random prime number generation for RSA

[hebaahmed]

How do I determine random prime numbers in RSA? I have already generated numbers but can't check if prime or not.

Here is the part of generation

    public static void main(String[] args) throws IOException {
int q=0;
int p=0;

int maxValue = 100;
int maxNumber = 2;

Random x = new Random ();

for (int k=1; k < maxNumber; k ++) {
p = (Math.abs(x.nextInt () % maxValue ));

System.out.println("p: "+p);

[jkmyoung]

You're looking for a Primality test. Look at some of the algorithms given, and decide which one you want to implement. Come back with your problems, if you have any.

[hebaahmed]

public static boolean isPrime(int n) {
    for(int i=2;2*i<n;i++) {
        if(n%i==0)
            return false;
    }
    return true;
}

public static void main(String[] args) throws IOException {
int q=0;
int p=0;

int maxValue = 100;
int maxNumber = 2;

Random x = new Random ();

for (int k=1; k < maxNumber; k ++) {
p = (Math.abs(x.nextInt () % maxValue ));

System.out.println("p: "+p);

I already got prime test algorithm but I want to make that if it's prime number ok show p=number that has been generated and being prime else break and generate another one and check again I try alot but i don't know the problem.

[jkmyoung]

You probably want a do-while loop

do{
Set your random number
} while (random number is not prime);

Then you can only leave the loop if your number IS prime.


Also, consider changing your for loop condition from
2*i<n TO
i*i <= n

Right now, 4 wouldn't work, because when i = 2, 2 * i = 2 * 2 = 4 which is not < n.

[hebaahmed]

Thanks for your reply. I can do it.