Question $4$

$($a$)$ Suppose transposition cipher is used to encrypt the message "ASYMMETRICKEY$123".$ The key used is $634215.$ If required, the message is padded with the character #$.$ Show your steps $()$ the encrypted message.

$(5$ marks$)$

$($b$)$ A certain algorithm creates message digests by adding the numeric values of all the letters in the message $($e$.$g$.A=1,B=2,$dots.$Z=26).$ The digest is taken as the result of the sum of the numeric values modulo $19.$

$($i$)$ Find the message digest of the message "ALPHABET".

$(4$ marks$)$

$($ii$)$ The message, together with the message digest, is sent through an insecure channel. At the receiver, the digest calculated from the received message is found to be $5.$ Should the message be accepted? Why?

$(2$ marks$)$

$($c$)$ Assume there is a server with domain name

wWw$.$networking.edu.hk$.$

$($i$)$ Show an HTTP version $1\mathrm{.}1$ request that needs to retrieve the document $/$usr$/$teacher$/$img$.$ The client accepts MIME version $1\mathrm{.}0,$ GIF or JPEG image. The length of the document is $2048$ bytes.

$(4$ marks$)$

$($ii$)$ Assume that the request to part $($i$)$ is successful. Show the HTTP response, assuming the request is sent at the date and time this exam is due.

$(5$ marks$)$