VpnGroupPasswordDecoder.java

import java.security.MessageDigest;
import java.util.Arrays;

import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import javax.swing.JOptionPane;
import javax.xml.bind.DatatypeConverter;
import javax.crypto.IllegalBlockSizeException;

public class VpnGroupPasswordDecoder {



    public static String decrypt(String cipherText) throws Exception {
        byte[] cipherTextBytes;
        try {
            cipherTextBytes = DatatypeConverter.parseHexBinary(cipherText);
        }
        catch (IllegalArgumentException e) {
            JOptionPane.showMessageDialog(null, "Invalid Encrypted Group Password",
                    "", JOptionPane.ERROR_MESSAGE);
            e.printStackTrace();
            return "";
        }
        if (cipherTextBytes.length < 48) {
            JOptionPane.showMessageDialog(null, "Invalid Encrypted Group Password",
                    "", JOptionPane.ERROR_MESSAGE);
            return "";
        }
        byte[] iv = new byte[8];
        System.arraycopy(cipherTextBytes, 0, iv, 0, 8);
        byte[] ht = new byte[20];
        System.arraycopy(cipherTextBytes, 0, ht, 0, 20);
        ht[19]++;
        byte[] h4 = new byte[20];
        System.arraycopy(cipherTextBytes, 20, h4, 0, 20);

        MessageDigest sha1 = MessageDigest.getInstance("SHA1");
        byte[] h2 = sha1.digest(ht);
        ht[19] += 2;
        byte[] h3 = sha1.digest(ht);
        byte[] key = new byte[24];
        System.arraycopy(h2, 0, key, 0, 20);
        System.arraycopy(h3, 0, key, 20, 4);
        byte[] ciphertextBytesWithoutHeader = new byte[cipherTextBytes.length - 40];
        System.arraycopy(cipherTextBytes, 40, ciphertextBytesWithoutHeader, 0, ciphertextBytesWithoutHeader.length);
        ht = sha1.digest(ciphertextBytesWithoutHeader);
        if (!Arrays.equals(h4, ht)) {
            JOptionPane.showMessageDialog(null, "Invalid Encrypted Group Password",
                    "", JOptionPane.ERROR_MESSAGE);
            return "";
        }

        SecretKey secretKey = new SecretKeySpec(key, "DESede");
        IvParameterSpec ivSpec = new IvParameterSpec(iv);
        Cipher decipher = Cipher.getInstance("DESede/CBC/PKCS5Padding");
        decipher.init(Cipher.DECRYPT_MODE, secretKey, ivSpec);
        byte[] plainTextBytes;
        try {
            plainTextBytes = decipher.doFinal(ciphertextBytesWithoutHeader);
        } catch (IllegalBlockSizeException e) {
            JOptionPane.showMessageDialog(null, "Invalid Encrypted Group Password",
                    "", JOptionPane.ERROR_MESSAGE);
            return "";
        }
        return new String(plainTextBytes, "UTF-8");
    }

    public static void main(String[] args) throws Exception {

        String groupPswd = "";

        System.out.println(decrypt(groupPswd));
    }
}