NonceFinder.java

import java.util.Base64;
import java.util.Scanner;
import java.util.concurrent.atomic.AtomicInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.nio.charset.StandardCharsets;

public class HashBreaker 
{
    public static void main(String[] args) throws NoSuchAlgorithmException {
        var scanner = new Scanner(System.in);
        System.out.println("Nonce Finder");
        System.out.print("Message        : ");
        
        String message = scanner.nextLine();
        System.out.print("Difficulty     : ");
        
        int d = scanner.nextInt();
        System.out.println("\nCalculating...");

        long startTime = System.currentTimeMillis();
        long nonce = bruteforce(d, message);
        long endTime = System.currentTimeMillis();

        System.out.println("\nValid Nonce  : " + nonce);
        System.out.println("Hash         : " + sha256(message + String.valueOf(nonce)));
        System.out.println("Time taken   : " + (endTime - startTime)/1000f + " seconds");
        scanner.close();
    }

    public static String sha256(String text) throws NoSuchAlgorithmException {
        MessageDigest digest = MessageDigest.getInstance("SHA-256");
        digest.update(text.getBytes(StandardCharsets.UTF_8));
        byte[] hash = digest.digest();
        
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < hash.length; i++)
            sb.append(Integer.toString((hash[i] & 0xff) + 0x100, 16).substring(1));
        return sb.toString().toUpperCase();
    }

    public static long bruteforce(int d, String s) throws NoSuchAlgorithmException {
        long nonce = 0;
        String pre = "";
        for (; pre.length()<d; pre += "0");
        while (!sha256(s + String.valueOf(nonce)).startsWith(pre)){
            nonce++;
        }
        return nonce;
    }
}