Bruteforce

import java.io.*;

class PrintAllKLengthStrings {

    static String users = "";

    public static void main(String[] args) {
        CHARACTERLIMIT = 500;
        char set1[] = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z','0','1','2','3','4','5','6','7','8','9'};
        for (int k = 0; k < CHARACTERLIMIT; k++) {
            printAllKLength(set1, k);
        }
        try {
            PrintWriter writer = new PrintWriter("foundusers2.txt", "UTF-8");
            writer.println(users);
            writer.close();
        } catch (IOException e) {}
    }
    static void printAllKLength(char set[], int k) {
        int n = set.length;
        printAllKLengthRec(set, "", n, k);
    }
    static void printAllKLengthRec(char set[], String prefix, int n, int k) {

        if (k == 0) {
            try {
            System.out.println(prefix);
            Process process = new ProcessBuilder("./epr.x", prefix, "1").start();
                String line = br.readLine();


                if (line != null) {
                    users = users + prefix + "\n";
                }

            } catch (IOException e) {}
            return;
        }
        for (int i = 0; i < n; ++i) {

            String newPrefix = prefix + set[i];
            printAllKLengthRec(set, newPrefix, n, k - 1);
        }
    }
}