Socket chat room Caesar cipher

client.py

import socket
import select
import sys
import string

key = 'abcdefghijklmnopqrstuvwxyz'

def encrypt(n, plaintext):
    """Encrypt the string and return the ciphertext"""
    result = ''

    for l in plaintext.lower():
        try:
            i = (key.index(l) + n) % 26
            result += key[i]
        except ValueError:
            result += l

    return result.lower()


server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
if len(sys.argv) != 3:
    print "script, IP address, port number"
    exit()
IP_address = str(sys.argv[1])
Port = int(sys.argv[2])
server.connect((IP_address, Port))

while True:
    sockets_list = [sys.stdin, server]
    read_sockets,write_socket, error_socket = select.select(sockets_list, [], [])
    for socks in read_sockets:
        if socks == server:
            message = socks.recv(2048)
            print message
        else:
            offset = 4
            text = message
            encrypted = encrypt(offset, text)
            message = sys.stdin.readline()
            server.send(encrypted)
            sys.stdout.write(encrypted)
            sys.stdout.flush()
server.close()

server.py

import socket
import select
from thread import *
import sys
import string

server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
 
# check argument yang dimasukkan lengkap dan benar
if len(sys.argv) != 3:
    print "Correct usage: script, IP address, port number"
    exit()
 
# perintah bind alamat ip address
IP_address = str(sys.argv[1])
 
# perintah bind port number
Port = int(sys.argv[2]) 
server.bind((IP_address, Port))
server.listen(100)

list_of_clients = []

key = 'abcdefghijklmnopqrstuvwxyz'

def decrypt(n, ciphertext):
    """Decrypt string dan return dengan hasil plaintext"""
    result = ''

    for l in ciphertext:
        try:
            i = (key.index(l) - n) % 26
            result += key[i]
        except ValueError:
            result += l
    return result
    
def clientthread(conn, addr):
 
    # sends a message to the client whose user object is conn
    conn.send("Welcome to chatroom!")
 
    while True:
            try:
                message = conn.recv(2048)
                if message:
                    offset = 4
                    text = message
                    decrypted = decrypt(offset, text)
                    print "<" + addr[0] + "> " + decrypted
 
                    # Calls broadcast function to send message to all
                    message_to_send = "<" + addr[0] + "> " + decrypted
                    broadcast(message_to_send, conn)
 
                else:
                    """message may have no content if the connection
                    is broken, in this case we remove the connection"""
                    remove(conn)
 
            except:
                continue

"""broadcast pesan ke semua client"""
def broadcast(message, connection):
    for clients in list_of_clients:
        if clients!=connection:
            try:
                clients.send(message)
            except:
                clients.close()
 
                # if the link is broken, we remove the client
                remove(clients)
 
"""The following function simply removes the object
from the list that was created at the beginning of 
the program"""
def remove(connection):
    if connection in list_of_clients:
        list_of_clients.remove(connection)
 
while True:
 
    conn, addr = server.accept()
 
    """List client yang terhubung pada room agar tetap terhubung"""
    list_of_clients.append(conn)
 
    # prints the address of the user that just connected
    print addr[0] + " connected"
 
    # creates and individual thread for every user 
    # that connects
    start_new_thread(clientthread,(conn,addr))    
 
conn.close()
server.close()