Consider the following polyalphabetic cipher with key pattern $\}\{2\}\}.$ Encrypt "football".

Plaintext letter: $abcd$ fghijklmn@pqrstuvwxyz

$C_1(k=5)$:$,$fghijklmn@pqrstuvwxyzabcde

$C_2(k=19)$:$,$tuvwxyzabcdefghijklmn@$pqrs$

Consider an $8-$bit block cipher.

a$.$ How many possible input blocks does this cipher have?

b$.$ How many possible mappings are there?

c$.$ If we view each mapping as a key, then how many possible keys does this cipher have?

Using Counter $($CTR$)$ mode of operation and the following $3-$bit block cipher for encryption$/$decryption$,$

encrypt the plaintext $000001111$ with initial counter value $011($denotes decimal $3).$ Then, perform decryption

to obtain the original plaintext message.

A specific $3-$bit block cipher