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Editorial Team
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Asked: 2026-05-26T10:31:20+00:00

I’m reading a lot since some weeks to implement an encrypt/decrypt algoritm for my

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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();
   }
}
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1 Answer

  1. Editorial Team

    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).

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