Python Notes: Diffie-Hellman (web demo)

The information presented here is intended for educational use.
The information presented here is provided free of charge, as-is, with no warranty of any kind.
Edit: 2019-11-27 (changes to "cgi.FieldStorage")

Overview

this application is composed of five files
1. one file of HTML (web page)
2. one file of JavaScript routines (only run client-side)
3. one optional file which only launches my server-side program
4. one file containing my Python3 program (only run server-side)
5. one file containing my Python3 functions (only run server-side)
clicking a button on the web page calls a JavaScript routine
- one JavaScript routine, named doReset(), works entirely client-side without annoying the server
- two JavaScript routines, named setDefaults1() and setDefaults2(), will call the server (via AJAX) expecting some sort of form initialization
- one JavaScript routine, named runDemo(), reads all the fields defined in the HTML form, tests them, then passes their contents on to the server (via AJAX) for processing
because this is a simple how-to demo, many of the routines are simplistic (e.g. no reliance on JavaScript libraries: jQuery, AngularJS, ReactJS although I was tempted to do so (the code would have been smaller but not readable by everyone)
there are numerous ways to pass data back to HTML but here I used XML because all browsers contain routines to perform XML parsing

file-1: HTML web page

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Diffie-Hellman Demo (via Python)</title>
 <script src="/js/dh_demo.js"></script>
</head>
<body style="font-family: Courier, Courier New, monospace; font-weight:bold">
 <h4>Diffie-Hellman Demo (via Python)</h4>
 <form id="form1">
  <div style="width:400px;text-align:right">
   Prime (p) <input id="p" type="text" size="20" maxlength="20"><br>
   Generate (g) <input id="g" type="text" size="20" maxlength="20"><br>
   Alice secret (a) <input id="a1" type="text" size="20" maxlength="20"><br>
   Bob's secret (b) <input id="b1" type="text" size="20" maxlength="20"><br>
   <hr>
   Alice sends (A) <input id="a2" type="text" size="20" maxlength="20" readonly="readonly"><br>
   Bob   sends (B) <input id="b2" type="text" size="20" maxlength="20" readonly="readonly"><br>
   Alice computes (X) <input id="a3" type="text" size="20" maxlength="20" readonly="readonly"><br>
   Bob   computes (Y) <input id="b3" type="text" size="20" maxlength="20" readonly="readonly"><br>
   <hr>
   <button type="button" id="but1" onclick="setDefaults1();">Defaults 1</button>
   <button type="button" id="but2" onclick="setDefaults2();">Defaults 2</button>
   <button type="button" id="but3" onclick="runDemo();">Run Demo</button>
   <button type="button" id="but4" onclick="doReset();">Reset</button>
  </div>
 </form><br>
 <div id="msg"></div><br>
 <hr>
 <div style="font-family:Calibri, Arial, sans-serif;font-weight:normal">
  <p>Neil Rieck<br>Waterloo, Ontario, Canada.<br>
   <a href="https://neilrieck.net" target="_blank">http://neilrieck.net</a></p>
 </div>
</body>
</html>

file-2: JavaScript

// title  : cgi-bin/dh_demo.js
// author : Neil Rieck
// created: 2019.09.01
// notes  : this demo done with plain JavaScript
//          (the next version will employ jQuery)
// ==============================================
var state=0;					// ajax state
var timeOutId1=0;				// timer ID
var debugFlag=0;
var nsr_domain=window.location.hostname;	// server address
var nsr_protocol=window.location.protocol;	// http or https
var p=0;					//
var g=0;					//
var a1=0;					//
var b1=0;					//
//
//	called by the RESET button
//
function doReset(){
    state=0;
    if (timeOutId1!=0){
        clearTimeout(timeOutId1);
        timeOutId1=0;
    }
    document.getElementById("p").value="";
    document.getElementById("g").value="";
    document.getElementById("a1").value="";
    document.getElementById("a2").value="";
    document.getElementById("a3").value="";
    document.getElementById("b1").value="";
    document.getElementById("b2").value="";
    document.getElementById("b3").value="";
    document.getElementById("msg").innerHTML="Reset Done";
}
//
//	called by the "SET DEFAULTS 1" button
//
function setDefaults1() {
    //alert("this is a test: 1");
    send_ajax("setDefaults1");
}
//
//	called by the "SET DEFAULTS 2" button
//
function setDefaults2() {
    //alert("this is a test: 2");
    send_ajax("setDefaults2");
}
//
//	read form fields
//
function readFields(){
    p = document.getElementById("p").value;
    g = document.getElementById("g").value;
    a1= document.getElementById("a1").value;
    b1= document.getElementById("b1").value;
}
//
//	called by the RUN DEMO button
//
function runDemo() {
    //alert("this is a test: 2");
    readFields();
    if (isStrInt(p)==false){
	document.getElementById("msg").innerHTML="error, 'p' is not an integer";
	return;
    }
    if (isStrInt(g)==false){
        document.getElementById("msg").innerHTML="error, 'g' is not an integer";
        return;
    }
    if (isStrInt(a1)==false){
        document.getElementById("msg").innerHTML="error, 'a' is not an integer";
        return;
    }
    if (isStrInt(b1)==false){
        document.getElementById("msg").innerHTML="error, 'b' is not an integer";
        return;
    }
    send_ajax("runDemo");
}
//
//	send_ajax (send an HTTP message via AJAX)
//
function send_ajax(action){
    switch(state){
    case 0:
	document.getElementById("msg").innerHTML="Connecting";
        var msg = "";				// init
	switch(action){
        case "runDemo":
	    msg=nsr_protocol+"//"+nsr_domain+"/cgi-bin/dh20?op="+ action + "&p="+p+"&g="+g+"&a1="+a1+"&b1="+b1;
	    start_ajax(msg);
	    break;
	case "setDefaults1":
	case "setDefaults2":
	    msg=nsr_protocol+"//"+nsr_domain+"/cgi-bin/dh20?op="+ action;
	    start_ajax(msg);
	    break;
	default:
	    document.getElementById("msg").innerHTML="programmer error (001)";
	    break;
	}
    default:
	document.getElementById("msg").innerHTML="Please wait (state:"+state+")";
	break;
    }
}
//
//	start_ajax (start an AJAX transaction)
//
function start_ajax(msg){
    response=null;
    if (typeof XMLHttpRequest == "undefined")
    XMLHttpRequest=function(){
	try { return new ActiveXObject("Msxml2.XMLHTTP.6.0")    } catch (e) {}
	try { return new ActiveXObject("Msxml2.XMLHTTP.3.0")    } catch (e) {}
	try { return new ActiveXObject("Msxml2.XMLHTTP")        } catch (e) {}
	try { return new ActiveXObject("Microsoft.XMLHTTP")     } catch (e) {}
	throw new Error("This browser does not support XMLHttpRequest or XMLHTTP.")
    }
    if (msg != ""){
	response=new XMLHttpRequest();
	if (response != null){
	    response.onreadystatechange=ajax_event_handler;
	    response.open("GET", msg, true);		// async=true
	    response.send(null);			// not null for POST
	}
	state = 1;
	init_timer1();
    }
}
//	ajax_event_handler (only executed if something is received) 
//
function ajax_event_handler(){
    if (response.readyState == 4){
	state=0;
	if (timeOutId1!=0){
	    clearTimeout(timeOutId1);
	    timeOutId1=0;
        }
        if (response.status == 200)
        {   document.getElementById("msg").innerHTML="Ready";
	    var resp$=response.responseText;
	    if (debugFlag==1) {
		alert("raw data:"+htmlEncode(resp$));
            }
	    if (window.DOMParser) {
		parser=new DOMParser();
		gXmlDoc=parser.parseFromString(resp$,"text/xml");
	    }else{
		gXmlDoc=new ActiveXObject("Microsoft.XMLDOM");
		gXmlDoc.async=false;
		gXmlDoc.loadXML(resp$);
	    }
	    var status= get_xml_data("status");
	    var fldP  = get_xml_data("p");
	    var fldG  = get_xml_data("g");
	    var fldA1 = get_xml_data("a1");
	    var fldB1 = get_xml_data("b1");
	    var fldMsg= get_xml_data("msg");
	    var fldA2 = get_xml_data("a2");
	    var fldB2 = get_xml_data("b2");
	    var fldA3 = get_xml_data("a3");
	    var fldB3 = get_xml_data("b3");
	    var status = 1;
	    if ((status == 1)||(status == 2)) {
//		document.getElementById("msg").innerHTML="raw data:"+htmlEncode(resp$);
		if (fldP !=null) document.getElementById("p").value=fldP;
		if (fldG !=null) document.getElementById("g").value=fldG;
		if (fldA1!=null) document.getElementById("a1").value=fldA1;
		if (fldB1!=null) document.getElementById("b1").value=fldB1;
		if (fldA2!=null) document.getElementById("a2").value=fldA2;
		if (fldB2!=null) document.getElementById("b2").value=fldB2;
		if (fldA3!=null) document.getElementById("a3").value=fldA3;
		if (fldB3!=null) document.getElementById("b3").value=fldB3;
//		document.getElementById("msg").innerHTML="p:"+fldP+" g:"+fldG+" b3:"+fldB3
		if (fldMsg!=null) document.getElementById("msg").innerHTML=fldMsg;
	    }else{
		document.getElementById("msg").innerHTML="error:"+status;
	    }
	}
    }
}
//
//	the server must answer back in 3 seconds
//
function init_timer1(){
    if (timeOutId1==0){					// if available
	timeOutId1=setTimeout("timer_job1();",3000);	// arm for 3 seconds
    }
}
//
//	this code is ONLY executed on TIMEOUT
//
function timer_job1(){
    state=0;						// reset state
    if (timeOutId1!=0){
	clearTimeout(timeOutId1);
	timeOutId1=0;
    }
    var txt="Error: the timer has expired";
    document.getElementById("msg").innerHTML=txt;
}
//
//	does string 'x' represent a positive integer?
//
function isStrInt(x){
x=x.replace(/\s/g,'');
    if (x=="") return false;
    try{
	y = parseInt(x);
    }catch(e){
	y = -1;
    }
    if(y>0){
    	return true;
    }else{
	return false;
    }
}
function htmlEncode(str) {
    return String(str).replace(/&/g, '&').replace(//g, '>').replace(/"/g, '"');
}
//
//	get_xml_data (look for one item)
//
function get_xml_data(tag){
    var x;
    try{
	x = gXmlDoc.getElementsByTagName(tag)[0].childNodes[0].nodeValue;
    }catch(e){
//	alert("t:"+tag+" e:"+e);
	x = null;
    }
    return(x);
}
//
//	get_xml_data2 (look for numerous similar items)
//
function get_xml_data2(xmlObj, tag){
    var x;
    try{
	x = xmlObj.getElementsByTagName(tag)[0].childNodes[0].nodeValue;
    }catch(e){
//	alert("t:"+tag+" e:"+e);
        x = null;
    }
    return(x);
}

file-3: Python3 (optional launcher)

#!/usr/bin/python3
# ----------------------
# Title  : cgi-bin/dh_20 (caveat: NO FILE EXTENSION)
# Author : Neil Rieck
# created: 2019-11-05
# purpose: ensure the use of compiled python (.pyc)
# notes  : this filename has no extension 
# ----------------------
import dh20_main	# use this auto-compiled module 
# dh20_main.main()	# enable if module only contains functions

file-4: Python3 (program)

#!/usr/bin/python3
# ------------------------------------------------------------
# Title  : dh20_main.py (this is the web version)
# Author : Neil Rieck
# created: 2019-08-31
# notes  :
# 1) Diffie-Hellman key-exchange demo
# 2) references:
#    https://en.wikipedia.org/wiki/Diffie-Hellman_key_exchange
# ------------------------------------------------------------
#
# uncomment this next line if your ISP is stuck at Python2
# from __future__ import print_function
#
import math, sys, cgi, cgitb, os, datetime
#
#	my Diffie-Hellman support routines live here
#
import dh20_module as dhm
#
#	declare a few variables
#
a2=""
b2=""
a3=""
b3=""
#
#	just a little hacking (probably only valid from HTTP GET)
#
try:
    query_string = os.environ['QUERY_STRING']
except:
    query_string ="BLANK"
#
#	extract CGI field data
#
fld  = cgi.FieldStorage(keep_blank_values=True)	# grab all CGI data
if 'p' in fld:
    p = fld.getvalue('p')
else:
    p = "?"
if 'g' in fld:
    g  = fld.getvalue('g')
else:
    g = "?"
if 'a1' in fld:
    a1 = fld.getvalue('a1')
else:
    a1 = "?"
if 'b1' in fld:
    b1 = fld.getvalue('b1')
else:
    b1 = "?"
if 'op' in fld:
    op = fld.getvalue('op')
else:
    op = "setDefaults1"
#
#	just a little hacking/logging (SELinux needs to allow it)
#
try:
    fn="dhlogs/dh20_"+datetime.datetime.now().strftime('%Y%m%d')+".txt"
    f = open(fn,"a")
    f.write("new event: %s\r\n" % datetime.datetime.now().strftime('%Y%m%d%H%M%S'))
    f.write("q  %s\r\n" % query_string)
    f.write("p  %s\r\n" % p)
    f.write("g  %s\r\n" % g)
    f.write("a1 %s\r\n" % a1)
    f.write("b1 %s\r\n" % b1)
    f.write("op %s\r\n" % op)
    f.write("f  %s\r\n" % fld)
    f.close()
except:
    z=""
#
#	send a response back to Apache
#	note: some popular return types include text/html, text/xml, text/plain
#
print('Content-Type: text/xml; charset=windows-1252')		# we're returning an XML response
print('')							# end of HTTP header
print('<?xml version="1.0" encoding="iso-8859-1"?>')		# start of XML content
print('<result>')						# user-defined by me (see JavaScript)
if op=="setDefaults1":
    print('<status>1</status>')
    print('<msg>Defaults from the Wikipedia article</msg>')
    print('<p>23</p><g>5</g><a1>4</a1><b1>3</b1>')
    print('<a2> </a2><b2> </b2><a3> </a3><b3> </b3>')
elif op=="setDefaults2":
    print('<status>1</status>')
    print('<msg>Using programmer defaults</msg>')
    print('<p>10007</p><g>5</g><a1>456</a1><b1>789</b1>')
    print('<a2> </a2><b2> </b2><a3> </a3><b3> </b3>')
elif op=="runDemo":
    try:
        (status, msg, a2, b2, a3, b3) = dhm.dh_machine(p,g,a1,b1)
    except Exception as e:
        status = 95
        #msg = 'yikes'
        msg = "{}".format(e)
    print('<status>'+ str(status) +'</status>')
    #
    # since we're sending back XML it might be wise to escape...
    # ...the contents of msg like so: html.escape()
    #
    print('<msg>'+ cgi.escape(msg) +'</msg>')
    print('<a2>'+ str(a2) + '</a2><b2>'+ str(b2) +'</b2><a3>'+ str(a3) +'</a3><b3>'+ str(b3) +'</b3>')
else:
    print('<status>1</status>')
    print('<msg>Defaults from the Wikipedia article</msg>')
    print('<p>23</p><g>5</g><a1>4</a1><b1>3</b1>')
    print('<a2> </a2><b2> </b2><a3> </a3><b3> </b3>')
print('</result>')
print('')
sys.exit()

file-5: Python3 (routines)

#!/usr/bin/python3 
# =========================================================
# file   : dh20_module.py
# author : Neil Rieck
# created: 2019-09-07
# purpose: python functions here are called by dh20_main.py
# =========================================================
#
# ===================================
#   test if n is prime
# ===================================
import math
def is_prime(n):
    n=int(n)
    if (((n % 2) == 0) and (n > 2)):
        return False
    for i in range(3, int(math.sqrt(n)) + 1, 2):
        if n % i == 0:
            return False
    return True
# ===========================================
#   test if n is a non-zero positive integer
# ===========================================
def is_nzp_integer(n):
    try:
        n=int(n)
    except:
        n=0
    if (n>0):
        return n
    else:
        return 0
# ===================================
#   get_primes hint
# ===================================
def get_primes_hints():
    a = "(hint) primes between 500 and 999: "
    for I in range(3,1000):
        if (is_prime(I)==True):
            a += i +" "
# ========================================================
#   Diffie-Hellman Key Exchange Demo
#   4   items are pass   in   (p,g,a1,b1)
#   2+4 items are passed back (status,msg,a2,b2,a3,b3)
# ========================================================
def dh_machine(p,g,a1,b1):
    if 1 != 1:			# enable to debug
        try:
            fn="dhlogs/dh20_"+datetime.datetime.now().strftime('%Y%m%d')+".txt"
            f=open(fn,"a")
            f.write("--- in function ---\r\n")
            f.write("p  %s\r\n" % p)
            f.write("g  %s\r\n" % g)
            f.write("a1 %s\r\n" % a1)
            f.write("b1 %s\r\n" % b1)
        except:
            z = 0
        f.write("1)\r\n")
    #
    # even though we have pre-tested this data in javascript,
    # test it again
    #
    if is_prime(p):
        p=int(p)
    else:
        return (99,"error, P is not a prime number",0,0,0,0)
    #
    if is_nzp_integer(g)>0:
        g=int(g)
    else:
        return (99,"error, G is not a positive integer",0,0,0,0)
    #
    if is_nzp_integer(a1)>0:
        a1=int(a1)
    else:
        return (99,"error, A is not a positive integer",0,0,0,0)
    #
    if is_nzp_integer(b1)>0:
        b1=int(b1)
    else:
        return (99,"error, B is not a positive integer",0,0,0,0)
    #
    #   Alice Sends Bob: a2 = g^a1 mod p
    #
    a2 = (g**a1) % p
    # 
    #   Bob Sends Alice: b2 = g^b1 mod p
    #
    b2 = (g**b1) % p
    #
    #   <<< Private computation's begin >>>
    #
    #   Alice Employs Bob's Shared Secret: s = b2^a1 mod p
    #
    a3 = (b2**a1) % p
    # 
    #   Bob Employs Alice's Shared Secret: s = a2^b1 mod p
    #
    b3 = (a2**b1) % p
    #
    # caveat: embedded HTML will mess up XML so needs to be escaped
    #
    msg = ("D-H solution: symmetric key = " + str(a3) +
        "<br>Evesdropper 'Eve' only sees: p, g, A, B")
    return (1,msg,a2,b2,a3,b3)
# ================================================================

External Links

https://en.wikipedia.org/wiki/Diffie-Hellman_key_exchange
back to: Python Notes (index)
- python notes: compiling + caching
https://neilrieck.net/dh_demo.html :: Diffie-Hellman key exchange demo (web) <<<--- click here to run real code

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Neil Rieck
Waterloo, Ontario, Canada.