Question $5$

$($a$)$ Name and briefly explain the THREE security goals for information to be secured from attacks.

$(3$ marks$)$

$($b$)$ In a system with $15$ users, every person has to communicate with every other person confidentially. Determine the total number of keys needed for each system below:

$($i$)$ Symmetric$-$key system

$(2$ marks$)$

$($ii$)$ Asymmetric$-$key system

$(2$ marks$)$

$($c$)$ Suppose transposition cipher is used to encrypt messages with key $562341.$ If required, the message is padded with character @$.$

Encrypt the message "NETWORKING". Show your steps AND ${?}_{(})$ the encrypted message.

$(4$ marks$)$

$($d$)$ In a network using Go$-$Back$-$N ARQ protocol with $m=4,$ the maximum window size is used. The values of the variables are $S_f=45,S_n=54,$ and $R_n=49.$ Assume that the network does not duplicate or reorder the packets. Determine the followings:

$($i$)$ The maximum sizes of the sender and receiver windows

$(2$ marks$)$

$($ii$)$ The sequence numbers of the packets still in transit

$(2$ marks$)$

$($iii$)$ The acknowledgment numbers of the ACKs still in transit

$(2$ marks$)$

$($iv$)$ The sequence number of the next packet to be sent at the sender

$(1$ mark$)$

$($v$)$ An ACK with acknowledgment number $48$ has just arrived at the sender$.$ Determine the values of $S_f$ and $S_n$ AFTER this ACK is received.

$(2$ marks$)$