Client - Server

Here is the code of client-server using a simple chat and implement encrypt text or you can download the code in this links: server.py and client.py

#!/usr/bin/python

# Server program
from socket import *

# Set the socket parameters
host = "localhost"
port = 21567
buf = 2048
addr = (host,port)

# Create socket and bind to address
UDPSock = socket(AF_INET,SOCK_DGRAM)
UDPSock.bind(addr)

#decrypt
def decrypt(a):
    length = len(a)
    for position in range (length):
        temp = ord(a[position])
        temp = temp - 1                                                        
        temp = chr(temp)
        data = a[:position] + temp + a[position + 1:]
    return data
  
print "Waiting message..."

# Receive messages
while 1:
    data,addr = UDPSock.recvfrom(buf)
    data1,addr = UDPSock.recvfrom(buf)
    if not data:
        print "Client has died!"
        break
    else:
        data, key = data.split(';')
        print "\nReceived message '", data1,"'"
        clave = raw_input('Give me the key to decrypt: ')
        if (clave == key):
            decrypt(data1)
            print "Mensaje: ",data
        else:
            break

# Close socket
UDPSock.close()

#!/usr/bin/python

# Client program
from socket import *

# Set the socket parameters
host = "localhost"
port = 21567
buf = 2048
addr = (host,port)

# Create socket
UDPSock = socket(AF_INET,SOCK_DGRAM)

def_msg = "Write something ";
print "\n",def_msg

#encrypt
def encrypt(data):
    length = len(data)
    for position in range (length):
       temp = ord(data[position])
       temp = temp + 1
       temp = chr(temp)
       data = data[:position] + temp + data[position + 1:]
    return data


# Send messages
while (1):
    data = raw_input('Introduce Message: ')
    clave = raw_input('Give me the key: ')
    data = data+";"+clave
    if not data:
        break
    else:
        if(UDPSock.sendto(data,addr)):
            print "Sending message.....\n"
            data1 = encrypt(data);            
            UDPSock.sendto(data1,addr)
            
# Close socket
UDPSock.close()

Now i'm try to explain the code:

host = "localhost"
port = 21567
buf = 2048
addr = (host,port)

In this part we set the parameters, we can change, for example port to another, the size of the buffer, and the host, but if we change in server or client, we need to change too in the other code. The parameter is used to do a connection between client and server.

# Create socket and bind to address
UDPSock = socket(AF_INET,SOCK_DGRAM)
UDPSock.bind(addr)

In this part we create the socket connection and bind the address, AF_INET & SOCK_DGRAM, represents the address family INET for the internet sockets.

#decrypt
def decrypt(a):
    length = len(a)
    for position in range (length):
        temp = ord(a[position])
        temp = temp - 1                                                        
        temp = chr(temp)
        data = a[:position] + temp + a[position + 1:]
    return data

The next part, is a function of decrypt, we saw this in other publication, is the same but we use to decrypt the message.

# Receive messages
while 1:
    data,addr = UDPSock.recvfrom(buf)
    data1,addr = UDPSock.recvfrom(buf)
    if not data:
        print "Client has died!"
        break
    else:
        data, key = data.split(';')
        print "\nReceived message '", data1,"'"
        clave = raw_input('Give me the key to decrypt: ')
        if (clave == key):
            decrypt(data1)
            print "Mensaje: ",data
        else:
            break

# Close socket
UDPSock.close()

The next part take the parameters and data sending, then separate the string in data (Information) and key (key to decrypt the text), then we compare between key to clave and if this is correct show the message and in the last part close the socket.

TUDPSock.sendto(data1, addr)

Execution: 

Links
http://chuwiki.chuidiang.org/index.php?title=Sockets_en_Python
http://python.about.com/od/networkingwithpython/ss/beg_web_client_5.htm
http://en.wikipedia.org/wiki/AF_INET