Numdroid cracker - ASIS CTF Finals 2014

ArrayTools.java

package sample;
import java.util.*;

public class ArrayTools
{
    public static Integer[] Range(final int n, final int n2) {
        final Integer[] array = new Integer[n2 - n];
        for (int i = 0; i < array.length; ++i) {
            array[i] = n + i;
        }
        return array;
    }

    public static <T> void each(final List<T> list, final EachAction<T> eachAction) {
        final Iterator<T> iterator = list.iterator();
        while (iterator.hasNext()) {
            eachAction.action((T)((Object)iterator.next()));
        }
    }

    public static <T> void each(final T[] array, final EachAction<T> eachAction) {
        for (int length = array.length, i = 0; i < length; ++i) {
            eachAction.action(array[i]);
        }
    }

    public static <T> void each_with_index(final T[] array, final EachIndexAction<T> eachIndexAction) {
        for (int i = 0; i < array.length; ++i) {
            eachIndexAction.action(i, array[i]);
        }
    }

    public static <T, S> List<S> map(final List<T> list, final MapAction<T, S> mapAction) {
        final ArrayList<S> list2 = new ArrayList<S>();
        final Iterator<T> iterator = list.iterator();
        while (iterator.hasNext()) {
            list2.add((S) mapAction.action((T)((Object)iterator.next())));
        }
        return list2;
    }

    public static <T, S> List<S> map(final T[] array, final MapAction<T, S> mapAction) {
        final ArrayList<S> list = new ArrayList<S>();
        for (int length = array.length, i = 0; i < length; ++i) {
            list.add((S) mapAction.action(array[i]));
        }
        return list;
    }

    public static <T> List<T> select(final List<T> list, final SelectAction<T> selectAction) {
        final ArrayList<T> list2 = new ArrayList<T>();
        for (final T t : list) {
            if (selectAction.action(t)) {
                list2.add((T) t);
            }
        }
        return list2;
    }

    public static <T> List<T> select(final T[] array, final SelectAction<T> selectAction) {
        final ArrayList<T> list = new ArrayList<T>();
        for (final T t : array) {
            if (selectAction.action(t)) {
                list.add((T) t);
            }
        }
        return list;
    }
}

EachAction.java

package sample;

public interface EachAction<T> {
    void action(T p0);
}

EachIndexAction.java

package sample;


public interface EachIndexAction<T> {
    void action(int p0, T p1);
}

MapAction.java

package sample;


public interface MapAction<T, S> {
    S action(T p0);
}

SelectAction.java

package sample;


public interface SelectAction<T> {
    boolean action(T p0);
}

Verify.java

package sample;

import java.security.*;
import java.util.*;

public class Verify
{
    private static String OneWayFunction(final String s) {
        final MessageDigest instance;
        final byte[] digest;
        final List<byte[]> map = ArrayTools.map(
                ArrayTools.select(
                    ArrayTools.map(
                        new String[] {"MD5", "SHA-1", "SHA-256", "SHA-384", "SHA-512" },
                        new MapAction<String, byte[]>() {

                            public byte[] action(final String algorithm) {
                                final byte[] digest;
                                try {
                                    final MessageDigest instance = MessageDigest.getInstance(algorithm);
                                    instance.update(s.getBytes());
                                    digest = instance.digest();
                                }
                                catch (NoSuchAlgorithmException ex) {
                                    System.out.println("NOT FOUND " + algorithm);
                                    return null;
                                }
                                return digest;
                            }
                }),
                new SelectAction<byte[]>() {
                    public boolean action(final byte[] array) {
                        return array != null;
                    }
                }),
                new MapAction<byte[], byte[]>() {
                    public byte[] action(final byte[] array) {
                        final byte[] array2 = new byte[8];
                        for (int i = 0; i < array2.length / 2; ++i) {
                            array2[i] = array[i];
                        }
                        for (int j = 0; j < array2.length / 2; ++j) {
                            array2[j + array2.length / 2] = array[-2 + (array.length - j)];
                        }
                        return array2;
                    }
                });
        final byte[] input = new byte[8 * map.size()];
        for (int i = 0; i < input.length; ++i) {
            input[i] = map.get(i % map.size())[i / map.size()];
        }

        try {
            instance = MessageDigest.getInstance("MD5");
            instance.update(input);
            digest = instance.digest();
        }
        catch (NoSuchAlgorithmException e) {
            return "";
        }
        StringBuilder hexString = new StringBuilder();
        for (int i = 0; i < digest.length; i++) {
            if ((0xff & digest[i]) < 0x10) {
                hexString.append("0" + Integer.toHexString((0xFF & digest[i])));
            } else {
                hexString.append(Integer.toHexString(0xFF & digest[i]));
            }
        }
        final String string2;
        string2 = hexString.toString();
        return string2;
    }

    public static String isOk(final String s) {
        String substring = s;
        if (s.length() > 7) {
            substring = s.substring(0, 7);
        }
        final String oneWayFunction = OneWayFunction(substring);
        //System.out.println("digest: " + substring + " => " + oneWayFunction);

        return oneWayFunction;
    }

    public static void main(String args[]) {
        for (int i=0; i<9999999; i++) {
            //DebugTools.log("digest: " + substring + " => " + oneWayFunction);
            String trying = String.format("%d", i);
            //System.out.println(trying);
            String oneWayFunction = isOk(trying);
            if (oneWayFunction.equals("be790d865f2cea9645b3f79c0342df7e")) {
                System.out.println("FOUND!! " + trying);
            }
        }
    }
}