$1.$ Use the Caesar cipher to encrypt and decrypt the message "HELLO," and the key $($shift$)$ value of this message is $15.(1$ mark$)$

$2.$ Use play fair cipher to encrypt and decrypt the message "COMPUTER" is the plaintext and "COMMUNICATION" is the encryption key. $(1$ mark$)$

$3.$ Use the Simple Columnar Transposition to find cipher text for plain text$=$ "Computer Science" $($key order $361524).(0\mathrm{.}5$ mark$)$

$4.$ Use S DES find K$1$ and K$2$ for the given key $1011000110.(0\mathrm{.}5$ mark$)$

$5.$ Using K$1$ and K$2$ obtained from question $4$ find the Encrypt the plain message to find cipher message. $(0\mathrm{.}5$ mark$)$ PT$=11010101$

$6.$ Using K$1$ and K$2$ obtained from question $4$ Decrypt the cipher message to find plain message. $(0\mathrm{.}5$ mark$)$