Network Programming (Secured Client-Server Chat Application)

1.1 INTRODUCTION
Several network systems are built to communicate with one another and are made
available through service-oriented architectures. In this project, we use the client
server architecture to develop a secured Client-Server chat application. A chat
application is created based on Transmission Control Protocol (TCP) where TCP is
connection oriented protocol and in the end, multithreading is used to develop the
application.
A client-server chat application consists of a Chat Client and a Chat Server and
there exists a two way communication between them. Here, Message Processor is
used to interpret message from the user, Message Interpreter is used to extract and
pass the received message. Message Maker is used to construct back the message
and Client Manager is used to maintain the clients list which the sender and
receiver at both sides use to interact with each other.
In general, the server process will start on some computer system; in fact, the
server should be executed before the client. Server usually initializes itself, and
then goes to wait state or sleep state where it will wait for a client request. After
that, a client process can start on either the same machine or on some other
machine. Whenever the client wants some service from the server, it will send a
request to the server and the server will accept the request and process it. After the
server has finished providing its service to the client, the server will again go back
to sleep, that is, waiting for the next client request to arrive. This process is
repeated as long as the server processes is running. Whenever such request comes,
the server can immediately serve the client and again go back to the waiting state
for the next request to arrive.

1.2 BACKGROUND OF THE STUDY
Client server model is the standard model which has been accepted by many for
developing network applications. In this model, there is a notion of client and
notion of server. As the name implies, a server is a process (or a computer in which
the process is running) that is offering some services to other entities which are
called clients. A client on the other hand is process (which is running) on the same
computer or other computer that is requesting the services provided by the server.
A chat application is basically a combination of two applications:
 Server application
 Client application
Server application runs on the server computer and client application runs on the
client computer (or the machine with server). In this chat application, a client can
send data to anyone who is connected to the server.
Java application programming interface (API) provides the classes for creating
sockets to facilitate program communications over the network. Sockets are the
endpoints of logical connections between two hosts and can be used to send and
receive data. Java treats socket communications much as it treat input and output
operations; thus programs can read from or write to sockets as easily as they can
read from or write to files.
To establish a server connection, a server socket needs to be created and attached
to a port, which is where the server listens for connections. The port recognizes the
Transmission Control Protocol service on the socket. For instance, the email server
runs on port 25, and the web server usually runs on port 80.
Server Execution: At server the side, a thread is created which receives numerous
clients’ requests. It also contains a list in which Client’s name and IP addresses are
stored. After that, it broadcast the list to all the users who are currently in chat
room and when a client logs out then server deletes that particular client from the
list, update the list and then broadcast the list to all available clients.
Client Execution: A client firstly must have to register itself by sending username
to the server and should have to start the thread so that system can get the list of all
available clients. Then any of two registered clients can communicate with each
other.

1.3 STATEMENT OF THE PROBLEM
The client-server communication model is used in a wide variety of software
applications. Where normally the server side is sufficiently protected and sealed
from public access, but client applications running on devices like notebooks and
desktops are considered insecure and exposed to security threats.
The main weakness of client-server chat application is that there is no security
provided to data which is transferred between clients. Any unauthorized client can
hack the client account and can change the data. This is the main objective of this
project (To develop a secured Client-Server Chat Application).

1.4 OBJECTIVES OF THE STUDY
The aim of this project is to develop a reliable and secure network programming
(Client-Server chat model) which can perform a multithreaded server client chat
connection is established between them then they can communicate with each
other.
Client: A client is a system that accesses or desires for a service made accessible
by a server.
Server: A server is a system (hardware or software) program running to provide
the service requests of other system programs.
Port: Port is a software mechanism that allows the centralized connected Servers
to listen for requests made by clients. Port is actually purposed as a gateway to
listen for the requested parameters by the server terminals or other machines. It is a
software address on a system that is on the network. Entire request response
proceeding among this Application is carries through machine ports.
Network: This refers to a system were computers are linked to share software,
data, hardware and resources for the benefit of users.
Interface: This may be software or hardware that upon an agreed method spells
out the manner a system component can exchange information with another system
component.
Secure socket layer (SSL): This refers to Secure Sockets Layer protocol that is
used for encryption of data for secure data transmission.
IP: This refers to Internet Protocol; it is the reasonable network address of device
on a network. It is notational called dotted-decimal (for instance: 128.1.0.1).
Thread: A thread is a section of code which is executing independently of others
threads in a same program. Java has a class Thread which is defined in java.lang
package. Thread is the most powerful feature that JAVA supports from other
programming languages.