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

48
chapter 4
and the cipher text is
E F D K L J B C A N O M H I G
which is quite different from the first version.
Other forms of transposition
So far we have written the text in rectangles row by row but there are
alternatives available and some of them would make it harder for a cryptanalyst
to solve the system. In each case the text of the message is broken
up into blocks of the appropriate length to fit the transposition box.
Regular transposition boxes
By a regular transposition box we mean a box that has columns of predictable
length. The text can be written into the box in various ways but care needs
to be taken, otherwise weaknesses may be introduced. Here are some
examples, not all of which can be recommended.
Table 4.10
A B C D E
P Q R S F
O X Y T G
N W V U H
M L K J I
(i) We could use a rectangle but write the text around the rectangle in a
‘spiral’ fashion (Table 4.10). Unfortunately this is definitely not recommended.
No matter what ordering of the columns is used in the transposition
the pentagraph EFGHI and the trigraph STU will appear in
unchanged form in the ‘cipher’. So, for example, if the message is
THISXMETHODXISXNOTXSECURE
and the transposition is 3-1-5-2-4 the box is as shown in Table 4.11,
Table 4.11
3 1 5 2 4
T H I S X
N O T X M
X R E S E
S U C E T
I X D O H