Exercise $1$: Exercise $2$:

Given the following Playfair matrix, answer the questions given below:

a$)$ Encrypt the plaintext: Ooredoo wallet. Divide plaintext into groups of two letters

and assume the padding character is the letter $\text{'}x\text{'}.$

Plaintest in $2-$Letter grour

Show the ciphertext for each $2-$letter group of the plaintext:

Ciphertext in $2-$Letter group$*$

Final Ciphertext:

b$)$ Decrypt the ciphertext: KGHNMCIUKLPDICAPQIMKKLBFAU

Plaintext:

Exercise $3$:

a$)$ Using Playfair cipher with the keyword DOHA METRO construct the Playfair

matrix. $($Hint: Use the following $55$ grid, and treat I and J as one letter $($as

shown in Exercise $1).)$

b$)$ Encrypt the plaintext: different stuff using the above matrix.

Plaintext in $2-$etter groups

Show the ciphertext for each $2-$letter group of the plaintext:

Ciphertext in $2-$Letter groups

Final Ciphertext:

c$)$ Decrypt the ciphertext: UGBAKDQIRLQOTBOHBNPDGWKMZAOBRH Exercise $4$:

a$)$ Using Hill cipher encrypt the plaintext "meeting cancelled". The key matrix, K and its

inverse are given as: $K=\left[\begin{array}{ccc}6& 24& 1\\ 13& 16& 10\\ 20& 17& 15\end{array}\right]$;$K^{-1}=\left[\begin{array}{ccc}8& 5& 10\\ 21& 8& 21\\ 21& 12& 8\end{array}\right]($modulo $26)$

b$)$ Using the above Hill cipher's information, decrypt the ciphertext: QRTWNFJVV

$($a$)$ Use a multiplicative cipher with key $=17$ to encrypt the message: "Running at

dawn is better". Use the table below to show details of your computations.

For your convenience, use the following alphabet and the assigned numbers.

$($Hint: add as many rows to the table as needed$)$

Ciphertext: $$

$($b$)$ Using the multiplicative cipher, with key$=17,$ decrypt the ciphertext: "PQFFE AUGA". Show yout

detailed steps in a table similar to that of Exercise $3.[$Hint: What is the multiplicative inverse of $17$ mod

$26?$ Refer to Chapter $2$ slides.$]$