Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
Implement the Playfair (Wheatstone) Cipher in a script.
The Playfair Cipher encrypts text by substitution of each 2−letter "digram" (grouping). Traditionally,
one would use a 5 x 5 letter scrambled alphabet code key square for the encryption and decryption.
C O D E S
A B F G H
I K L M N
P Q R T U
V W X Y Z
Each letter of the alphabet appears once, except "I" also represents
"J". The arbitrarily chosen key word, "CODES" comes first, then all the
rest of the alphabet, skipping letters already used.
To encrypt, separate the plaintext message into digrams (2−letter
groups). If a group has two identical letters, delete the second, and
form a new group. If there is a single letter left over at the end,
insert a "null" character, typically an "X".
THIS IS A TOP SECRET MESSAGE
TH IS IS AT OP SE CR ET ME SA GE
For each digram, there are three possibilities.
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
1) Both letters will be on the same row of the key square
For each letter, substitute the one immediately to the right, in that
row. If necessary, wrap around left to the beginning of the row.
or
2) Both letters will be in the same column of the key square
For each letter, substitute the one immediately below it, in that
row. If necessary, wrap around to the top of the column.
or
3) Both letters will form the corners of a rectangle within the key
square. For each letter, substitute the one on the other corner the
rectangle which lies on the same row.
The "TH" digram falls under case #3.
G H
M N
T U
(Rectangle with "T" and "H" at corners)
T −−> U
H −−> G
The "SE" digram falls under case #1.
C O D E S (Row containing "S" and "E")
S −−> C (wraps around left to beginning of row)
E −−> S
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To decrypt encrypted text, reverse the above procedure under cases #1
Appendix I. Exercises 438