Introduction to Cryptography with Coding Theory, 3rd Edition

C.2 Examples for Chapter 2

Example 1

A shift cipher was used to obtain the ciphertext kddkmu.

Decrypt it by trying all possibilities.

>> allshift(’kddkmu’)


kddkmu
leelnv
mffmow
nggnpx
ohhoqy
piiprz
qjjqsa
rkkrtb
sllsuc
tmmtvd
unnuwe
voovxf
wppwyg
xqqxzh
yrryai
zsszbj
attack
buubdl
cvvcem
dwwdfn
exxego
fyyfhp
gzzgiq
haahjr
ibbiks
jccjlt

As you can see, attack is the only word that occurs on this list, so that was the plaintext.

Example 2

Encrypt the plaintext message cleopatra using the affine function $7x+8$:

>> affinecrypt(’cleopatra’,7,8)

ans =


’whkcjilxi’

Example 3

The ciphertext mzdvezc was encrypted using the affine function $5x+12$. Decrypt it.

SOLUTION

First, solve $y\equiv 5x+12(\mathrm{m}\mathrm{o}\mathrm{d}26)$ for $x$ to obtain $x\equiv 5^{-1}(y-12)$. We need to find the ...