decryption.cpp

#include <iostream>
using namespace std;


unsigned long powmod( unsigned long base, int exp, int mod ) {

  base %= mod;

  unsigned long result = 1;
  while ( exp > 0 ) {

    if ( exp & 1 ) {
      result = ( result * base ) % mod;
    }
    base = ( base * base ) % mod;
    exp >>= 1;
  }

  return result;
}


int main( void ) {

  unsigned long msg;
  int d, n;

  cout << "MESSEAGE: ";
  cin >> msg;

  cout << "d (Secret key): ";
  cin >> d;

  cout << "n (Public key): ";
  cin >> n;

  cout << "\nDECODED MESSAGE >> " << powmod( msg, d, n )  << endl;

  return 0;
}