How to generate random bytes in Java?

There are many ways to generate random bytes in Java.

         a. Using nextBytes() method of Random class

         b. Using nextBytes method of SecureRandom class

         c. Using SecureRandom.getInstanceStrong().nextBytes(bytes)

         d. Using ThreadLocalRandom.current().nextBytes(bytes)

        

Using nextBytes() method of Random class

Random class provides nextBytes() method, it takes a byte array as argument and fill the input byte array with random bytes.

                 Random random = new Random();

                 byte[] b = new byte[20];

                 random.nextBytes(b);

                

RandomDemo.java

package com.sample.Random;

import java.util.Random;

public class RandomDemo {
  private static void printArray(byte[] byteArr) {
    for (byte b : byteArr) {
      System.out.print(b + ",");
    }
  }

  public static void main(String args[]) {
    Random random = new Random();
    byte[] b = new byte[20];
    random.nextBytes(b);
    printArray(b);
  }
}

Sample Output

-56,108,-21,-73,83,1,-2,-117,-52,-66,-96,-51,89,-79,100,-106,121,68,79,81,

Using nextBytes method of SecureRandom class

'java.security.SecureRandom' class provides nextBytes method, it takes byte array as argument and fill the byte array with random bytes. I would prefer to use Secure Random to generate random numbers than Random. It is because, Random class uses System clock to generate the seed, where as SecureRandom class uses random data from operating system (For ex interval betweem keystorkes, cpul cycles etc.,) to generate the seed. SecureRandom is more secure as compared to Random.

                 SecureRandom random = new SecureRandom();

                 byte[] b = new byte[20];

                 random.nextBytes(b);

                

RandomDemo.java

package com.sample.Random;

import java.security.SecureRandom;

public class RandomDemo {
  private static void printArray(byte[] byteArr) {
    for (byte b : byteArr) {
      System.out.print(b + ",");
    }
  }

  public static void main(String args[]) {
    SecureRandom random = new SecureRandom();
    byte[] b = new byte[20];
    random.nextBytes(b);
    printArray(b);
  }
}

Sample Output

9,62,-83,114,6,70,68,-63,-94,33,-100,-75,-91,-36,-60,-26,27,-116,64,-113,

Using SecureRandom.getInstanceStrong().nextBytes(bytes)

SecureRandom.getInstanceStrong() is avialble in Java1.8. 'SecureRandom.getInstanceStrong()' returns a secure random object that was selected by using the algorithms/providers specified in the securerandom.strongAlgorithms (Ex: Windows-PRNG:SunMSCAPI,SHA1PRNG:SUN)

Get the algorithms specified in 'securerandom.strongAlgorithms'

By using below statements, you can get the algorithms specified in 'securerandom.strongAlgorithms' property

                 String randomAlgorithms = Security.getProperty("securerandom.strongAlgorithms");

                 System.out.println(randomAlgorithms);

Below statements are used to generate random bytes.

                 SecureRandom random = SecureRandom.getInstanceStrong();

                 byte[] b = new byte[20];

                 random.nextBytes(b);

        

RandomDemo.java

package com.sample.Random;

import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

public class RandomDemo {
  private static void printArray(byte[] byteArr) {
    for (byte b : byteArr) {
      System.out.print(b + ",");
    }
  }

  public static void main(String args[]) throws NoSuchAlgorithmException {
    SecureRandom random = SecureRandom.getInstanceStrong();
    byte[] b = new byte[20];
    random.nextBytes(b);
    printArray(b);

  }
}

Sample Output

73,-22,-8,-24,-80,-50,113,-33,-34,-69,104,-66,-74,-114,-104,-118,102,-26,-106,-72,

Prefer to use 'SecureRandom.getInstanceStrong()' over all other methods

Using nextBytes method of ThreadLocalRandom class

Below statements are used to generate random numbers.

                 ThreadLocalRandom threadLocalRandom = ThreadLocalRandom.current();

                 byte b[] = new byte[20];

                 threadLocalRandom.nextBytes(b);

RandomDemo.java

package com.sample.Random;

import java.security.NoSuchAlgorithmException;
import java.util.concurrent.ThreadLocalRandom;

public class RandomDemo {
  private static void printArray(byte[] byteArr) {
    for (byte b : byteArr) {
      System.out.print(b + ",");
    }
  }

  public static void main(String args[]) throws NoSuchAlgorithmException {

    ThreadLocalRandom threadLocalRandom = ThreadLocalRandom.current();
    byte b[] = new byte[20];
    threadLocalRandom.nextBytes(b);
    printArray(b);
  }
}

Sample Output

-14,5,0,-105,37,65,95,-45,112,-79,72,-59,68,42,36,2,25,-70,13,-99,

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