Question

I need help on encryption/decryption on Android application.

I explain the situation. I’m actually wrote an application that used content generated and encrypted by an iPhone application.

And for securisation, the user provide his own passphrase to correctly encrypt/decrypt data between different platform…

But, I encount a problem with the encryption/decryption of this passphrase on Android.

I have two functions:

public byte[] crypt(String pStringToCrypt) throws Exception{

    byte[] key = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
    System.arraycopy(this.passphrase.getBytes(), 0, key, 0, this.passphrase.getBytes().length);
    SecretKeySpec skeySpec = new SecretKeySpec(key, "AES");

    Cipher cipher = Cipher.getInstance("AES/ECB/PKCS7Padding");
    cipher.init(Cipher.ENCRYPT_MODE, skeySpec);
    byte[] encrypted = cipher.doFinal(pStringToCrypt.getBytes());
    return encrypted;

}

for the crypting of String, and this function:

public String decrypt(byte[] pCryptedStringtoDecrypt) throws Exception{
    byte[] key = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
    System.arraycopy(this.passphrase.getBytes(), 0, key, 0, this.passphrase.getBytes().length);
    SecretKeySpec skeySpec = new SecretKeySpec(key, "AES");

    Cipher cipher = Cipher.getInstance("AES/ECB/PKCS7Padding");
    byte[] encrypted = pCryptedStringtoDecrypt;
    cipher.init(Cipher.DECRYPT_MODE, skeySpec);
    byte[] original = cipher.doFinal(encrypted);
    String originalString = new String(original);
    return originalString;
}

for the decrypting of String.

When I use the crypt method to encrypt data, I haven’t error and the string was encrypted:

encrypted = [26, 119, -108, -24, 81, -128, 18, 35, -96, 10, -38, 69, 111, 40, 109, 107]

If I try to transform this byte into a string, I obtain this string:

encryptedString = "w��Q�#�\n�Eo(mk"

I think the crypting phase was good.
Now when I try to decrypt this encrypted String, the application crashed:

javax.crypto.IllegalBlockSizeException: last block incomplete in decryption
    at org.bouncycastle.jce.provider.JCEBlockCipher.engineDoFinal(JCEBlockCipher.java:711)
    at javax.crypto.Cipher.doFinal(Cipher.java:1090)
    at org.vincentsaluzzo.lightrpc.common.security.AES256.decrypt(AES256.java:61)
    at com.vincentsaluzzo.LoginBox.model.SettingsManager.getUserPassphrase(SettingsManager.java:67)
    at com.vincentsaluzzo.LoginBox.mainActivity.onCreate(mainActivity.java:26)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1586)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1638)
    at android.app.ActivityThread.access$1500(ActivityThread.java:117)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:928)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:123)
    at android.app.ActivityThread.main(ActivityThread.java:3647)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:507)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
    at dalvik.system.NativeStart.main(Native Method)

And I don’t understand why this error appear…

Do you have some solutions ? or some explanation for me ?

Answer 1

Ok, with the help of all yours comments, I solved my problem.

I explain. I have transformed my two methods to be the more simplest possible:

public byte[] crypt(byte[] toCrypt) throws Exception {
    byte[] key = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
    System.arraycopy(this.passphrase.getBytes(), 0, key, 0, ((this.passphrase.getBytes().length < 16) ? this.passphrase.getBytes().length : 16));
    SecretKeySpec skeySpec = new SecretKeySpec(key, "AES");

    Cipher cipher = Cipher.getInstance("AES/ECB/PKCS7Padding");
    cipher.init(Cipher.ENCRYPT_MODE, skeySpec);
    byte[] encrypted = cipher.doFinal(toCrypt);
    return encrypted;
}

and

public byte[] decryptt(byte[] toDecrypt) throws Exception {
    byte[] key = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
    System.arraycopy(this.passphrase.getBytes(), 0, key, 0, ((this.passphrase.getBytes().length < 16) ? this.passphrase.getBytes().length : 16));
    SecretKeySpec skeySpec = new SecretKeySpec(key, "AES");

    Cipher cipher = Cipher.getInstance("AES/ECB/PKCS7Padding");
    cipher.init(Cipher.DECRYPT_MODE, skeySpec);
    byte[] original = cipher.doFinal(toDecrypt);
    return original;
}

Shortly, I remove all conversion into string in this methods.
And I have inspect my methods in my other project in Objective-C.
I found the problem: Encoding String !

Sometime, if I encrypt/decrypt, I get the same bytes array, but sometime, the array was different, because I stock this encrypted byte array into a String in the SharedPreference.
It could be possible, of Course, but I just put the byte into a new String(bytes) and the basic encoding are the UTF-8, so that’s the problem.

I resolv this with base 64 encoding. I use the Base64 Class found here: http://sourceforge.net/projects/iharder/files/base64/2.3/

and before save the encrypted bytes array into SharedPreferences, I encode them into a Base64 encoding, and same for the decryption process.