Thread: I wanted to ask if i am doing this RSA assignment right i got it to work! (Assignments purpose was to use methods)

Assignments purpose was to use methods
Question 2 in the PDF file provided

import java.math.BigDecimal;
import java.util.*;
 
public class HussainDilawerA3Q2 {
 
  public static void main(String[] args) {
 
    long p = randomPrime();  // Generate p prime number
    long q = randomPrime();  // Generate q prime number 
    long n = p*q;
    long phi = (p-1)*(q-1);
    long e = generateEKey(phi);
    long d = generateDKey(phi, e);
    [ATTACH]1901[/ATTACH]
    System.out.println("e: "+e+", d: "+d+".");
 
    long enCr = encrypt(2216056, e, n);
    long dCr = decrypt(enCr, d, n);
    System.out.println(enCr);
    System.out.println(dCr);
    System.out.println(d);
 
 
 
    //this method call will give you the actall mesage after decryption
    long dCr1 = decrypt(2216056, 53972333, 269895149);   // its message is 109
    long dCr2 = decrypt(254084839, 53972333, 269895149);   // its message is 101
    System.out.println(dCr1);
    System.out.println(dCr2);
  }
 
  // Method to generate prime number in range 100 - 20000
  public static long randomPrime(){
 
    // Generate Random Number up to 20000
    long min = 100;
    long max = 20000;
    Random rnd = new Random();
    long rand = min+((long)(rnd.nextDouble()*max));
 
 
    // Keep checking until the random number is prime
    boolean test = isPrime(rand);
 
    while (test == false){
      rand = min+((long)(rnd.nextDouble()*max));
      test = isPrime(rand);
    }
 
    // return result
    return rand;
  }
 
 
  // Method to check if random number is prime
  public static boolean isPrime(long rand) {
 
    for (int i=2; i < rand; i++){ 
      if(rand%i == 0 && rand != i) {
        return false;
 
      }
    }
    return true;
  }
 
  public static long generateEKey(long phi){
 
    boolean result = false;
    for (long e=2; e<phi; e++){
 
      boolean test = isPrime(e);
      if(phi%e == 0 || test == false) {
        result = false;
      }
      else {
        return e;
      }
    }return 0;
  } 
 
  public static long generateDKey(long phi, long e){
    for(long k = 1; k < phi; k++){
      long d = (k*phi+1)/e;
      if((d*e) % phi==1){
        return d;
      }
    }
 
    return 0;
  }
 
  public static long encrypt(long message, long e, long n){
 
	long result = expandedPowerMod(message, e, n);
 
	return result;  
  }
 
  public static long decrypt(long message, long d, long n){
 
  long result = expandedPowerMod(message, d, n);  
 
	return result;  
  }
 
  public static long expandedPowerMod(long base1,long exponent, long modulus)
  {
      long result = 0;
 
      if (base1 < 1 || exponent < 0 || modulus < 1)   // just and error check for valid values
          return -1;
 
      result = 1;
      while (exponent > 0)
      {
          if ((exponent % 2) == 1)    // first iteration only 
          {
              result = (result * base1) % modulus;
          }
          base1 = (base1 * base1) % modulus;    // multiplied and added in orignal values after taking modulus
 
          BigDecimal bd = new BigDecimal(Math.floor(exponent / 2 ));    // these two steps will convert long to decimal as after division there might be some decimal values
          exponent = bd.setScale(0, BigDecimal.ROUND_HALF_UP).longValue();
      }
      return result;
  }
}

It compiles, it runs, you use methods like the assignment asks. Not sure what else you're looking for. Open your assignment file and for each paragraph as yourself "Did I do that?" (In your best Steve Urkel voice). If you can answer yes, turn it in and see what your teacher says.

Im basically not understanding what the whole RSA thing is like i made the program according to and with the help of what was given to me in the assignment. Yet i dont know that well what the question is really for, and i wanted to ask someone who can tell me where the code overflows because sometimes somewhere even the long values cant hold some values so the program overflows and returns me a 0.

--- Update ---

I have done that lol, and in Steve Urkels voice. i did it and it runs properly i just want to make sure its done exactly like it says because i cant afford to lose marks on this puppy

Thank-you for that i will definitely look into that and i used the BigDecimal class for the expandedPowerMod, then i converted it to long so i think i am safe on that part. Just not sure on the part where i generate the e and d key, but thank you very much