import java.util.Random;

public class Crypto {
    public static String encrypt(String plainText, String key) {
        char[] encText = plainText.toCharArray();
        Random r = new Random(key.length());
        Random[] randoms = new Random[key.length()];
        int i = -1;
        while (++i < randoms.length)
            randoms[i] = new Random(key.charAt(i) + r.nextInt());
        i = -1;
        while (++i < encText.length)
            encText[i] += randoms[i % randoms.length].nextInt(2 * Short.MAX_VALUE);
        return new String(encText);
    }

    public static String decrypt(String encText, String key) {
        char[] plainText = encText.toCharArray();
        Random r = new Random(key.length());
        Random[] randoms = new Random[key.length()];
        int i = -1;
        while (++i < randoms.length)
            randoms[i] = new Random(key.charAt(i) + r.nextInt());
        i = -1;
        while (++i < plainText.length)
            plainText[i] -= randoms[i % randoms.length].nextInt(2 * Short.MAX_VALUE);
        return new String(plainText);
    }

    //Demo
    public static void main(String[] args) {
        String text = "This is just some sample text to make the attack easy. I like using simple letters and stuff. This scoring system is very simple. A smarter scoring system would be able to decrypt anything.";
        String key1 = "01234567";
        String encrypted1;
        encrypted1 = encrypt(text, key1);
        System.out.println("Encrypted text: '" + encrypted1 + "'");

        // Let's attack!
        int keylen = key1.length(); // assuming we've already guessed or just know the key length
        Random r = new Random(keylen);
        Random randoms[] = new Random[keylen];
        StringBuilder key = new StringBuilder(keylen);
        for (int i=0; i<keylen; ++i) {
            key.append("\0");
        }

        for (int i=0; i<keylen; ++i) {
            char winner = '\0';
            int score = 0;
            for (char guess = 0; guess < Short.MAX_VALUE; ++guess) {
                key.replace(i, i+1, Character.toString(guess));
                String decrypted = decrypt(encrypted1, key.toString());
                StringBuilder tmp = new StringBuilder(decrypted.length()/keylen);
                for (int j=0; j<decrypted.length()/keylen; j += keylen) {
                    tmp.append(Character.toString(decrypted.charAt(i+j)));
                }
                int _score = calcScore(tmp.toString());
                if (_score > score) {
                    score = _score;
                    winner = guess;
                }
            }
            key.replace(i, i+1, Character.toString(winner));
        }

        System.out.println("Decrypted:");
        System.out.println(decrypt(encrypted1, key.toString()));

    }

    public static int calcScore(String s) {
        int score = 0;
        for (int i=0; i<s.length(); ++i) {
            char ch = s.charAt(i);
            short c = (short)ch;
            if (c < 32 || c >= 127) {
                return -1;
            } else if (ch == 'e') {
                score += 12;
            } else if (ch == 't') {
                score += 10;
            } else if (ch == 'a') {
                score += 9;
            } else if (ch == 'o') {
                score += 8;
            } else if (ch == 'i' || ch == 'n') {
                score += 7;
            } else if (ch == 's' || ch == 'h' || ch == 'r') {
                score += 6;
            } else if (Character.isLowerCase(ch)) {
                score += 4;
            } else if (Character.isUpperCase(ch)) {
                score += 2;
            } else {
                score += 1;
            }
        }
        return score;
    }
}