Rick Sanchez Rick Sanchez - 3 months ago 15
Android Question

How to decrypt in Java (Android) text that was encrypted with Crypt in Laravel?

I need to decrypt some data that I receive from the server, and the programmer who made the API directed me to this Encrypter class, to see what he used to encrypt.

Now based on that class, I found that the algorithm used is AES128 CBC, and that the string I receive is Base64 encoded and contains other data, not just the ciphertext.

Namely that if I receive the following String:

eyJpdiI6InJsSzRlU3pDZTBBUVNwMzdXMjVcL0tBPT0iLCJ2YWx1ZSI6Ik5JOENsSVVWaWk2RGNhNlwvWjJNeG94UzVkclwvMGJOREQreWUyS1UzclRMND0iLCJtYWMiOiJhZTZkYjNkNGM2ZTliNmU0ZTc0MTRiNDBmMzFlZTJhNTczZWIxMjk4N2YwMjlhODA1NTIyMDEzODljNDY2OTk2In0


after base64 decoding I get:

{"iv":"rlK4eSzCe0AQSp37W25\/KA==","value":"NI8ClIUVii6Dca6\/Z2MxoxS5dr\/0bNDD+ye2KU3rTL4=","mac":"ae6db3d4c6e9b6e4e7414b40f31ee2a573eb12987f029a80552201389c466996"}


Based on
line 99
of
Encrypter
class (
iv = base64_decode($payload['iv']);
), I performed another base64 decode on the
iv
and the
value
, and got an
iv
of length 16. Those I passed as parameters to the function below:

public static String decrypt(String iv, String encryptedData) throws Exception {
byte[] keyValue = "zy2dEd1pKG5i3WuWbvOBolFQR84AYbvN".getBytes();
Key key = new SecretKeySpec(keyValue, "AES");
Cipher c = Cipher.getInstance("AES/CBC/PKCS7Padding");
c.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(iv.getBytes()));
byte[] decordedValue = Base64.decode(encryptedData.getBytes(), Base64.DEFAULT);
byte[] decValue = c.doFinal(decordedValue);
return new String(decValue);
}


But I'm getting the following error:

10-06 19:13:33.601 12895-12895/? W/System.err: java.security.InvalidAlgorithmParameterException: expected IV length of 16
10-06 19:13:33.601 12895-12895/? W/System.err: at com.android.org.conscrypt.OpenSSLCipher.engineInitInternal(OpenSSLCipher.java:281)
10-06 19:13:33.601 12895-12895/? W/System.err: at com.android.org.conscrypt.OpenSSLCipher.engineInit(OpenSSLCipher.java:323)
10-06 19:13:33.601 12895-12895/? W/System.err: at javax.crypto.Cipher.init(Cipher.java:751)
10-06 19:13:33.601 12895-12895/? W/System.err: at javax.crypto.Cipher.init(Cipher.java:701)
10-06 19:13:33.601 12895-12895/? W/System.err: at com.example.kushtrim.testproject.MainActivity.decrypt(MainActivity.java:62)
10-06 19:13:33.601 12895-12895/? W/System.err: at com.example.kushtrim.testproject.MainActivity.onCreate(MainActivity.java:45)
10-06 19:13:33.601 12895-12895/? W/System.err: at android.app.Activity.performCreate(Activity.java:5990)
10-06 19:13:33.601 12895-12895/? W/System.err: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
10-06 19:13:33.601 12895-12895/? W/System.err: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
10-06 19:13:33.601 12895-12895/? W/System.err: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
10-06 19:13:33.601 12895-12895/? W/System.err: at android.app.ActivityThread.access$800(ActivityThread.java:151)
10-06 19:13:33.601 12895-12895/? W/System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
10-06 19:13:33.602 12895-12895/? W/System.err: at android.os.Handler.dispatchMessage(Handler.java:102)
10-06 19:13:33.602 12895-12895/? W/System.err: at android.os.Looper.loop(Looper.java:135)
10-06 19:13:33.602 12895-12895/? W/System.err: at android.app.ActivityThread.main(ActivityThread.java:5254)
10-06 19:13:33.602 12895-12895/? W/System.err: at java.lang.reflect.Method.invoke(Native Method)
10-06 19:13:33.602 12895-12895/? W/System.err: at java.lang.reflect.Method.invoke(Method.java:372)
10-06 19:13:33.602 12895-12895/? W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
10-06 19:13:33.602 12895-12895/? W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)


Note: The String
iv
has length of 16, but
iv.getBytes()
returns an array of length 26.

Could someone point me to where I went wrong, and how do I fix it.
Thanks/

EDIT

After the comment, I made some changes, that resolved the above error:

Before I was base64 decoding
iv
, converting the bytes to String, then passing that String to the decrypt method, which in return called the getBytes() on it. Somehow this made the byte array have a length of 26.

Sending the byte array I obtained after base64 decoding to the decrypt method fixed the problem.

Now the method is as follows:

public static String decrypt(byte[] iv, String encryptedData) throws Exception {
byte[] keyValue = "zy2dEd1pKG5i3WuWbvOBolFQR84AYbvN".getBytes();
Key key = new SecretKeySpec(keyValue, "AES");
Cipher c = Cipher.getInstance("AES/CBC/PKCS7Padding");
c.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(iv));
byte[] decordedValue = Base64.decode(encryptedData.getBytes(), Base64.DEFAULT);
byte[] decValue = c.doFinal(decordedValue);
return new String(decValue);
}


Now I have another weird problem:

The text I encrypted on the first place was
KushtrimPacaj
, but the decrypted text is
s:13:"KushtrimPacaj";
.
Where is that other part coming from ? 13 perhaps represents the length of
KushtrimPacaj
?

Edit

Here's the working code, in case anyone needs it :

https://gist.github.com/KushtrimPacaj/43a383ab419fc222f80e

Answer

You can see in the padAndMcrypt() function, that the given $value is serialized using PHP's serialize() function. You can re-implement the unserialize() function in Java or you can split the byte array yourself if you're always encrypting strings in PHP.

int firstQuoteIndex = 0;
while(decValue[firstQuoteIndex] != (byte)'"') firstQuoteIndex++;
return new String(Arrays.copyOfRange(decValue, firstQuoteIndex + 1, decValue.length-2));

Full code:

public static String decrypt(byte[] keyValue, String ivValue, String encryptedData) throws Exception {
    Key key = new SecretKeySpec(keyValue, "AES");
    byte[] iv = Base64.decode(ivValue.getBytes("UTF-8"), Base64.DEFAULT);
    byte[] decodedValue = Base64.decode(encryptedData.getBytes("UTF-8"), Base64.DEFAULT);

    Cipher c = Cipher.getInstance("AES/CBC/PKCS7Padding"); // or PKCS5Padding
    c.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(iv));
    byte[] decValue = c.doFinal(decodedValue);

    int firstQuoteIndex = 0;
    while(decValue[firstQuoteIndex] != (byte)'"') firstQuoteIndex++;
    return new String(Arrays.copyOfRange(decValue, firstQuoteIndex + 1, decValue.length-2));
}

Verifying the MAC is always a good idea, because it prevents some attacks such as the padding oracle attack. It is also a very good way to detect general modifications of ciphertexts.

Full code with MAC verification:

public static String decrypt(byte[] keyValue, String ivValue, String encryptedData, String macValue) throws Exception {
    Key key = new SecretKeySpec(keyValue, "AES");
    byte[] iv = Base64.decode(ivValue.getBytes("UTF-8"), Base64.DEFAULT);
    byte[] decodedValue = Base64.decode(encryptedData.getBytes("UTF-8"), Base64.DEFAULT);

    SecretKeySpec macKey = new SecretKeySpec(keyValue, "HmacSHA256");
    Mac hmacSha256 = Mac.getInstance("HmacSHA256");
    hmacSha256.init(macKey);
    hmacSha256.update(ivValue.getBytes("UTF-8"));
    byte[] calcMac = hmacSha256.doFinal(encryptedData.getBytes("UTF-8"));
    byte[] mac = Hex.decodeHex(macValue.toCharArray());
    if (!secureEquals(calcMac, mac))
        return null; // or throw exception

    Cipher c = Cipher.getInstance("AES/CBC/PKCS7Padding"); // or PKCS5Padding
    c.init(Cipher.DECRYPT_MODE, key, new IvParameterSpec(iv));
    byte[] decValue = c.doFinal(decodedValue);

    int firstQuoteIndex = 0;
    while(decValue[firstQuoteIndex] != (byte)'"') firstQuoteIndex++;
    return new String(Arrays.copyOfRange(decValue, firstQuoteIndex + 1, decValue.length-2));
}

/* Constant-time compare to prevent timing attacks on invalid authentication tags. */
public static boolean secureEquals(final byte[] known, final byte[] user) {
    int knownLen = known.length;
    int userLen = user.length;

    int result = knownLen ^ userLen;
    for (int i = 0; i < knownLen; i++) {
        result |= known[i] ^ user[i % userLen];
    }
    return result == 0;
}
Comments