Java Android - Decrypted byte array 255 bytes long instead of "Hello World!".length

Question

I'm reading a lot since some weeks to implement an encrypt/decrypt algoritm for my Android application. I'm implementing a license key that is downloaded from my website and stored in the external storage of my Android device. the application read the content of the file and decrypt it using the server public key (yes i know that i should with client private key but it's ok for my purpose). The problem is that the final string has a lot of black square with question mark inside. i've read a lot of other posts here on stackoverflow, but i think that the "only" problem is that, even if there should be 10 chars in the string, the string is long 255 bytes (with 2048 bit RSA key) and the remaining chars are filled with black "". Why the newPlainText var is not long as "Hello World!" ? Here below my code... Many thanks in advance!

public boolean licenseValid() throws IOException, InvalidKeyException, NoSuchAlgorithmException, NoSuchPaddingException, IllegalBlockSizeException, BadPaddingException{
    java.io.File file = new java.io.File(Environment.getExternalStorageDirectory().toString() ,
            "/folder/file.lic");
    byte[] fileBArray = new byte[(int)file.length()];
    FileInputStream fis = new FileInputStream(file);

    // Read in the bytes
    int offset = 0;
    int numRead = 0;
    while (offset < fileBArray.length
           && (numRead=fis.read(fileBArray, offset, fileBArray.length-offset)) >= 0) {
        offset += numRead;
    }

    // Ensure all the bytes have been read in
    if (offset < fileBArray.length) {
        throw new IOException("Could not completely read file "+file.getName());
    }

    fis.close();

    // Decrypt the ciphertext using the public key
PublicKey pubKey = readKeyFromFile();
Cipher cipher = Cipher.getInstance("RSA");
cipher.init(Cipher.DECRYPT_MODE, pubKey);
byte[] newPlainText = cipher.doFinal(fileBArray);

    // THE FOLLOWING TOAST PRINTS MANY <?> AND THAN THE DECRYPTED MESSAGE. THE TOTAL NUMBER OF CHARACTERS IS 255, EVEN IF I CHANGE ENCRYPTED TEXT!
toast(String.valueOf(cipher.doFinal(fileBArray).length));

    if (new String(newPlainText, "utf-8").compareTo("Hello World!") == 0)
        return true;
    else
        return false;
}

PublicKey readKeyFromFile() throws IOException {
    Resources myResources = getResources();
    //public key filename "pub.lic"
    InputStream is = myResources.openRawResource(R.raw.pub);
ObjectInputStream oin =
    new ObjectInputStream(new BufferedInputStream(is));

try {
        BigInteger m = (BigInteger) oin.readObject();
        BigInteger e = (BigInteger) oin.readObject();
        RSAPublicKeySpec keySpec = new RSAPublicKeySpec(m, e);
    KeyFactory fact = KeyFactory.getInstance("RSA");
    PublicKey pubKey = fact.generatePublic(keySpec);
    return pubKey;
  } catch (Exception e) {
    throw new RuntimeException("Spurious serialisation error", e);
  } finally {
    oin.close();
   }
}

Nikolay Elenkov · Accepted Answer · 2011-10-25 02:08:26Z

2

If you encrypt with RSA the input and output are always the same length as the key. In your case, that should be 256 bytes (=2048 bits), so first check your code, you are missing a byte. When the input is shorter, you need to apply a padding, and it looks like your server and client are using a different one. Cipher.getInstance("RSA") will use the platform default, which is probably different for Android and Java SE. You need to specify the padding explicitly in both programs for this to work. Something like this:

Cipher cipher = Cipher.getInstance("RSA/None/PKCS1Padding");

BTW, you really don't want to distribute the private key with your app, so using the public key is the right thing to do. (Whether your whole encryption scheme is secure is another matter though).

answered Oct 25, 2011 at 2:08

Nikolay Elenkov

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5 Comments

Vale Over a year ago

Trying with: "RSA/None/PKCS1Padding" It gives me the following error: Exception in thread "main" java.security.NoSuchAlgorithmException: Cannot find any provider supporting RSA/None/PKCS1Padding. Probably what you assume could be the real problem... What can i try now?

Nikolay Elenkov Over a year ago

Is this on Android or on your server? You may need to install the BouncyCastle provider and use it explicitly like so: Cipher.getInstance("RSA/NONE/PKCS1Padding", "BC");

Vale Over a year ago

Hi, i create the pub.key, priv.key and file.lic on my laptop and than i put the pub.key in the res->raw folder of the android project and the file.lic in the folder on the external memory of my Android device. i read on the web that Android is not embedding BouncyCastle. I suppose it is only a matter of Padding... what can i try?

Nikolay Elenkov Over a year ago

Did you actually try it? It's a lot more effective than reading random stuff on the Web. BouncyCastle is part of Android, and the above code works.

Vale Over a year ago

SOLVED! THANKS! - Bouncycastle’s default RSA implementation is: “RSA/None/NoPadding”, whereas Sun’s default security provider implementation is “RSA/None/PKCS1Padding”. So, no decryption possible when just using Cipher.getInstance("RSA"). I downloaded the bouncycastle Jar and imported in Eclipse to the server side project. Than i used Cipher.getInstance("RSA/NONE/PKCS1Padding", "BC"); on Android device and i visualized the correct decrypted message. Thanks @all!

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