They are given in Tables 7.2 and 7.3. Notice that in
the encipher table (Table 7.2) it makes no difference whether we call the