Stripe CTF 2014 Java Miner.

MIner.java

import java.lang.ProcessBuilder;
import java.io.File;
import java.io.FileWriter;
import java.io.FileReader;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.security.MessageDigest;
import javax.xml.bind.DatatypeConverter;

public class Miner {

    public static String DIRECTORY = "level1";

    public static void solve(String difficulty, String directory) throws Exception {
        // Create an entry in the ledger.
        FileWriter ledger = new FileWriter(directory + "/LEDGER.txt", true);
        ledger.write("user-fn27nmtz: 1\n");
        ledger.close();

        run("git add LEDGER.txt", directory);
        String tree = run("git write-tree", directory);
        String parent = run("git rev-parse HEAD", directory);
        long timestamp = System.currentTimeMillis() / 1000;

        int counter = 0;

        StringBuilder sb = new StringBuilder();
        sb.append(String.format("tree %s\n", tree));
        sb.append(String.format("parent %s\n", parent));
        sb.append(String.format("author LOL <lol@biz.info> %d +0000", timestamp) + "\n");
        sb.append(String.format("commiter LOL <lol@biz.info> %d +0000", timestamp) + "\n");
        sb.append("\n");

        String body = sb.toString();
        String content = String.format("commit %d\0", body.length() + 10) + body;

        MessageDigest digest = MessageDigest.getInstance("SHA-1");
        digest.update(content.getBytes("UTF-8"));

        while (true) {
            MessageDigest digestClone = (MessageDigest)digest.clone();
            String counterString = formatCounter(counter);
            digestClone.update(counterString.getBytes("UTF-8"));

            String sha1 = DatatypeConverter.printHexBinary(digestClone.digest());

            if (sha1.compareTo(difficulty) < 0) {
                System.out.println("found: " + sha1);

                FileWriter tmpbody = new FileWriter(directory + "/body");
                tmpbody.write(body + counterString);
                tmpbody.close();

                puts(run("git hash-object -t commit -w body", directory));
                puts(run(String.format("git reset --hard %s", sha1), directory));
                break;
            }

            counter++;
        }
    }

    public static String formatCounter(int counter) {
        char[] digits = new char[10];
        for (int d = 9; d >= 0; d--) {
            digits[d] = Character.forDigit(counter % 10, 10);
            counter /= 10;
        }
        return new String(digits);
    }


    public static String run(String command, String directory) throws Exception {
        ProcessBuilder pb = new ProcessBuilder(command.split(" "));
        pb.directory(new File(directory));
        Process p = pb.start();
        p.waitFor();
        if (p.exitValue() != 0) throw new Exception("command: " + command + " failed.");

        BufferedReader reader = new BufferedReader(new InputStreamReader(p.getInputStream()));
        return reader.readLine();
    }

    public static String readDifficulty() throws Exception {
        BufferedReader br = new BufferedReader(new FileReader("level1/difficulty.txt"));
        return br.readLine();
    }

    public static int commit(String directory) throws Exception {
        Process p = Runtime.getRuntime().exec("git push origin master".split(" "), null, new File(directory));
        p.waitFor();

        return p.exitValue();
    }

    public static void reset(String directory) throws Exception {
        puts(run("git log", directory));
        puts(run("git fetch origin master", directory));
        puts(run("git reset --hard origin/master", directory));
    }

    public static void puts(String s) {
        System.out.println(s);
    }

    public static void main(String[] args) {
        try {
            String difficulty = readDifficulty().toUpperCase();
            reset(DIRECTORY);
            while (true) {
                solve(difficulty, DIRECTORY);

                if (commit(DIRECTORY) == 0) {
                    break;
                } else {
                    reset(DIRECTORY);
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}