Question

Exercise 3.2 (Symmetric and Asymmetric Encryption) t In this exercise, you will send an encrypted message from a socket client to a socket server. The message will be encrypted using AES symmetric encryption algorithm in the client. The server receives this ciphertext and decrypt it using the same symmetric key. Before communication over symmetric encryption, you need to first securely distribute the symmetric key between sender and receiver. You can use asymmetric encryption to help distribute the symmetric key. The client can generate the symmetric key and encrypts it with server's public key. The server can receive the encrypted key and decrypts it using its own private key. After the key distribution, the message transmission can start. Write a python program to create a socket server and socket client to accomplish this process

Answer 1

Language Used: Python 2.7

Download PyCrypto module from PyCryptoPlus .

I have shown both the server side and the client side implementations. Please find the code below:

Server Side:

import os
import time
import itertools
import socket
import hashlib
import threading
import sys
import Crypto.Cipher.AES as AES
from Crypto.PublicKey import RSA
from CryptoPlus.Cipher import IDEA

#get server address and port number
hosts = raw_input("Server Addr - > ")
ports = int(input("Port no - > "))
#check server port
checkflag = False
doneflag = False

def anim():
for char in itertools.cycle(['.....','.......','..........','............']):
if doneflag:
break
sys.stdout.write('\rCHECKING NOT USED PORT AND IP ADDRESS '+char)
sys.stdout.flush()
time.sleep(0.1)
sys.stdout.write('\r -----SERVER HAS STARTED. WAITING FOR CLIENT-----\n')
try:
#set up socket
serv = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
serv.bind((hosts,ports))
serv.listen(5)
checkflag = True
except BaseException:
print "-----Check Port or Server Address -----"
checkflag = False

if checkflag is True:
# Quit
shut_down = False
# printing "Server Started"
threadload = threading.Thread(target=anim)
threadload.start()

time.sleep(4)
doneflag = True
# client and address binding
cl,adrs = serv.accept()
print ("CLIENT CONNECTED. ADDRESS ->",adrs)
print ("\n-----WAITING FOR PUBLIC KEY & HASH -----\n")

# Public Key
getpbkey = cl.recv(2048)

#convert string to KEY
servpublic_key = RSA.importKey(getpbkey)

# public key hashing in server side for validation
hashobj = hashlib.sha1(getpbkey)
hexdigest = hashobj.hexdigest()

if getpbkey != "":
print (getpbkey)
cl.send("YES")
get_hash = cl.recv(1024)
print ("\n----- PUBLIC KEY HASH ----- \n"+get_hash)
if hexdigest == get_hash:
# create session key
key128 = os.urandom(16)
#encrypt CTR MODE session key
enc = AES.new(key128,AES.MODE_CTR,counter = lambda:key128)
encryptor = enc.encrypt(key128)
#hash sha1
enc_object = hashlib.sha1(encryptor)
enc_digest = enc_object.hexdigest()

print ("\n----- SESSION KEY -----\n"+enc_digest)

# session key and public key encryption
En = servpublic_key.encrypt(encryptor,16)
print ("\n-----ENCRYPTED PUBLIC AND SESSION KEY-----\n"+str(En))
print ("\n-----HANDSHAKE DONE-----")
cl.send(str(En))
while True:
#message from client
newmsg = cl.recv(1024)
#decoding the message from HEX to decrypt the ecrypted version of the message only
Decoded = newmsg.decode("hex")
#making enc_digest(session_key) as the key
k = enc_digest[:16]
print ("\nENCRYPTED MESSAGE FROM CLIENT -> "+newmsg)
#decrypt message from the client
idea_Decrypt = IDEA.new(k, IDEA.MODE_CTR, counter=lambda: k)
d_Msg = idea_Decrypt.decrypt(Decoded)
print ("\n**New Message** "+time.ctime(time.time()) +" > "+d_Msg+"\n")
msg = raw_input("\nMessage To Client -> ")
if msg != "":
idea_Encrypt = IDEA.new(k, IDEA.MODE_CTR, counter=lambda : k)
e_Msg = ideaEncrypt.encrypt(msg)
e_Msg = e_Msg.encode("hex").upper()
if e_Msg != "":
print ("ENCRYPTED MESSAGE TO CLIENT-> " + e_Msg)
cl.send(e_Msg)
cl.close()
else:
print ("\n-----PUBLIC KEY HASH DOESNOT MATCH-----\n")

Client Side

import time
import hashlib
import itertools
import socket
import threading
import sys
from Crypto import Random
from Crypto.PublicKey import RSA
from CryptoPlus.Cipher import IDEA

#animate loading
flag = False
def anim():
for char in itertools.cycle(['.....','.......','..........','............']):
if flag:
break
sys.stdout.write('\rCONFIRMING CONNECTION TO SERVER '+char)
sys.stdout.flush()
time.sleep(0.1)

# private and public key
rand_generator = Random.new().read
K = RSA.generate(1024,rand_generator)
pub = K.publickey().exportKey()
priv = K.exportKey()

#hash the public key
hash_obj = hashlib.sha1(pub)
hex_digest = hash_obj.hexdigest()

#Set up socket
serv = socket.socket(socket.AF_INET,socket.SOCK_STREAM)

#host and port
hosts = raw_input("Server Address To Be Connected -> ")
ports = int(input("Port of The Server -> "))
#bind port and the address
serv.connect((hosts, ports))
# Server Started Message
threadload = threading.Thread(target=anim)
threadload.start()

time.sleep(4)
flag = True

def send(t,name,K):
mess = raw_input(name + " : ")
K = K[:16]
#merg the message and name
whole = name+" : "+mess
idea_Encrypt = IDEA.new(K, IDEA.MODE_CTR, counter=lambda : K)
e_Msg = idea_Encrypt.encrypt(whole)
#convert encrypted message to HEX to readable
e_Msg = e_Msg.encode("hex").upper()
if e_Msg != "":
print ("ENCRYPTED MESSAGE TO SERVER-> "+e_Msg)
serv.send(e_Msg)
def recv(t,K):
newmsg = serv.recv(1024)
print ("\nENCRYPTED MESSAGE FROM SERVER-> " + newmsg)
K = K[:16]
decoded = newmsg.decode("hex")
idea_Decrypt = IDEA.new(K, IDEA.MODE_CTR, counter=lambda: K)
d_Msg = idea_Decrypt.decrypt(decoded)
print ("\n**New Message From Server** " + time.ctime(time.time()) + " : " + d_Msg + "\n")

while True:
serv.send(pub)
confirm = serv.recv(1024)
if confirm == "YES":
serv.send(hex_digest)

#connected message
msg = serv.recv(1024)
enc = eval(msg)
decrypt = K.decrypt(enc)
# hash sha1
enc_object = hashlib.sha1(decrypt)
enc_digest = enc_object.hexdigest()

print ("\n-----ENCRYPTED PUBLIC KEY AND SESSION KEY FROM SERVER-----")
print (msg)
print ("\n-----DECRYPTED SESSION KEY-----")
print (enc_digest)
print ("\n-----HANDSHAKE COMPLETE-----\n")
alais = raw_input("\nYour Name -> ")

while True:
threadsend = threading.Thread(target=send,args=("------Sending Message------",alais,enc_digest))
threadrecv = threading.Thread(target=recv,args=("------Recieving Message------",enc_digest))
threadsend.start()
threadrecv.start()

threadsend.join()
threadrecv.join()
time.sleep(0.5)
time.sleep(60)
serv.close()