Notre Dame UniversityFaculty Of Natural And Applied Sciences

Department of computer Science

DEVELOPING ACHAT SERVER

A senior study inComputer Science

By

Waleed Farah

Fall 2000

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Abstract

Teleconferencing or Chatting, is a method of using technology to bring people and

ideas “together” despite of the geographical barriers. The technology has been

available for years but the acceptance it was quit recent. Our project is an example of

a chat server. It is made up of 2 applications the client application, which runs on the

user’s Pc and server application, which runs on any Pc on the network. To start

chatting client should get connected to server where they can practice two kinds of

chatting, public one (message is broadcasted to all connected users) and private one

(between any 2 users only) and during the last one security measures were taken.

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Acknowledgments

Many People contributed to the success of this project. Although a single

sentence hardly suffices, I would like to thank some of the key people:

Mr. Youssif Simonouglo, Professional Technical Support Sita; Mr. Maroun Attallah,

Director of N.D.U Computer Center, and all N.D.U Computer Center Staff.

For their helpful comments and suggestions, I thank:

Mr. Armen Balian, N.D.U Analyst and Programmer.

Mr. Victor Sawma, N.D.U Analyst and Programmer.

Special thanks to Dr. Marie khair for supervisoring this project all the semester.

Another special thanks go to my teachers, friends, and my family, whose

patience and encouragement were the catalyst of this project.

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Dedication

This book is dedicated to you, the reader, for deciding to embark on fascinating

journey of discovery and exploration so that you can implement innovate windows

sockets application.

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Table of Contents

1. Introduction 11.1. Why Chat Server? 21.2. Why Visual Basic? 21.3. General OverView 3

2. Network Programming 42.1. Network application 52.2. Protocol Stacks 5 2.2.1. TCP 6 2.2.2. IP 62.3. Service Port 72.4. Sockets 7 2.4.1. What is WinSock? 8 2.4.1. How WinSock Works 8 2.4.3. Family Protocols 102.5. Client – Server Application 102.6. Blocking Vs. Non-Blocking 112.7. System Requirement 12

3. Windows Socket With Visual Basic 13 3.1. Properties Of Windows Socket Control 14 3.1.1. Host Name Property 14 3.1.2. Remote Port Property 15 3.1.3. Local Port Property 15 3.1.4. Protocol Property 15 3.1.5. Action Property 16 3.1.6. State Property 16 3.1.7. RecvLen Property 17 3.1.8. Send Data Property 17 3.1.9. Address Family 17 3.1.10. SocketType property 17 3.1.11. BufferSize Property 18 3.1.12. Binary Property 18 3.2. Method of Windows Socket Control 18 3.2.1. Accept Method 18 3.2.2. Close Method 19 3.2.3. Listen Method 19 3.2.4. Connect Method 19 3.3. Events 19 3.3.1. Read Event 20 3.3.2. Disconnect Event 20 3.3.3. Connect Event 20

4. Algorithm Used4.1. Project Structure 21 4.1.1. Control String 22 4.1.2. Client 23 4.1.3. Server 24

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4.2. Private Chatting 25

5. Problem Encountered 26 5.1. Multi User 27 5.2. How To Allow Multi User 27 5.3. Private Chatting 30 5.4. Keep Track Of Users 30 5.5. How To Keep Track Of Users 30 5.6. Private Chatting 31 5.6.1. Open Window 31 5.6.2. Keep Track Of Windows 32

6. Encryption 33 6.1. Data Encryption/Decryption 34 6.2. XOR Encryption 35

Conclusion 38References 39

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List Of Figures

Figure 4.1.2.1. Client Form 23Figure 4.1.3.1. Server Form 24Figure 4.2.1. Private Form 25Figure 5.2.1 Multi Users 29

List Of Tables

Table 2.4.1. Windows Socket Layer 9Table 3.1.1. Server States 16

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CHAPTER 1

INTRODUCTION

In this chapter we are going to deal with the major points behind choosing a chatserver as a senior project and why visual basic is the programming language Ichoose, with a general overview about the project and how it works. So turn thispage quickly and get yourself ready to find how it is working.

1.1. Why Chat Server ?

One year ago I was reading an article about Teleconferencing / chatting, which

as we said it is a method of using technology to bring people and ideas “together”

despite of the geographic barriers. From that time and on I started wondering if I can

participate in that technology and how? The idea of creating a chat server was

initiated and it has two dimensions, one to have an experience in network

programming and the other is to let people conduct meetings with others in different

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locations. Such that it crosses time zones, can reach many people, and reduce the

paper flood.

1.2. Why Visual Basic ?

Two main reasons let me choose Visual Basic as the programming language

for this project. The first is that Visual Basic 6 is Microsoft’s latest and greatest

language for Visual programming. Although writing programs can be a tedious chore

at times, Visual Basic reduces the effort required on your part and makes

programming enjoyable. Visual Basic makes many aspects of programming as simple

as dragging graphic objects onto the screen with your mouse.

The second reason which is the main one is that we are going to use a control

called windows socket or WinSock as you will see in the following chapters, and this

control is some how has a low level interface. The only high level languages that

deal with windows sockets is Visual Basic and Visual C++, and since Visual Basic is

more easy to learn in small period of time than Visual C++, it was chosen to be the

programming language.

1.3. General Overview

As a matter of fact there are several varieties of chatting. The simplest

computer chatting is a method of sending, receiving, and storing typed messages with

a network of users. This network could be WAN (Wide Area Network) or LAN(Local

Area Network). Our chatting system will deal only with LAN’s (static IP address) and

it is made up of two applications one runs on the server side (any computer on the

network you choose it to be the server) while the other is delivered and executed on

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the client PC. Every time the client wants to chat he runs the client application, enter

his user name, host name where the server application is running, and hits the connect

button and start chatting. The system is many-to-many arrangement; every–one is

able to “talk” to anyone else. Messages may be broadcasted to all receivers

(recipients are automatically notified of incoming messages) or sent to special

individuals (private chatting through server) where during this operation all messages

are encrypted at the sender side and decrypted at the recipient to disallow any hackers

to the server from reading these private messages. For this system to be physically

realized you should be familiar with programming and networking. Visual Basic is

our programming language, TCP/IP is our network protocol, and finally “windows

sockets” is our programming interface to have access to network functionality. This

is a first glance at our senior project and the rest of explanations and ideas are eagerly

waiting for you in the next chapters.

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CHAPTER 2

NETWORK PROGRAMING

In this chapter, we are going to give some light on the main concepts and ideas we use

to build our project. What we mean by network application, protocols, ports, and how

do they interact with each other. Windows sockets, how it works and how we are

going to use it in our application. All these information will be available for you in

this chapter.

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2.1. Network Application

Network application exchange data between physically separated machines.

For this to occur the machines must be connected by a transmission media. There are

many different types of communication links and new ones continue to be developed.

Coaxial cables, phone lines, digital phone lines, fiber optic cable, satellite beam, and

infrared waves are all used as transmission media for exchange data between

computers.

A network includes a group of computers connected by a physical link

allowing data to be exchanged between them. A local are network on LAN is a

network of computers in close physical proximity, usually a single building, but can

be a group of adjacent buildings. Over the last decades LANs have become an

important component of the computer workplace.

2.2. Protocol Stacks

Very Early in the history of computer network development the concept of

separating the problem into multiple levels was adapted. With a multilevel

architecture each layer can handle a different aspect of networking and provide that

functionality to the above layer. TCP/IP is a specific implementation of a multi level

network architecture. In both, the first and second chapter, we are always repeating

the same sentence, which is TCP/IP protocol. It is now the time to dissect this

sentence.

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2.2.1. TCP

TCP (the “Transmission Control Protocol “) has the responsibility for

breaking up the message into datagrams, reassembling them at the other end,

resending anything that gets lost, and putting things back in the right order. It may

seem that TCP is doing all the work. And in small network it is true. With TCP, there

is no maximum message length. When a message is passed to the TCP protocol, if it

is too large to be sent in one peace, the message is broken up into chunks or packets

and sent one at a time to the destination address. The TCP packet contains the

addressing information. The TCP message also contains a packet number and total

number of packets. Because of the nature of the TCP/IP protocol, the packet may

travel different paths and may arrive in a different order than sent. TCP reassemble

the packets in the proper order and requests the retransmission of any missing or

corrupted packets. TCP enables you to create and maintain a connection to a remote

computer. By using the connection, both computers can stream data between each

other.

2.2.2. IP

As the number of computers networked become larger, a system becomes

necessary to give remote computers the capability to recognize other remote

computers; thus the IP addressing method was born. Therefore, simply an IP address

uniquely identifies any computer connected to a network. This address is made up of

32 bits divided into 4 four bytes. But since the number of connected computers is too

large and since it is difficult to remember all their IP addresses, the Domain Name

Service (DNS) was designed. It has the job of transforming the unique computer

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names (host name) into an IP address. Therefor, whenever in our project we run the

client application and enter the host name, this means that we are writing the IP

address of the remote computer we want to connect to indirectly. In general, TCP/IP

is a set of protocols developed to allow cooperating computers to share resources

across the network.

2.3. Service Port

Till now, we have seen that TCP/IP forms the backbone for communication

between computers, but do you know how these computers speak to each other?

The answer is Ports. A port is a special location in the computer’s memory that

exists when two computers are communicating via TCP/IP. Application uses a port

number to communicate and the sending and receiving computers use this same port

to exchange data. To make the job of communication easier, some port numbers have

been standardized, ex, (www Port 80, Ftp Port 20, 21, Etc). Our application uses a

constant named IP-echoport = 7.

2.4. Sockets

The world is defining itself as a largely Intel-processor, windows-based set of

desktops communicating with back end servers of various types. Hardware and

software technology advances are pushing PC’s into the role of every where

communications devices. For software applications to take advantage of increasingly

sophisticated and feature-rich communications technology, they require an

Application Programming Interface (API) which provides a simple and uniform

access to this technology. WinSock has been this interface for TCP/IP on windows

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systems for the last 3 years. It is now set to become the definitive applications

interface for all windows-based communication-capable applications.

2.4.1. What is WinSock ?

Windows Socket or “WinSock” as it has become known, is an open

network API standard. It was first designed to create a standard programming

interface for TCP/IP on all versions of Microsoft Windows including Windows 3.x.

The major benefit of WinSock to date have come in 3 areas. First, by providing an

open API standard rather than a closed propriety API, it has helped foster the success

of TCP on Microsoft operating system. Second, application developers have been

able to easily create applications that will run on many different vendors’ TCP/IP

network stack. Third, it has made it much easier for end users to select both

applications and network stacks – they need only look for WinSock compatibility.

There are now hundreds of WinSock-capable applications and an ever-increasing

number of users mandating and relying on this open standard for the connectivity

delivered by their TCP/IP applications.

2.4.2. How WinSock Works

The previous sections described what information a program needs to

communicate over a TCP/IP network. The next step is for a program to create what

is called a socket; a communication end-point that can be linked to a telephone.

However, creating a socket by itself doesn’t let you exchange information (having a

telephone doesn’t mean that you can talk by simply taking it off the hook). You need

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to establish a connection with the other program. Table 2.4.1. shows how winsock

works

Table 2.4.1 the windows socket layer

Wireless stackTCP/IP Stack

Wireless

Service provider Interface

(SPI)

Name Space

SPI

TCP/IP

Protocol Specific

Annex

TCP/IP

SPI

Winsock 2 DLLS

Application Layer

The winsock DLL is a layer between an application and the TCP/IP stack. The

application tells the Winsock DLL what to do. These instructions then go to the

TCP/IP stack, and the TCP/IP stack passes them along to the network. To do this,

you need a socket address of the application that you want to connect to. This

address consists of 3 parts: protocol family, Internet protocol, and the service port

number. We have already covered the service port and protocol, but what is protocol

family?

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2.4.3. Family Protocol

It is a number used to logically designate the group that a given protocol

belongs to. Since the socket is general enough to be used with any protocols, the

protocol family tells the underling network software which protocol is being used by

the socket. In our project it is given the value AF_INET, which s a global constant

with the integer value of 2.

2.5. Client-Server Application

When the program wishes to use TCP to exchange data, one of the programs

should take the role of a client while the other must take the role of a server. The

client application initiates what is called active open. It creates a socket and actively

attempts to connect to server program. On the other hand, the server application

creates a socket and passively listens for incoming connections from client,

performing what is called passive open. When the client wants to connect a server, it

sends a connection request. The server is notified that some process is trying to

connect with it. By accepting the connection, the server completes what is called a

virtual circuit, a logical communication pathway between the two programs.

To review, there are five significant steps that a program that uses TCP must take to

establish and complete a connection.

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• The server side would follow these steps:

1. Create a socket

2. Listen for incoming connections from clients

3. Accept the client connection

4. Send and receive information

5. Close the socket when finished, terminating the conversation

• In case of the client, these steps are followed:

1. Create a socket

2. Specify the address and service port of the server program

3. Establish the connection with the server

4. Send and receive information

5. Close the socket when finished, terminating the conversation

2.6. Blocking vs. Non-blocking Mode

A socket can operate in blocking or non-blocking mode. This mode is

associated with the socket and affects the operation of all relevant commands on that

socket. The socket commands affected by the blocking mode are: Accept ( ),

Connect ( ), Disconnect ( ), Read ( ), Write ( ).

In general, in blocking mode, commands wait to return until the required

network activity completes, while in the non-blocking mode, commands fail if the

required network activity has not completed when the command is issued. In non-

blocking mode, issuing a command can be used as a method of polling for the

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associated network event completion. It is highly recommended that in WinSock

application to use the non-blocking mode as my application does.

2.7. System Requirements

The SocketWrench control requires Microsoft Windows 3.1 or later, Visual

Basic 2.0 or later and networking software that supports the Windows Sockets 1.1

specification. For 16-bit platforms, the Visual Basic or ActiveX control should be

installed in the system directory. For 32-bit platforms, the ActiveX control should be

placed in the system directory under Windows 95, or the system32 directory under

Windows NT.

Windows 3.1 does not come with TCP/IP software as part of the operating

system, so you will need a third-party product for that platform. One of the most

popular TCP/IP stacks for this platform is written by Trumpet Software International,

and can be used over local and dial-up networks. Windows for Workgroups 3.11 does

not include TCP/IP software, but Microsoft does offer one for free. However, it does

not support dial-up networks, so if that is a requirement, you will still need a third-

part product.

Windows 95 and Windows NT both have TCP/IP software included with the

operating system. Both local and dial-up networks are supported, with dial-up

networking installed as part of the Windows Remote Access Services subsystem. If

you are using Windows 95, it is recommended that you obtain the Dial-Up

networking 2.1 update that includes a number of enhancements to the product. You

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can obtain further information about this update by searching the KnowledgeBase on

the Microsoft website.

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CHAPTER 3

WINDOWS SOCKETS WITH VISUAL BASIC

With the acceptance of the TCP/IP as a standard platform-independent network

protocol, and the explosive growth of the internet, the Windows Socket API

(Application Program Interface) has emerged as the standard for network

programming in the Window’s environment. There are two general approaches that

you can follow when creating a program that uses Windows Sockets. The first is to

code directly against the API. The second-which we use in our project-is to use a

component, which provides a higher level interface to the library by setting properties

and responding to events, which we will describe later in the next chapters. So this

chapter has the responsibility of describing this control and how we can make use of it

in our project. Therefore, psyche yourself up to get motivated to explore the beauty

of Windows Socket Control.

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3.1. Properties of Windows Socket Control

Winsock/socket wrench enables you to create clients and servers using the

same control. These dual functioning enables you to specify through property setting

the type of application you will be building. The socket control uses a number of the

same properties, whether you are creating a client or a server, thereby all but

eliminating the learning curve needed to create applications. The next couple of

sections describe some of the properties we used and available to the socket control

and how they can be called in an application.

3.1.1. Host Name Property

The Host Name Property returns or sets the remote host. Remote host

can be both read from and written to and is available both at design time and run

time. The value returned is a string and can be specified either as IP address such as

(204.246.66.6) or as DNS name (www.microsoft.com). The following is an example

of the syntax to read the property:

RetVa1 = winsock.Hostname

To set the property, you use: winsock.Hostname = “127.0.0.2”

Sitting this property at run time enables the user to select the remote host when the

application starts or based on some criteria.

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3.1.2. Remote Port Property

The Remote Port Property returns or sets the remote port number. This

property can be both read from and written to and is available both at design time

and run time. The value returned is a long integer. An example of the syntax to read

property is:

RetVar = winsock.remoteport

To set the property, use the following:

Winsock.remoteport = 7

This property can be used to select the application that is to be contacted at the

remote host.

3.1.3. Local Port Property

The Local Port Property returns or sets the local port number. Local Port

can be both read from and written to and is available both at design time and run

time. The value returned is a long integer. The following is an example of syntax to

read the property:

RetVar = winsock.localportTo set the property, you use the following:

Winsock.localport = 7

3.1.4. Protocol Property

The Protocol Property returns or sets the protocol of either UDP or TCP.

This property can be both read from and written to and is available both at design

time and run time. The value returned is 0 (or the constant SckTCP Protocol) or 1

(SckUDPProtocol). Example:

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RetVar = winsock.ProtocolTo set the property, you use:

Winsock.localport = SckTCPProtocol

3.1.5. Action Property

Setting this property causes the socket to take some action, such as creating

a socket connecting to a remote system or closing a connection. Example:

Winsock.Action = action

3.1.6. State Property

The State Property returns the state of the control as expressed by

enumerated list. This property is read-only and is unavailable at design time. The

state property is set by using various methods and events. The syntax to read the

property: RetVar = winsock.state

Table 3.1.1. shows the setting for the state property:

Table 3.1.1. server states

Constant Value DescriptionSckClosed 0 DefaultSckOpen 1 Open

SckListening 2 ListeningSckConnection Pending 3 Connection Pending

SckResolving Host 4 Resolving HostSckHost Resolved 5 Host Resolved

SckConnecting 6 ConnectingSckConnected 7 Connected

SckClosing 8 ClosingSckError 9 Error

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3.1.7. RecvLen Property

Sets the maximum number of bytes to read, or return the number of bytes

read. Example:

Winsock.Recvlen = [[ number of characters to be sent ]]

3.1.8. Send Data Property

Writes data to the remote server. Example:

Winsock.Senddata = [[ data to be sent ]]

3.1.9. AddressFamily property

This property is part of the socket address, and should always be set

to a value of AF_INET, which is global constant with the integer

value of 2.

3.1.10 SocketType property

This property specifies the type of socket that is to be created. It

may be either of type SOCK_STREAM or SOCK_DGRAM. The stream-

based socket uses the TCP protocol, and data is read and written on the

socket as a stream of bytes, similar to how data in file is accessed. The

datagram-based socket uses the UDP (uniform datagram protocol) protocol,

and data is read and written in discrete units called datagrams. The socket

we use is a stream one.

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3.1.11. BufferSize Property

This property is used only for stream based (TCP) sockets. It

specifies the amount of memory, in bytes, that should be allocated for the

socket’s send and receive buffer.

3.1.12. Binary Property

This property determines how data should be read from the socket.

If set to a value of True, then the data is received unmodified. If set to False,

the data is interpreted as text, with the carriage return and linefeed characters

stripped from the data stream. Each receive returns exactly one line of text.

3.2. Methods of Windows Socket Control

Methods are predefined functions used to perform various tasks on the control.

There are methods that open and close a connection and methods that accept a request

for the connection. The following are the most important ones of them:

3.2.1. Accept Method

The Accept Method is used for the TCP server applications only. It accepts

the request for connection from client system. For the Accept Method to be used, the

control must be in a listening state. This method is used in conjunction with the

connection request. Example:

Private sub winsock connection request (ByVal request ID as integer)

Winsock (Numsockets).Accept request ID

End sub

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3.2.2. Close Method

This method is used to terminate a TCP connection.

Syntax:

Winsock.close

3.2.3. Listen Method

The Listen Method is invoked on the server application to have the server

application wait for a TCP request for connection from client system.

Syntax:

Winsock.listen

3.2.4. Connect Method

The Connect Method requests a connection to remote computer according

to host name and remote port.

Winsock.connect

3.3. Events

Events are triggers that invoke the methods. The winsock control generates

events that also can be used. Some of these events, such as the connection request,

happen at the server as a result of an action taken by the client. The events generated

by the winsock control make it possible for an unattached system to participate in a

network communications.

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3.3.1. Read Event

The Read Event is generated when data is available to be read.

Syntax:

Sub winsock-read ([[ index as integer,]] datalength as integer)

3.3.2. Disconnect Event

The Disconnect Event occurs when the remote computer closes the

connection.

Syntax:

Winsock-disconnect (index as integer)

3.3.3. Connect Event

The Connect Event occurs after the connection with remote computer has

been made.

Syntax:

Private sub winsock-connect (index as integer)

End sub

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CHAPTER 4

ALGORITHM USED

Knowing sockets, protocols, and network programming does not mean that you can

easily create a chat server. How you are going to send and receive data. What kind of

synchronization exists between the server and the client application. This chapter will

show you how this server is build and what algorithm it follow, try to enjoy what you

are reading.

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4.1. Project Structure

Since we are using the TCP/IP facilities, we are obliged to divide the project

into two applications. In the next few sections we are going to explain how these

applications works and how they are synchronized with each other.

4.1.1. Control Strings

A set of “Control Strings” was created, such that whenever the client or the

server receives one of these control strings, it should do a certain task. To distinguish

these strings from normal strings (chatting sentences) they are given the following

format: “*ControlCommand “.

For example if the client sends to server the string “*Out walid”, the server upon

receiving this message, the message parsing operation starts. It looks at the first

character if it is a “ * ” immediately it recognizes that this is a control string. It

continues parsing until it reads the control command which is in this case “ Out “. It

matches this control to the group of commands it has. Where it finds out that this

client ( walid ) is trying to disconnect himself, so the function “ User_Out ” is called

where the disconnection operation is completed. Else if the string does not begin with

a star, then it is a normal string and therefore it should be broadcasted to other

connected users to be displayed in their public window. In fact there are many

controls in both server and client where each has its own job and some of these

controls are common between both server and client.

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4.1.2. Client

The client form is divided into four sections. A text box of the username, a

text box of the host where the server application is running, another text box of the

public sent data, and a list of the connected users. To get connected to server you just

enter a username, the host name you want to connect to, and finally hit the connect

button figure 4.1.2.1. shows the client form.

Figure 4.1.2.1 Client Form

During this connection operation, the client will send a control “*In username”

indicating to the server that there is a new chatter. The server will parse this string and

call the “User_In” function which adds this username to his list and retransmit another

identical control string to all connected users containing all the online usernames in

Connect button

Users on line

Received DataUser status

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order to update the “connected users list” in the client’s form. The same thing

happens when the user wants to disconnect himself where instead of sending “*In

username”, “*Out username” is sent and “User_Out” function is called which

disconnects the user and do all the updates.

4.1.3. Server

Till now it is clear that the server application has dual job, one to receive

data and another to transmit data. In fact it plays the role of a “ traffic officer ”

( putting everything in order the right order ). Since all the chatting operations are

passed through the server, then the main job of server is responding to the control

strings sent by clients as we have seen in the “in” and “out” requests. Figure 4.1.3.1

shows the server form.

Figure 4.1.3.1. server form

Number of connected

Users on line

Connected Users

Encrypted Data

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It plays an important role during the private chatting as we are going to see in the next

chapter.

4.2. Private chatting

When ever a client wants to initiate a private session, in the list of connected

users, he double clicks the username he wants to chat with. Two phenomenon will

take place. First, open a local-private session window. Second, to tell the server that

he wants to chat privately in order to open a remote private window. When these

conditions are satisfied, private chatting can be practiced by just writing in the send

text box and then hitting the enter button in your keyboard. Figure 4.2.1. shows the

private session form.

Figure 4.2.1. private session form

Private Data

Terminate Private Session

Send Data

Data To be Send

Destination Client

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CHAPTER 5

PROBLEMS ENCOUNTERED

To say, “I am going to write a chat application without having problems” means that

you are not really working with that application. In our previous four chapters, our

main concentration was to show and explain network programming, sockets and how

do they work. Now, considering that all the needed information was properly

delivered for you, it is time to shake your brain neurons and start solving problems.

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5.1. Multi-user

For two programs to use the TCP/IP facilities, one should take the role of client

and the other should take the role of server. The client initiates a socket and tries to

send a connection request on that port. For the connection to take place, the client

should be on the connection mode and the server should be on the listening mode.

Supposing that all the required conditions for a connection are satisfied and

both applications are ready to establish that connection. The first client sends a

connection request for the server that is in the listening mode so it detects a

connection. It accepts that connection and the socket shifts itself from the listening

mode to the connected mode. Now another user enters and sends another connection

request for the server. Automatically, this request will be rejected since the server is in

the connected mode and it should be in the listening mode. So, what shall we do in

order to allow other users to connect to server?

5.2. How To Allow Multi-user

Following an extensive research and reading, it seems that Biology in general

and cloning in specific is the solution. What is cloning? And how are we going to use

it?

Cloning: a group of cells or organisms, derived from a single ancestral cell or

individual and all genetically alike.

In fact, we are not going to use genes and cells. All what we are going to do is

to create a socket with all its parameters. Each time a client requests a connection, the

server clones from that socket and provides the client with a socket descriptor (like

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array structure) so the operation works likes this. When the server application first

runs, it creates an array of sockets and puts the socket of index zero in the listening

mode. When a client requests a connection, the connection will be accepted (since

socket [0] is listening) immediately. The server takes that connection that was caught

by socket [0] and scans other socket indexes for available sockets. The time it finds a

socket descriptor, it puts that client on that socket, updates the number of last socket

being used, and returns to listening mode at socket [0] such that when another user

tries to access the server, he will be accepted as shown in (figure 5.2.1.). Since all the

connection operations take place in Accept event, the code should look like this:

Private sub socket-accept (index as integer, socket id as integer)

Dim 1 as integer

For I = 1 to last socket

If not socket [I]. connected then Exit for

Next I

If I > last socket then

Last socket = last socket + 1 : I = last socket

Load socket (I)

End if

Socket [I].address family = AF-INET

Socket [I].protocol = IPPROTO – IP

Socket [I]. sockettype = sock-stream

Socket [I]. binary = true

Socket [I]. buffersize = 1024

Socket [I]. blocking = false

Socket [I]. accept = socket id End sub.

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figure 5.2.1. MultiUser

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5.3. Private chatting

Another facility of our chat server is the private chatting. Whenever you are

connected to a server, you will have two kinds of chatting; public chatting (your

messages appears to all users on-line) and private chatting (exchange of data between

clients privately). By default, you will be connected to a public window, plus another

window containing all the users on-line with you. Private chatting is initiated by

double clicking on the user name in the list. Immediately a small window between

you and the remote client appears indicating that you are in the private mode and that

you can start chatting.

5.4. Keep track of users

In fact, we have two kinds of private chatting; DCC that is a direct IP to IP

connection and another one where all the messages pass through a server which

has the job of delivery. In our system, we will use the server technique, but the

problem here is the delivery process and how it works. Therefore the server

should know each user, to which socket they are connected, and the available

window number.

5.5. How To Keep Track Of Users

To keep track of a user and on which socket he is connected, we maintain a

data base made up of one table containing two fields; username and socket

number. Each time a user is connected to a server, his username and socket

number are added to database automatically. Whenever the user double clicks a

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username to run the private chatting, the server fires a query that selects the

destination socket number (according to usernames) and the source socket

number. It sends the destination socket to source and the source socket to the

destination. From that time on, these socket addresses are contcatinated with the

sent message and passed by the server such that it knows where it comes from and

where it should go.

5.6. Private window

To chat privately you should have two windows one on the source client’s

screen and the other on the destination client’s screen. The problem is where does

these windows come from, on which bases they were created, and if I send a

message to which opened window on the screen it belongs.

5.6.1. Open Window

As a matter of fact we have created three identical forms for private

chatting and each of which has its own identity number i.e. form1, form2, form3.

When a client asks for private session, the system will call a function “

choose_window “. This function contains an array of type integer, where the

indexes of this array are equivalent to the identity number of forms, so it scans the

array, if it contains “1” (window is in use) it skips that window and jump to next

window, if it is “0” (window is available) it exits the function and return the index

of that available window to client to open it and the identity window number is

stored in a variable in that opened window. The same thing happens for the

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remote window but here the order to open a window comes directly from the

server.

5.6.2. Keep Track Of Windows

When the open window in the source client ends and before sending the

first message, the source client waits an acknowledgment control string from the

server containing in it the window number that was opened in the destination

client’s screen. The next time it receives that number, it calls a function

“First_Message” which has the responsibility of sending the first message where

the destination socket number and the destination window number are attached to

that first message such that when the destination client receives this message, he

parse it and know the window number from which it comes. Now each time a

private message are exchanged these variables are concatenated with it.

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CHAPTTER 6

ENCRYPTION

Data being sent over communication lines may be protected by scrambling the

message-that is, putting them in code that can be broken only by the person receiving

the message. After completing the private chatting, its time to deal with encryption or

decryption, where two users can chat privately without fearing that their words are

published to others.

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6.1. Data Encryption / Decryption

We are living in the information age. Computer thrive on information, with

more and more data being obtained and processed every day. Data about you, your

business, your bank account, your bills, and your credit is being processed everyday.

Information is a commodity that is sometimes valued more than gold and silver, and

that’s not likely to change any time soon.

Most information is freely accessible, available to anyone who wants it. The

internet is the world’s largest source of information, an unlimited amount of data that

is, for the most part, free for the asking.

But some information is not meant for public eyes and has to be secured, lest

it fall into the wrong hands. Details of military operations, financial information,

medical records, and som forms of communication are all type of information that

need to be kept secret. This is where data encryption (also known as cryptography)

comes in.

Data encryption is nothing new. it goes back to ancient civilization that used

hieroglyphic to build messages. We’re all familiar with the strange symbols used by

the ancient Egyptians. These hieroglyphics were an early form of encryption, and a

relatively good one. It wasn’t until the Rosetta stone was found in the recent times

that these hieroglyphics could be accurately deciphered. Although data encryption

has always been important, never has it been as necessary and widespread as it is

today.

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A wide range of encryption techniques have been devised, Some are simple,

whereas others are incredibly complex. Obviously, the simpler the encryption

techniques. The easier it is to “crack“. Also, as computers get faster and more

efficient, they become more capable of cracking existing encryption techniques.

Therefore, newer and better methods of encrypting data are constantly being invented.

And, since in our project the main concern is on network programming and not

encryption, we are going to use a simple way of encryption (as demonstration) which

is the XOR encryption.

6.2. XOR Encryption

One simple way to encrypt and decrypt data is by using the XOR Encryption

method. basically. The characters in the data stream and those of code key are

XORed together to create and encrypted character. The process is exactly the same

for data being decrypted.

The code key is a string made up of any alphanumeric characters you want to

use. It can be any number of characters long, but the longer the key is, the more

secure the encryption.

XOR Encryption is not a tight method of encryption, meaning that it can be

broken relatively easily. However, if you want to encrypt messages or documents so

that they cannot be viewed with text editor or browser, then XOR Encryption offers a

simple way to do so

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The following code shows a function called XOREncryption that can be used for both

encryption and decryption. You need only pass it the key (a string) and the data to be

encrypted or decrypted.

Public Function XOREncryption ( strCodekey As string, strDataIn As string) As

string

Dim IonDataPter As Long

Dim intXORValue1 As Integer

Dim intXORValue2 As Integer

Dim strDataOut As string

For IonDataPtr = 1 To Len (strDataIn)

‘ the first value to XOR comes from the data to be encrypted

intXORvalue1 = Asc(Mid$(strDataIn, IonDataPtr, 1))

‘ the second value to XOR comes from the code key.

intXORvalue2 = Asc(Mid$(strCodeKey, ((LonDataPter Mod

Len(strCodeKey))+1, 1))

‘ the two values are XORed together to create a decrypted character.

StrDataOut = strDataOut + Chr(intXORValue1 Xor intXORValue2)

Next IonDataPtr

‘ the XOREcryption function returns the encrypted or decrypted data.

XOREncryption = strDataOut

End Function

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To make the XOREncryption in our application more complex, I avoided using the

same constant key each time the private chatting is fired. Instead, whenever a user

double clicks another username for private chatting the source will automatically

create a random alphanumeric key of 15 characters and has the following format. Six

digits constitute the hour, minutes, seconds when the source user enters the server.

The rest nine characters are random characters generated by source also. When key

generated and before sending the first message to destination, the sources sends that

key to server and server send it to destination where it is used during that private

session. For more security and since the key will pass through the server without

being encrypted, the possibility of cracking this key becomes higher. To overcome

this problem, we have created a function that is called by one of the clients where

after each unit of time say ten minutes it will generate another key and send it to the

other chatter in such a way if a hacker cracks the key after ten minutes another key

will be generated and the hacker will spend his time cracking only.

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CONCLUSION

As a matter of fact this project took me one year to be completed and this year is

divided into two sections, six months of collecting information and six months of

writing Visual Basic code.

First, the theoretical section, where I have learned lots of things in networking, such

as the TCP/IP protocol (number one in network protocols) and how it works, windows

sockets, and network programming which is one of the best programming domains in

computers.

Second, the technical section which let me become more familiar with a new

programming language such as Visual Basic and its network components.

Having a chat server as a senior project, obliged you and indirectly to go into the tiny

details in networking and no one will teach you these details. Lots of experience was

gained and another beautiful face of networking was discovered.

The psychological part I have learned from that project is that, nothing in life comes

easily and at the same time nothing is impossible only it needs time, patience, and

hard working.

Finally I would like to thank my university N.D.U and all my instructors for helping

me to reach this level.

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REFERENCES

Books Used

1. James Martin, [1989], “ Local Area Network Architecture and Implementations “.

2. H. L. Capron, [1996], “ Computers Tools For an Information age “.

3. Rob Thayer, [1998], “ Visual Basic 6 “.

4. Pat Bonner, [1996], “Network Programming With Windows Sockets ”.

5. Catalyst Development Corporation, [1999], “ An Introduction To TCP/IP

programming with windows sockets “.

6. Charles L. Hedrick, [1987], “ Introduction To Internet Protocols “.

7. Greg Perry, [1998], “ Sams Teach Yourself Visual Basic 6 In 21 Days “.

8. Star Dust Technology, [1997], “Introducing Windows sockets version 2 “.

9. Novel Education, [1997], “ Course 200 Networking Technologies “.

Software Used

1. Visual Basic 6

2. Socket Wrench Control

3. Microsoft Access 2000