Alice and Bod have decided to use a symmetric encryption algorithm. They have some

assumptions about their messages:

$-$ Messages only contain capital letters $($i$.$e$.$ A to Z$)$

$-$ The length of their shared key must be greater than or equal to the length of the plaintext

$-$ They assign each letter a number as follows: $($A$,0),($B$,1),($C$,2),($D$,3),...,($Z$,25)$

Their algorithm combines the key and the message using modular addition. The numerical values

of corresponding message and key letters are added together, modulo $26.$ For example, if the plain

text is $$HELLO$$ and the key is $$SECRET$$ then the encrypted message is calculated as following:

Since the length of the plaintext is $5,$ we just need the first $5$ letters of the key $($i$.$e$.$SECRE$),$ then

for each letter, we should add corresponding letters in both the plaintext and the key modulo $26.$

Plaintext: H $(7)$ E $(4)$ L $(11)$ L $(11)$ O $(14)$

Key: S $(18)$ E $(4)$ C $(2)$ R $(17)$ E$(4)$

Cipher: Z $(25)$ I $(8)$ N$(13)$ C$(2)$ S $(18)$

a$)$ Write a program in Python, C$/$C$++$ or JavaScript to take both the plaintext and the key as

its input, then print out the cipher. Assume the plaintext is your name, for the

key$=$THISISANEXAMPLEKEYINCOMPUTERSECURITYEXAM$$ what is the

output? Add the screenshots of your program along the input and output and submit it as a

PDF file. You should also submit the source code $[10$ marks$].$

b$)$ Explain how one can decrypt the encrypted message using the encryption algorithm in part

$($a$)?$ Write a program, in JavaScript, C$/$C$++$ or Python to take both the plaintext and the key

as its input, then print out the plaintext. Test it using the results of the previous program.

Add the screenshots of your program along the input and output and submit it as a PDF

file. You should also submit the source code $[10$ marks$].$

c$)$ Under what conditions $($specify two$)$ can a cipher be broken, meaning that an attacker can

decipher the encrypted message without having the key? Support your reasoning with

example$($s$)[6$ marks$].$

d$)$ Identify THREE key factors for defining a robust key to enhance algorithm security?

Explain your reasons $[6$ marks$].$