Decrypt AES Key with RSA in Android

Question

I am trying to encrypt a small file with AES (128 bits) and then encrypt the AES key with RSA (1024 bits). This works fine.

As the logical next step I try to decrypt the AES key with RSA.

Decrypting with RSA returns a 128 byte block, but my AES key is just 16 byte long. Upon research I read that I need to use RSA with Padding so i used RSA/ECB/PKCS1Padding.

But this always gives me the following exception -

javax.crypto.BadPaddingException: error:04000089:RSA routines:OPENSSL_internal:PKCS_DECODING_ERROR
at com.android.org.conscrypt.NativeCrypto.RSA_private_decrypt(Native Method)
at com.android.org.conscrypt.OpenSSLCipherRSA$DirectRSA.doCryptoOperation(OpenSSLCipherRSA.java:402)
at com.android.org.conscrypt.OpenSSLCipherRSA.engineDoFinal(OpenSSLCipherRSA.java:314)
at javax.crypto.Cipher.doFinal(Cipher.java:2055)

My KeyPair generation logic -

        KeyPairGenerator keyGen = null;
        try {
            keyGen = KeyPairGenerator.getInstance("RSA");
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        }
        keyGen.initialize(1024);
        byte[] publicKey = keyGen.genKeyPair().getPublic().getEncoded();

        byte[] privateKey = keyGen.genKeyPair().getPrivate().getEncoded();


        Util.save("privateKey", rsa.encryptBASE64(privateKey), this);
        Util.save("publicKey", rsa.encryptBASE64(publicKey), this);

My Encryption logic -

        public static byte[] encryptByPublicKey(byte[] data, String key)
            throws Exception {

        byte[] keyBytes = decryptBASE64(key);


        X509EncodedKeySpec x509KeySpec = new X509EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        Key publicKey = keyFactory.generatePublic(x509KeySpec);


        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
        cipher.init(Cipher.ENCRYPT_MODE, publicKey);

        return cipher.doFinal(data);
    }

My Decryption logic -

        public static byte[] decryptByPrivateKey(byte[] data, String key)
            throws Exception {

        byte[] keyBytes = decryptBASE64(key);


        PKCS8EncodedKeySpec pkcs8KeySpec = new PKCS8EncodedKeySpec(keyBytes);
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        Key privateKey = keyFactory.generatePrivate(pkcs8KeySpec);


        Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
        cipher.init(Cipher.DECRYPT_MODE, privateKey);

        return cipher.doFinal(data);
    }

Any help will be appreciated.

Could anyone also guide me on how to convert the decrypted RSA block to AES key ?

To test this is whatI do in my code. Encryption and Decryption are back to back steps -

String aesKeyCipherBase64 = rsa.encryptBASE64(rsa.encryptByPublicKey(secretKey.getEncoded(), myPublicKeyString));

byte[] aesKeyRecovered = rsa.decryptByPrivateKey(rsa.decryptBASE64(aesKeyCipherBase64),myPrivateKeyString);

The Base64 Util Method -

public static byte[] decryptBASE64(String key)  {
        return Base64.decode(key, Base64.NO_PADDING|Base64.NO_WRAP|Base64.NO_PADDING|Base64.URL_SAFE);
    }

public static String encryptBASE64(byte[] key)  {
        return Base64.encodeToString(key, Base64.NO_PADDING|Base64.NO_WRAP|Base64.NO_PADDING|Base64.URL_SAFE);
    }

       public static void save(String key, String value, Activity activity) {


        SharedPreferences sharedPref = activity.getSharedPreferences("myapp",Context.MODE_PRIVATE);
        SharedPreferences.Editor editor = sharedPref.edit();
        editor.putString(key, value);
        editor.commit();
    }

        String myPublicKeyString = Util.getPublicKey(this);
        String myPrivateKeyString = Util.getPrivateKey(this);

    public static String getPrivateKey(Activity activity) {

        SharedPreferences sharedPref = activity.getSharedPreferences("myapp",Context.MODE_PRIVATE);
        return sharedPref.getString("privateKey", null);
    }

    public static String getPublicKey(Activity activity) {

        SharedPreferences sharedPref = activity.getSharedPreferences("myapp",Context.MODE_PRIVATE);
        return sharedPref.getString("publicKey", null);
    }

Answer 1

I changed the following -

byte[] publicKey = keyGen.genKeyPair().getPublic().getEncoded();

byte[] privateKey = keyGen.genKeyPair().getPrivate().getEncoded();

to -

KeyPair kp = keyGen.genKeyPair();

byte[] publicKey = kp.getPublic().getEncoded();
byte[] privateKey = kp.getPrivate().getEncoded();

I was generating a new KeyPair each to access the public & private key.

Thank you @president-james-moveon-polk for pointing me towards the right direction.