hop is to node $4.$ The encryption of the shaded area has to be done by the source node in such a

way that

Each router in the network can only find out the address of the next hop, but is

clueless about the remaining path.

After router $4$ receives the packet, it will

$($i$)$ decrypt the routing header and retrieve the next hop address, i$.$e$.3$;

$($ii$)$ left shifts the entire header $($to push out address $4,$ see below$)$ and append a random

number $x\text{'}$ to the end in such a way that people can't tell if $x\text{'}$ is part of the encrypted

path or not;

$($iii$)$ send the packet to router $3.$

Note that the value of appended $x^{\text{'}}$ is not important. Its purpose is to hide the number of hops

left in the remaining path from an intruder. The other nodes along the path will act on the

routing header in the same way.

We assume the following keys are available:

$($a$)$ Write down the encrypted path header by source node $s$ for the following routing path:

$s-$ router $3-$ router $2-$ router $1-$ router $4-d$