Implementing-cryptography-using-python

264 Chapter 9 ■ Mastering Cryptography Using Python
# turn on the use of PKI
useClientPKI = True;
# let the user know PKI certs are found
print ("Client certificate found ...")
if result == 20:
# turn off Diffie-Hellman
useDHKey = False;
if result == 21:
# turn on Diffie-Hellman
useDHKey = True;
print ("DH Key Exchange ...")
serverSecret = get_dh_sharedkey()
# if any encryption is used, change the message to 'secure' message
if useClientPKI == True or useDHKey == True:
print ("Received secured message: " + msg)
else:
print ("Received message: " + msg)
Modifying the Client File
The client will call the gen _ client _ DH() method to generate the certificates
when the application starts up. The certificates need to be in place prior to the
client requiring them, which takes place when the user types addDH on the client.
In addition to the Diffie-Hellman certificates that get created, we also need to
implement the Diffie-Hellman methods to examine the server’s public key and
use it to create a shared secret using the client’s private key. The same process
happens on the server.
The following code was added to the client file:
import Chapter9.ch9_crypto_chat as ct
clientDH = clientDH = ct.gen_client_DH()
clientSecret = 0;
def get_dh_sharedsecret():
key = int(open('server_public_dh_key.pem').read())
clientDH.generateSharedKey(key)
clientDH.getSharedKey()
clientDH.generateSharedKey(key)
shared_key = clientDH.sharedSecret
return (shared_key)