Simple encrypted UDP chat in Python.

chat.py

#!/usr/bin/env python3

import rsa
import socket
import sys
import threading
import time

myPublic, myPrivate = rsa.newkeys(1024, exponent=65537)
otherPublic = None

dest = ('127.0.0.1', 5000 if len(sys.argv) > 1 else 5001)

def outFunc():
    udp = socket.socket(socket.AF_INET,socket.SOCK_DGRAM)

    while not otherPublic:
        msg = str(myPublic.n).encode()
        udp.sendto(b'\x01' + msg, dest)
        time.sleep(1)

    while True:
        msg = input().encode()
        msg = rsa.encrypt(msg, otherPublic)
        udp.sendto(b'\x00' + msg, dest)

def inFunc():
    # Otherwise we would modify a local copy of `otherPublic`.
    global otherPublic

    udp = socket.socket(socket.AF_INET,socket.SOCK_DGRAM)
    udp.bind(('', 5001 if dest[1] == 5000 else 5000))

    while True:
        msg, client = udp.recvfrom(1024)

        if msg[0] == 0:
            msg = rsa.decrypt(msg[1:], myPrivate).decode()
            print(client, msg)
        elif msg[0] == 1:
            msg = int(msg[1:].decode())
            otherPublic = rsa.PublicKey(msg, 65537)

            # Send our public key to prevent a deadlock.
            msg = str(myPublic.n).encode()
            udp.sendto(b'\x01' + msg, dest)

inT = threading.Thread(target=inFunc)
inT.start()
outT = threading.Thread(target=outFunc)
outT.start()