WIRTE TWO PROGRAMS ONE A CLIENT PROGRAM AND ONE A SERVER PROGRAM. WRITE IN C$++$

Objective

You will be introduced to public$-$key cryptography and

learn to use datagram sockets.

PUBLIC$-$KEY CRYPtogRAPHY

Public$-$key cryptography allows people to exchange secret

signed messages without exchanging keys to a secure

channel. To achieve that goal, it assigns to each user a

public key K$\_($p$)$ and a secret key K$\_($s$)$ that satisfy the three

following properties:

Messages encrypted with the public key K$\_($p$)$ of a user

can only be decrypted using the secret key K$\_($s$)$ of that

user.

Messages encrypted with the secret key K$\_($s$)$ of a user

can only be decrypted using the public key K$\_($F$)$ of that

user.

There is no possible algorithm that can compute in a

reasonable amount of time the secret key K$\_($s$)$ of a user

knowing her public key K$\_($P$)$

Your client$/$server Pair

You are to write two programs:

A client program that will connect with your server

and ask for the public key of a given user.

A server program that will read a list of pairs $($user

ID$,$ public key$)$ from a file, wait for connection

requests from client processes, and send them back

either a string containing the requested public key or

a code notifying them that the user is not in the

database.

The server program

Your server should start by prompting the user for a file

name:

Enter a file name: keys$2.$txt

This file will contain pairs of user IDs and public keys as

in:

jorge@uh$.$edu alb$234$c$4567$df$123$abc

eva@rice.edu $5672$c$4567$df$123$abc$54$

Both entries will never contain any white spaces. It

will then store the contents of the file into a table. Your

server should then prompt for a port number as in:

Enter the server port number: $2468$

It will then:

Create a datagram socket in the Internet domain,

Do a bind$()$ to bind the socket to the specified port

number.

Enter an infinite loop where it will repeatedly do a

recvfrom$()$ to receive user IDs from its clients then a

sendto $()$ to reply with the corresponding public keys.

For debugging and grading purposes, your server

should print out the user ID it receives and its status as:

jorge@uh$.$edu: Found a public key

rick@tamu.edu: Not in the database

Your server will keep accepting client requests until

it receives a Done message from one of its clients. In that

case, it should terminate after printing:

Terminated by user request.

THE CLIENT PROGRAM

Your client should start by prompting the user for a

server hostname and a server port number as:

Enter the server hostname: localhost

Enter the server port number: $2468$

It will then prompt for a user ID as:

Enter a user ID: jorge@uh$.$edu

It should then create a datagram socket, do a

sendto $()$ to send the user ID to the server then a

recvfrom$()$ to get its reply. Once this reply has arrived,

the client should display the outcome of the query as:

The public key for jorge@uh$.$edu is alb$234...$

The user rickgtamu.edu is not in the database.

Unlike the server, each client will only handle a single

request.

Hints

Use a single$-$threaded server to keep things simple.

If you do your assignment on the WSL you should

know that the hostname of your PC does not contain a

domain, say, something like cs$.$uh$.$edu. Use

instead localhost.

Please name the two files you will submit

FirstName$\_$LastName$\_$client. $\{$c$,$ cpp$\}$ and

FirstName$\_$LastName$\_$server$\{$c$,.$cpp$\}.$