$($a$)$ Given the ciphertext and key below, find the plaintext if the Playfair cipher was used. $x$ is the special character used for padding and i and $j$ are treated as the same character.

C $=$ eiioqoyldc

$K=$ security

$P=,($write your final answer here; show your calculations below$)$

$($b$)($i$)$ A polyalphabetic cipher such as the above Vigen$$re is stronger against letter frequency analysis when compared to a monoalphabetic cipher like Caesar. Explain why.

$($ii$)$ Despite being stronger than monoalphabetic ciphers, the Vigen$$re cipher is still subject to letter frequency attacks. Explain why.

$($iii$)$ Can the modified Vigen$$re cipher in this question be used as a one$-$time pad. If yes, then explain how. If no$,$ then explain why not.

$($c$)$ Draw a matrix that shows the relationship between security services and attacks.