Max Hernandez: RSA-based digital signatures

The homework for this week is implementing a RS-Authentication using a web service and this is what i did.

My implementation

This class allows user to connect with a computer called server and receive a challenge value used to authenticate with this. Once the client send its challenge ciphered it is compared with the value calculated with the private keys saved in the server database.

Server.py

#! /usr/bin/python
# -*- coding: latin-1 -*-

from rsa import RSA
import cgi, cgitb
import random

def open_keys(nameFile, username):
    fl = open(nameFile, "r")
    for i in fl:
        i = i.split(",")
        if i[0].lower() == username.lower() :
            fl.close()
            return int(i[1]), int(i[2])
    fl.close()
    return False

def save_value(nameFile, value):
    fl = open(nameFile, "w")
    fl.write(str(value))
    fl.close()

def load_value(nameFile):
    fl = open(nameFile, "r")
    value = fl.read()
    fl.close()
    return int(value)

def standar_function(x):
    return 3*x+20;

def verify(user, y):
    original_value = load_value("value.dat")
    if original_value == y:
        return True
    else:
        return False

def main():
    entrada = cgi.FieldStorage()
    print "Content-type: text\n"
    if entrada.has_key("x") and entrada.has_key("user"):
        rsa = RSA()
        try:
            n, e = open_keys( "users.dat", str(entrada["user"].value) )
        except:
            print "False"
            return 
        y = rsa.descifrar(n, e, int(entrada["x"].value))
        print str( verify(str(entrada["user"].value), y) )

    else:
        x = random.randint(1, 1000)
        save_value("value.dat", standar_function(x))
        print x

main()

This code is what connect with the server. I implemented this part in javascript language because is easy to share in the web and although is a Web-script the private keys do not get out of the computer in any time.

users.dat

Max, 233273, 73109

elisa, 124837172984042613022163, 124837172984042613022163

cliente.html

function standar(x){
    return (3*x)+20;
}

function pot_bin(x, n, mod){
    if(n == 1){
        return x;
    }else if( n%2 == 0){
        return Math.pow(pot_bin(x, n/2, mod), 2)%mod;
    }else{
        return (x*pot_bin(x, n-1, mod))%mod;
    }
}

function start(form){
  $.post( "http://localhost/cgi-bin/servidor_crip.py", {},
      function( data ) {
          y = standar(parseInt(data));
          n = parseInt( document.forms[form].elements["n"].value)
          d = parseInt( document.forms[form].elements["d"].value)
          user = String( document.forms[form].elements["user"].value)

          num = pot_bin(y, d, n).toPrecision(100);
          num = num.substring(0, num.indexOf("."));
          $.post( "http://localhost/cgi-bin/servidor_crip.py", {"x":num, "user":user},
              function( data ) {
                  if( data.indexOf("True") != -1 ){
                       document.getElementById("res").innerHTML=("
The user:'"+user+"' has been authenticated.
");
                  }else if( data.indexOf("False") != -1 ){
                       document.getElementById("res").innerHTML=("
The user:'"+user+"' has not been authenticated.
");
                  }else{
                       document.getElementById("res").innerHTML=("
Error with the server.
");
                  }
              }
            );

      }
    );
}

Changes made in old RSA code:

    def exp_bin(self, x, n, module):
        if n == 1:
            return x
        elif n%2 == 0:
            return ((self.exp_bin(x, n/2))**2)%module
        else:
            return (x*self.exp_bin(x, n-1))%module

Service running

This is the interface of the client running: