Cryptfs from Secret's Android App

Crypfs.java

//package ly.secret.android.net;

/**
 * This is source code derived from the Cryptfs class found in Secret.ly's android app.
 *
 * Comments and variable names are my own.
 *
 * Here's how I generated this file:
 * 1. Downloaded the Secret.ly APK via this link:
 *    http://storage.evozi.com/apk/dl/14/05/21/ly.secret.android.apk?vc=1600040
 * 2. Unzipped the APK
 * 3. Used dex2jar (https://code.google.com/p/dex2jar/) to convert the
 *    classes.dex to a jar file
 * 4. Unpacked the jar
 * 5. Looked for instances of "aes", "sha-1", "md5", etc. in the ly.secret package:
 *    $ find . -name "*.class" | grep "ly" | xargs javap -c | less
 * 6. Noticed the ly/secret/android/net/Crypts.class
 * 7. Used JD-GUI (http://jd.benow.ca/) to generate some source code
 * 8. Manually renamed variables and cleaned up code to make it readable.
 */

import java.security.MessageDigest;
import javax.crypto.Cipher;
import javax.crypto.Mac;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;

final class Crypts {
	private static final byte[] ZERO_PADDING = new byte[64];

	static byte[] encrypt(byte[] inputArray, byte[] keyValue) throws Exception {
		byte[] sha256HashOfKeyValue =
				MessageDigest.getInstance("SHA-256").digest(keyValue);
		// Why are they padding with zeros?
		byte[] inputPaddedWithZeros = mergeArrays(inputArray, ZERO_PADDING);

		// Using a fixed zero-value IV for CBC
		byte[] IV = new byte[16];

		// This throws a NoSuchAlgorithmException for the default Java 6 and 7 
		// providers due to the PKCS7 padding
		Cipher aesCbcCipher = Cipher.getInstance("AES/CBC/PKCS7Padding");
		
		// Maybe they're using a different provider or this class is never used.
		// I had to replace PKCS7 with PKCS5 to get it to work:
		// Cipher aesCbcCipher = Cipher.getInstance("AES/CBC/PKCS5Padding");

		aesCbcCipher.init(Cipher.ENCRYPT_MODE,
					new SecretKeySpec(sha256HashOfKeyValue, "AES"),
					new IvParameterSpec(IV));
		byte[] ciphertext = aesCbcCipher.doFinal(inputPaddedWithZeros);

		// Using the same key for MAC as encryption
		SecretKeySpec hmacMD5Key = new SecretKeySpec(sha256HashOfKeyValue, "HmacMD5");
		Mac localMac = Mac.getInstance(hmacMD5Key.getAlgorithm());
		localMac.init(hmacMD5Key);

		// Forgot to HMAC the IV.
		// Also they are MACing the plaintext, not the ciphertext.
		localMac.update(inputPaddedWithZeros);
		byte[] hmac = localMac.doFinal();
		return mergeArrays(ciphertext, hmac);
	}

	// This method was generated from com.google.common.primitives.Bytes
	// Merged and made readable manually.
	static byte[] mergeArrays(byte[]... arraysToMerge) {
		int totalLen = 0;
		for (int i = 0; i < arraysToMerge.length; i++) {
			totalLen += arraysToMerge[i].length;
		}
		byte[] outputArray = new byte[totalLen];
		int outputIndexPosition = 0;
		for (int i = 0; i < arraysToMerge.length; i++) {
			System.arraycopy(arraysToMerge[i], 0,
					outputArray, outputIndexPosition,
					arraysToMerge[i].length);
			outputIndexPosition += arraysToMerge[i].length;
		}
		return outputArray;
	}
	
	public static void main(String[] args) throws Exception {
		byte[] ciphertext = encrypt("hello".getBytes(), "world".getBytes());
	}
}