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Creating An Antidote For Android Simplelocker Ransomware

17 Jun 2014 • 6 min read

chemistry bottlesIn yesterday's blog post (How To Dissect Android Simplelocker Ransomware) we dissected the new Android Simplelocker ransomware.

In this blog post we'll be creating an antidote for the ransomware to decrypt any files it encrypts.

The process of creating the antidote is actually very simple because the ransomware comes with a built-in decrypt method and cipher password. This means we're able to create our own Java class and copy the decryption code from the ransomware into our antidote class.

So let's jump right in and start creating our antidote for Simplelocker!

NB: if you haven't already read the previous post (How To Dissect Android Simplelocker Ransomware) it might be worth having a quick read through that now, since I'll be referencing Java classes that were analysed in that post.

First, we want to examine the AesCrypt class, which is shown in full below:

package org.simplelocker;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.security.MessageDigest;
import java.security.spec.AlgorithmParameterSpec;
import javax.crypto.Cipher;
import javax.crypto.CipherInputStream;
import javax.crypto.CipherOutputStream;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class AesCrypt
{
  private final Cipher cipher;
  private final SecretKeySpec key;
  private AlgorithmParameterSpec spec;

  public AesCrypt(String paramString)
    throws Exception
  {
    MessageDigest localMessageDigest = MessageDigest.getInstance("SHA-256");
    localMessageDigest.update(paramString.getBytes("UTF-8"));
    byte[] arrayOfByte = new byte[32];
    System.arraycopy(localMessageDigest.digest(), 0, arrayOfByte, 0, arrayOfByte.length);
    this.cipher = Cipher.getInstance("AES/CBC/PKCS7Padding");
    this.key = new SecretKeySpec(arrayOfByte, "AES");
    this.spec = getIV();
  }

  public void decrypt(String paramString1, String paramString2)
    throws Exception
  {
    FileInputStream localFileInputStream = new FileInputStream(paramString1);
    FileOutputStream localFileOutputStream = new FileOutputStream(paramString2);
    this.cipher.init(2, this.key, this.spec);
    CipherInputStream localCipherInputStream = new CipherInputStream(localFileInputStream, this.cipher);
    byte[] arrayOfByte = new byte[8];
    while (true)
    {
      int i = localCipherInputStream.read(arrayOfByte);
      if (i == -1)
      {
        localFileOutputStream.flush();
        localFileOutputStream.close();
        localCipherInputStream.close();
        return;
      }
      localFileOutputStream.write(arrayOfByte, 0, i);
    }
  }

  public void encrypt(String paramString1, String paramString2)
    throws Exception
  {
    FileInputStream localFileInputStream = new FileInputStream(paramString1);
    FileOutputStream localFileOutputStream = new FileOutputStream(paramString2);
    this.cipher.init(1, this.key, this.spec);
    CipherOutputStream localCipherOutputStream = new CipherOutputStream(localFileOutputStream, this.cipher);
    byte[] arrayOfByte = new byte[8];
    while (true)
    {
      int i = localFileInputStream.read(arrayOfByte);
      if (i == -1)
      {
        localCipherOutputStream.flush();
        localCipherOutputStream.close();
        localFileInputStream.close();
        return;
      }
      localCipherOutputStream.write(arrayOfByte, 0, i);
    }
  }

  public AlgorithmParameterSpec getIV()
  {
    return new IvParameterSpec(new byte[16]);
  }
}

We can see that the decrypt() method is clearly provided within this class. We also already know the decryption password (as set in the Constants class), so all we need to is call this method on any files ending in .enc.

Let's create a new Java class called SimplelockerAntidote with a method called getEncryptedFiles() which will scan the current directory for all files ending with .enc:

public static String[] getEncryptedFiles() {

		File dir = new File(System.getProperty("user.dir"));

		Collection<String> files  =new ArrayList<String>();

		if(dir.isDirectory()){
			File[] listFiles = dir.listFiles();

			for(File file : listFiles){
				String filename = file.getName();
				if(
					file.isFile() 
					&& (filename.lastIndexOf(".") >= 0)
					&& (filename.substring(filename.lastIndexOf(".")).toLowerCase().equals(".enc"))
				) {
					files.add(file.getName());
				}
			}
		}
		
		return files.toArray(new String[]{});
	}

This method returns an array of String files names (of all files ending in .enc in the current directory) which we can then feed into the decrypt() method. Line 3 sets the current directory (where the Java program is running). Line 10 iterates over all files in the directory. Finally, line 15 checks if the current file ends in .enc where line 17 adds it to the return array if it matches.

Next, create a main method which calls getEncryptedFiles() and sets the decryption password:

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

		// set default cipher password
		String cipher_password = "jndlasf074hr";

		// overwrite cipher password if set by first argument
		if(args.length == 1)
		{
			cipher_password = args[0];
		}

		// create new SimplelockerAntidote object
		SimplelockerAntidote sa = new SimplelockerAntidote(cipher_password);

		// get array of filenames to decrypt from current directory
		String[] files = sa.getEncryptedFiles();

		// iterate through files in the array
		for (int i = 0; i < files.length; i++) {

			// set input and output filenames
			// and remove the .enc file extension
			String inputFilename = files[i];			
			String outputFilename = inputFilename.substring(0,inputFilename.length()-4);

			System.out.println("Decrypting "+outputFilename);

			// call decrypt on the current file
			sa.decrypt(inputFilename,outputFilename);
		}

		System.out.println("Decryption complete.");

	}

Now all we need to do is copy-and-paste the constructor method from AesCrypt along with AlgorithmParameterSpec getIV(), encrypt() and decrypt().

This completes the antidote. The full Java class file is shown below:

// Simplelocker antidote, written by Simon Bell, SecureHoney.net

import javax.crypto.*;
import javax.crypto.interfaces.*;
import javax.crypto.spec.*;
import java.io.*;
import java.security.*;
import java.security.cert.*;
import java.security.interfaces.*;
import java.security.spec.*;
import javax.xml.bind.DatatypeConverter;
import java.util.ArrayList;
import java.util.Collection;

public class SimplelockerAntidote {

	private final Cipher cipher;
	private final SecretKeySpec key;
	private AlgorithmParameterSpec spec;

	public SimplelockerAntidote(String password) throws Exception {

		MessageDigest digest = MessageDigest.getInstance("SHA-256");
		digest.update(password.getBytes("UTF-8"));
		byte[] keyBytes = new byte[32];
		System.arraycopy(digest.digest(), 0, keyBytes, 0, keyBytes.length);

		cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
		key = new SecretKeySpec(keyBytes, "AES");
		spec = getIV();
	}

	public AlgorithmParameterSpec getIV() {
		return new IvParameterSpec(new byte[16]);
	}

	public void encrypt(String paramString1, String paramString2) throws Exception {
		FileInputStream localFileInputStream = new FileInputStream(paramString1);
		FileOutputStream localFileOutputStream = new FileOutputStream(paramString2);
		this.cipher.init(1, this.key, this.spec);
		CipherOutputStream localCipherOutputStream = new CipherOutputStream(localFileOutputStream, this.cipher);
		byte[] arrayOfByte = new byte[8];
		while (true) {
			int i = localFileInputStream.read(arrayOfByte);
			if (i == -1) {
				localCipherOutputStream.flush();
				localCipherOutputStream.close();
				localFileInputStream.close();
				return;
			}
			localCipherOutputStream.write(arrayOfByte, 0, i);
		}
	}

	public void decrypt(String paramString1, String paramString2) throws Exception {
		FileInputStream localFileInputStream = new FileInputStream(paramString1);
		FileOutputStream localFileOutputStream = new FileOutputStream(paramString2);
		this.cipher.init(2, this.key, this.spec);
		CipherInputStream localCipherInputStream = new CipherInputStream(localFileInputStream, this.cipher);
		byte[] arrayOfByte = new byte[8];
		while (true) {
			int i = localCipherInputStream.read(arrayOfByte);
			if (i == -1) {
				localFileOutputStream.flush();
				localFileOutputStream.close();
				localCipherInputStream.close();
				return;
			}
			localFileOutputStream.write(arrayOfByte, 0, i);
		}
	} 

	public static String[] getEncryptedFiles() {

		File dir = new File(System.getProperty("user.dir"));

		Collection<String> files  =new ArrayList<String>();

		if(dir.isDirectory()){
			File[] listFiles = dir.listFiles();

			for(File file : listFiles){
				String filename = file.getName();
				if(
					file.isFile() 
					&& (filename.lastIndexOf(".") >= 0)
					&& (filename.substring(filename.lastIndexOf(".")).toLowerCase().equals(".enc"))
				) {
					files.add(file.getName());
				}
			}
		}
		
		return files.toArray(new String[]{});
	}

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

		// set default cipher password
		String cipher_password = "jndlasf074hr";

		// overwrite cipher password if set by first argument
		if(args.length == 1)
		{
			cipher_password = args[0];
		}

		// create new SimplelockerAntidote object
		SimplelockerAntidote sa = new SimplelockerAntidote(cipher_password);

		// get array of filenames to decrypt from current directory
		String[] files = sa.getEncryptedFiles();

		// iterate through files in the array
		for (int i = 0; i < files.length; i++) {

			// set input and output filenames
			// and remove the .enc file extension
			String inputFilename = files[i];			
			String outputFilename = inputFilename.substring(0,inputFilename.length()-4);

			System.out.println("Decrypting "+outputFilename);

			// call decrypt on the current file
			sa.decrypt(inputFilename,outputFilename);
		}

		System.out.println("Decryption complete.");

	}
}

Once compiled, this Java program will scan the current directory for all files ending in .enc and attempt to decrypt them using the pre-set password.

It's also possible to override the default cipher password by calling the program as follows:

java SimplelockerAntidote <cipher-password>

The complete Java program can be downloaded here: SimplelockerAntidote.java.

It would be fairly straightforward from here to convert this Java program into an Android app. This app could then search for all .enc files on the SD card and attempt to decrypt them - thus providing a direct Android antidote app.

The antidote for this ransomware was incredibly easy to create because the ransomware came with both the decryption method and the decryption password. Therefore producing an antidote was more of a copy-and-paste job than anything.

It's also worth noting that while this antidote doesn't detect the decryption password automatically, it could be possible to do so. However, future versions of the ransomware will probably not reveal the decryption password so easily and will likely receive it from the C&C server.

Since the Simplelocker ransomware is a proof-of-concept, the antidote provided here is simply a solution to this proof-of-concept. Future versions of advanced smartphone ransomware will likely prove significantly harder to reverse engineer.

Image credit: "Chemistry" by Paul, flickr.com/photos/howzey/4955405374.

About the author

Simon BellSimon Bell is an award-winning Cyber Security Researcher, Software Engineer, and Web Security Specialist. Simon's research papers have been published internationally, and his findings have featured in Ars Technica, The Hacker News, PC World, among others. He founded Secure Honey, an open-source honeypot and threat intelligence project, in 2013. He has a PhD in Information Security and a BSc in Computer Science.

Follow Simon on Twitter: @SimonByte