a$)$ An internal web server with an IP address $119\mathrm{.}40\mathrm{.}114\mathrm{.}70$ is located inside a corporate network Y$.$ A packet filter firewall is situated at a chokepoint between the network and the rest of the Internet. Can the packet filter block all incoming email containing the phrase "Computer and Network Security"? If yes, show a packet filtering ruleset that provides this functionality; if no$,$ explain why a packet filter cannot do it$.$

$(3$ marks$)$

b$)$ A software detects distributed Denial$-$of$-$Service $($DDoS$)$ attacks by examining the source of Internet Protocol $($IP$)$ address on all incoming packets. Any IP address that accounts for more than $\backslash (1\backslash \%\backslash )$ of the traffic over the last hour will be considered harmful. All packets from that address is subsequently blocked for the next $24$ hours. Is this a good solution for defeating DDoS? Discuss your answer.

$(3$ marks$)$

c$)$ Ali sends an encrypted message to Ahmed using the combination of one$-$time pads, $6$columnar transposition, and Caesar substitution algorithms with the shift of $5,$ as shown in Figure $1.$ The generated ciphertext of this system is:

$"$Q S H S V F O W X R N O L R L L$".$

Derive the original plaintext.

$(4$ marks$)$

Figure $1.$