DES Chat App
Group Members:Yusra Ali Shaikh (13573)Khizar Waseem
(16738)Muhammad Ahmed (14571) December 21, 2014
ProblemTo develop Client-Server chat application in which users
can create a group chat by either creating a server or joining an
already created server by some other user. The main problem is to
apply a security algorithm which will encrypt and decrypt the text
messages in order to provide secure data transfer.
Real Life ExampleThe real life example of this problem can be an
e-mail system which takes messages as input from users, convert it
into cipher text (encrypted text) and then sends it over the
internet to reach the receiver. The receiver system then converts
that received cipher text into normal text based on some algorithm
or key and that is how emails are secured from hacking.
(Fig. 1)
ComplexityTo encrypt and decrypt messages we use DES algorithm.
Since DES is a block cipher, that is, the data is encrypted in form
of blocks, therefore, the size of a single message will always be a
64-bit block encrypted with a key of 64-bit key.
Since the size of message and key is fixed, this algorithm has
complexity of O(1) for a single block of text and O(m) for multiple
blocks where m is the number of messages encrypted.
AlgorithmDES (Data Encryption Standard) is a block cipher
encryption algorithm. It is a symmetric key algorithm which means
it shares same key with sender and receiver of the message to
encrypt/decrypt. DES takes 64-bit plaintext as input and encrypts
it with a 64-bit key and sends the resulting cipher text to
receiver. (Fig. 2)Key Generation:A 64-bit binary number is used
which goes through a number of steps of Permutation, Circular
Left-Shifts etc. to generate 16 sub-keys. These sub-keys are used
in each of 16 rounds of encryption and decryption.
Encryption:After generation of 16 sub-keys, a block of 64-bits
is encrypted by applying following steps: IP(Initial Permutation) A
Function (16 rounds) Swapping of left and right 32-bits
IP-1(Inverse Initial Permutation)
Decryption:The decryption part is same as the encryption except
the order of sub-keys application. In decryption, the generated 16
sub-keys are applied in reverse order i.e. K16 first, then K15,
then K14 and so on.IP
Algorithm CorrectnessThe correctness of this algorithm can be
proved in a way that, the fact of DES being an encryption algorithm
and that too being symmetric key, it will encrypt a plain text and
always decrypt its cipher text into correct plaintext because the
steps of decryption are no different from encryption except that
they are reversed only. Therefore, this algorithms correctness is
proved.
Pseudo CodeCipher (plainBlock[64], RoundKeys[16, 48],
cipherBlock[64]){ permute (64, 64, plainBlock, inBlock,
InitialPermutationTable) split (64, 32, inBlock, leftBlock,
rightBlock) for (round = 1 to 16) { mixer (leftBlock, rightBlock,
RoundKeys[round]) if (round!=16) swapper (leftBlock, rightBlock) }
combine (32, 64, leftBlock, rightBlock, outBlock) permute (64, 64,
outBlock, cipherBlock, FinalPermutationTable)}mixer (leftBlock[48],
rightBlock[48], RoundKey[48]){ copy (32, rightBlock, T1) function
(T1, RoundKey, T2)exclusiveOr (32, leftBlock, T2, T3) copy (32, T3,
rightBlock)}swapper (leftBlock[32], rigthBlock[32]){ copy (32,
leftBlock, T) copy (32, rightBlock, leftBlock) copy (32, T,
rightBlock)}function (inBlock[32], RoundKey[48], outBlock[32]){
permute (32, 48, inBlock, T1, ExpansionPermutationTable)
exclusiveOr (48, T1, RoundKey, T2) substitute (T2, T3,
SubstituteTables) permute (32, 32, T3, outBlock,
StraightPermutationTable)}substitute (inBlock[32], outBlock[48],
SubstitutionTables[8, 4, 16]){ for (i = 1 to 8) { row 2 x inBlock[i
x 6 + 1] + inBlock [i x 6 + 6] col 8 x inBlock[i x 6 + 2] + 4 x
inBlock[i x 6 + 3] + 2 x inBlock[i x 6 + 4] + inBlock[i x 6 + 5]
value = SubstitutionTables [i][row][col] outBlock[[i x 4 + 1] value
/ 8; value value mod 8 outBlock[[i x 4 + 2] value / 4; value value
mod 4 outBlock[[i x 4 + 3] value / 2; value value mod 2 outBlock[[i
x 4 + 4] value }}
Cost AnalysisThe cost of implementing this algorithm depends on
number of factors, having said that, weve compared the time and
speed of this algorithm on two different platforms i.e. C# and C++
and both perform differently as we increase or decrease the size of
data. Graphs in following figures show the comparison:
ImplementationSince DES algorithm is used in chat application,
the implementation part breaks into following two major parts: Chat
App DES AlgorithmChat App:The chat application is a GUI based
application developed on C# WinForms platform. Socket Programming
is used to send and receive messages on the LAN network.There are 3
classes in this app, Form1.cs, Server.cs and Client.cs.
Form1.cs is the main class i.e. the program starts from this
class.Server.cs class is called when user clicks on Create Server
button. This class has all the functionality required to start
server like getting ready to accept incoming client connections on
a certain port number and starting new thread for each client that
connects to server. As the server connects to a client, it can
start chatting with the client and the messages sent/received are
encrypted/decrypted on backend.Client.cs class is started when user
decides to join an existing server on the network by giving
IP(Internet Protocol) and Port number of server. In this class, an
object of DES class is created to apply the algorithm in the chat
app.
DES Algorithm:There is another class called DES.cs in which the
DES algorithm is implemented. The methods are created in a way that
they take plain and cipher texts as input and output
encrypted/decrypted text. Client.cs and Server.cs classes create
object of this (DES.cs) class and invoke methods when user wants to
send/receive messages.
Correctness RunsFor checking the correctness of this algorithm
we used multiple strings as test cases which we matched with some
of the online DES encryption tools to check if the algorithm is
correctly working in encrypting and decrypting messages.
Test Cases:Since this is a chat application, we used common
messages to start a conversation like hello, hey etc. and a
randomly generated 64-bit binary key which is shared between client
and server as soon as connection is established between them.The
expected output while testing this algorithm should be the original
message that is passed through algorithm as first it will be
encrypted and then if it decrypts correctly then itll surely return
the same message.
Correctness Runs ResultsIn the test case we passed hello123
string which makes a 64-bit binary as 01101000 01100101 01101100
01101100 01101111 00110001 00110010 00110011 and we encrypted it
with a 64-bit key encrypt1 i.e. 01100101 01101110 01100011 01110010
01111001 01110000 01110100 00110001 gives 01100101 01101110
01010100 01000011 01110000 01110101 01001000 01001111 as 64-bit
cipher text. Then we decrypted this cipher text with same key and
we got 01101000 01100101 01101100 01101100 01101111 00110001
00110010 00110011 which is same 64-bit binary as the original
messages.
Cost RunsTo check the cost, we run test cases with texts H,HEL
and HELLO on two different platforms i.e. C# and C++ and check how
they perform in terms of time as the data size increases.
Cost Runs Results
As depicted in above figures, the performance in C# gets better
as data increases and the time decreases.
References1. Fig. 1 (Page 2)
http://www-01.ibm.com/software/webservers/hostondemand/library/infocentergafinal/hod/en/doc/redbook/images/6182ax013.gif
2. Fig. 2 (Page 3)
http://azmadagreen.blogspot.com/2009_08_01_archive.html 3. Pseudo
code (Page 4)
http://highered.mheducation.com/sites/dl/free/0072870222/385981/Student_Solution_Chap_06.pdf
Running CodeSince this program is written in C# language, it
requires you to have Visual Studio IDE to run its code. Below are
the instructions to run the code of this program:1. First, import
the project folder in Visual Studio 2013.2. Press F5 button to run
the code.3. A form will show up with two buttons, each for Creating
or Joining Server respectively.4. When you go for Create Server
button, a new window will be shown in which there is an IP address
of your network and textArea and textField for chatting with
clients joining this server.5. If Join Server button is clicked,
the user will be shown a new window where he/she may provide
existing servers IP and port number and click Connect button. This
will result in a message that youre connected to server and Keys
are exchanged between client and server as soon as the connection
is established, and the key is used for later encryption/decryption
of messages.
There are following three main classes in this project: DES.cs
Server.cs Client.cs
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