Solved

getting the mod from a double number in C++

Posted on 2008-06-25
4
1,000 Views
Last Modified: 2010-04-21
Hi, Im trying to do the RSA Algorithm as in this page:
http://pajhome.org.uk/crypt/rsa/rsa.html

but I have a problem with data types (in, double, lon int)

My code is like this:
double p,q,n,m,e,d;
double coded,decoded,msg;
 
 
unsigned int mcd(unsigned int a, unsigned int b){
     return (b == 0)? a : mcd(b, a % b);
}
 
void main(){
  p=7;
  q=9;
  n=p*q;//n for Encryption  and DesEncryption 
 
  m=(p-1)*(q-1);
 
  for(e=2;e<m;e++)//getting e as public key
    if(mcd(m,e)==1)
      break;
 
  for(int x=0;x<m;x++)
  {
    if(fmod((1+x*m),e)==0)
    {
      d=(1+x*m)/e;//getting d as private key
      break;
    }
  }
}
   msg=6;//original message
  //Here I try to Encryp the message
  coded=fmod(pow(mensaje,e),n);//cant use % coz are doubles....
  //coded is 62
 
  //Now I try yo DesEncryp the message
   decoded=fmod(pow(cifrado,d),n);//It is suppose that i must result 6 as msg, but it doesn't. It give me 21.

Open in new window

0
Comment
Question by:manganzon
  • 2
  • 2
4 Comments
 
LVL 3

Expert Comment

by:Darrylsh
ID: 21869429
Why are you using Doubles.  You probably should be using a Big Number library depending on the size of intergers you need, but for learning purposes, you could use a 64 bit int which on most systems would be type long long.
0
 

Author Comment

by:manganzon
ID: 21871922
Im using double coz I need a large rangen and long int is not enough. I need very large numbers like 65^70.

Whats the range of  64bits Int? and how do I declare a variable of it?  and wich library do you mean?
0
 
LVL 3

Accepted Solution

by:
Darrylsh earned 125 total points
ID: 21871943
unsigned a 64 bit integer is 0 to 18,446,744,073,709,551,615
They are declare like:

long long MyVar;

If you need 65^70 you will need a large number library.  I'd recommend GMP which can be found here http://gmplib.org/

Doubles won't work  because of rounding, if you are doing cryptography, you need integers.


0
 

Author Closing Comment

by:manganzon
ID: 31470730
thanks;
0

Featured Post

Free Tool: Postgres Monitoring System

A PHP and Perl based system to collect and display usage statistics from PostgreSQL databases.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Many companies are looking to get out of the datacenter business and to services like Microsoft Azure to provide Infrastructure as a Service (IaaS) solutions for legacy client server workloads, rather than continuing to make capital investments in h…
Since pre-biblical times, humans have sought ways to keep secrets, and share the secrets selectively.  This article explores the ways PHP can be used to hide and encrypt information.
The viewer will be introduced to the technique of using vectors in C++. The video will cover how to define a vector, store values in the vector and retrieve data from the values stored in the vector.
The Email Laundry PDF encryption service allows companies to send confidential encrypted  emails to anybody. The PDF document can also contain attachments that are embedded in the encrypted PDF. The password is randomly generated by The Email Laundr…

839 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question