Storing credentials in the Android app

Question

How can we safely storing credentials data for access to the smtp-server in Android app? These data are constants and only the developer should know them. At the moment they are stored in the code, but this is not safe, because they can be seen by decompiling the application.

Is it possible to use Android Keystore System for this purpose and how? And most importantly, will Android Keystore be effective on rooted devices?

Answer 1

In android applications you may store data in the SharedPreferences but since this data is actually stored in a file, anyone with root access to a phone may access it. That means a security leak if you want to store credentials or any other sensitive data.

In order to avoid other persons to see this data in plain text a solution is to encrypt the data before storing it. From API 18 Android introduced the KeyStore which is able to store keys in which you encrypt and decrypt the data.

Problem until API 23 is that you were not able to store AES keys in KeyStore so the most reliable key for encryption was RSA with private and public key.

So the solution I came up with was:

For APIs below 23

• You generate an RSA private and public key and save it in KeyStore, generate an AES key, encrypt it with RSA public key and save it to SharedPreferences.
• Every time you need to save encrypted data in SharedPreferences with the AES key, you get the encrypted AES key from SharedPreferences, decrypt it with RSA private key and encrypt the data you want to save to SharedPreferences with the already decrypted AES key.
• To decrypt the data the process is pretty much the same, get encrypted AES key from SharedPreferences, decrypt it with the RSA private key, get the encrypted data from SharedPreferences you want to decrypt, and decrypt it with the decrypted AES key.

For API 23 and above

• just generate and store an AES key in KeyStore, and access it whenever you want for data encryption/decryption.

Also added a generated IV for the encryption.

Code:

public class KeyHelper{


    private static final String RSA_MODE =  "RSA/ECB/PKCS1Padding";
    private static final String AES_MODE_M = "AES/GCM/NoPadding";

    private static final String KEY_ALIAS = "KEY";
    private static final String AndroidKeyStore = "AndroidKeyStore";
    public static final String SHARED_PREFENCE_NAME = "SAVED_TO_SHARED";
    public static final String ENCRYPTED_KEY = "ENCRYPTED_KEY";
    public static final String PUBLIC_IV = "PUBLIC_IV";


    private KeyStore keyStore;
    private static KeyHelper keyHelper;

    public static KeyHelper getInstance(Context ctx){
        if(keyHelper == null){
            try{
                keyHelper = new KeyHelper(ctx);
            } catch (NoSuchPaddingException | NoSuchProviderException | NoSuchAlgorithmException | InvalidAlgorithmParameterException | KeyStoreException | CertificateException | IOException e){
                e.printStackTrace();
            }
        }
        return keyHelper;
    }

    public KeyHelper(Context ctx) throws  NoSuchPaddingException,NoSuchProviderException, NoSuchAlgorithmException, InvalidAlgorithmParameterException, KeyStoreException, CertificateException, IOException  {
        this.generateEncryptKey(ctx);
        this.generateRandomIV(ctx);
        if(android.os.Build.VERSION.SDK_INT < android.os.Build.VERSION_CODES.M){
            try{
                this.generateAESKey(ctx);
            } catch(Exception e){
                e.printStackTrace();
            }
        }
    }


    private void generateEncryptKey(Context ctx) throws  NoSuchProviderException, NoSuchAlgorithmException, InvalidAlgorithmParameterException, KeyStoreException, CertificateException, IOException {

        keyStore = KeyStore.getInstance(AndroidKeyStore);
        keyStore.load(null);

        if(android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.M){
            if (!keyStore.containsAlias(KEY_ALIAS)) {
                KeyGenerator keyGenerator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, AndroidKeyStore);
                keyGenerator.init(
                        new KeyGenParameterSpec.Builder(KEY_ALIAS,
                            KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)
                                .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
                                .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
                                .setRandomizedEncryptionRequired(false)
                                .build());
                keyGenerator.generateKey();
            }
        } else{
            if (!keyStore.containsAlias(KEY_ALIAS)) {
                // Generate a key pair for encryption
                Calendar start = Calendar.getInstance();
                Calendar end = Calendar.getInstance();
                end.add(Calendar.YEAR, 30);
                KeyPairGeneratorSpec spec = new   KeyPairGeneratorSpec.Builder(ctx)
                        .setAlias(KEY_ALIAS)
                        .setSubject(new X500Principal("CN=" + KEY_ALIAS))
                        .setSerialNumber(BigInteger.TEN)
                        .setStartDate(start.getTime())
                        .setEndDate(end.getTime())
                        .build();
                KeyPairGenerator kpg = KeyPairGenerator.getInstance(KeyProperties.KEY_ALGORITHM_RSA, AndroidKeyStore);
                kpg.initialize(spec);
                kpg.generateKeyPair();
            }
        }


    }

    private byte[] rsaEncrypt(byte[] secret) throws Exception{
        KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry) keyStore.getEntry(KEY_ALIAS, null);
        // Encrypt the text
        Cipher inputCipher = Cipher.getInstance(RSA_MODE, "AndroidOpenSSL");
        inputCipher.init(Cipher.ENCRYPT_MODE, privateKeyEntry.getCertificate().getPublicKey());

        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        CipherOutputStream cipherOutputStream = new CipherOutputStream(outputStream, inputCipher);
        cipherOutputStream.write(secret);
        cipherOutputStream.close();

        return outputStream.toByteArray();
    }

    private  byte[]  rsaDecrypt(byte[] encrypted) throws Exception {
        KeyStore.PrivateKeyEntry privateKeyEntry = (KeyStore.PrivateKeyEntry)keyStore.getEntry(KEY_ALIAS, null);
        Cipher output = Cipher.getInstance(RSA_MODE, "AndroidOpenSSL");
        output.init(Cipher.DECRYPT_MODE, privateKeyEntry.getPrivateKey());
        CipherInputStream cipherInputStream = new CipherInputStream(
            new ByteArrayInputStream(encrypted), output);
        ArrayList<Byte> values = new ArrayList<>();
        int nextByte;
        while ((nextByte = cipherInputStream.read()) != -1) {
            values.add((byte)nextByte);
        }

        byte[] bytes = new byte[values.size()];
        for(int i = 0; i < bytes.length; i++) {
            bytes[i] = values.get(i).byteValue();
        }
        return bytes;
    }

    private void generateAESKey(Context context) throws  Exception{
        SharedPreferences pref = context.getSharedPreferences(SHARED_PREFENCE_NAME, Context.MODE_PRIVATE);
        String enryptedKeyB64 = pref.getString(ENCRYPTED_KEY, null);
        if (enryptedKeyB64 == null) {
            byte[] key = new byte[16];
            SecureRandom secureRandom = new SecureRandom();
            secureRandom.nextBytes(key);
            byte[] encryptedKey = rsaEncrypt(key);
            enryptedKeyB64 = Base64.encodeToString(encryptedKey, Base64.DEFAULT);
            SharedPreferences.Editor edit = pref.edit();
            edit.putString(ENCRYPTED_KEY, enryptedKeyB64);
            edit.apply();
        }
    }


    private Key getAESKeyFromKS() throws  NoSuchProviderException, NoSuchAlgorithmException, InvalidAlgorithmParameterException, KeyStoreException, CertificateException, IOException, UnrecoverableKeyException{
        keyStore = KeyStore.getInstance(AndroidKeyStore);
        keyStore.load(null);
        SecretKey key = (SecretKey)keyStore.getKey(KEY_ALIAS,null);
        return key;
    }


    private Key getSecretKey(Context context) throws Exception{
        SharedPreferences pref = context.getSharedPreferences(SHARED_PREFENCE_NAME, Context.MODE_PRIVATE);
        String enryptedKeyB64 = pref.getString(ENCRYPTED_KEY, null);

        byte[] encryptedKey = Base64.decode(enryptedKeyB64, Base64.DEFAULT);
        byte[] key = rsaDecrypt(encryptedKey);
        return new SecretKeySpec(key, "AES");
    }

    public String encrypt(Context context, String input) throws NoSuchAlgorithmException, NoSuchPaddingException, NoSuchProviderException, BadPaddingException, IllegalBlockSizeException, UnsupportedEncodingException {
        Cipher c;
        SharedPreferences pref = context.getSharedPreferences(SHARED_PREFENCE_NAME, Context.MODE_PRIVATE);
        String publicIV = pref.getString(PUBLIC_IV, null);

        if(android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.M){
            c = Cipher.getInstance(AES_MODE_M);
            try{
                c.init(Cipher.ENCRYPT_MODE, getAESKeyFromKS(), new GCMParameterSpec(128,Base64.decode(publicIV, Base64.DEFAULT)));
            } catch(Exception e){
                e.printStackTrace();
            }
        } else{
            c = Cipher.getInstance(AES_MODE_M);
            try{
                c.init(Cipher.ENCRYPT_MODE, getSecretKey(context),new GCMParameterSpec(128,Base64.decode(publicIV, Base64.DEFAULT)));
            } catch (Exception e){
                e.printStackTrace();
            }
        }
        byte[] encodedBytes = c.doFinal(input.getBytes("UTF-8"));
        return Base64.encodeToString(encodedBytes, Base64.DEFAULT);
    }





    public String decrypt(Context context, String encrypted) throws NoSuchAlgorithmException, NoSuchPaddingException, NoSuchProviderException, BadPaddingException, IllegalBlockSizeException, UnsupportedEncodingException {
        Cipher c;
        SharedPreferences pref = context.getSharedPreferences(SHARED_PREFENCE_NAME, Context.MODE_PRIVATE);
        String publicIV = pref.getString(PUBLIC_IV, null);


        if(android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.M){
            c = Cipher.getInstance(AES_MODE_M);
            try{
                c.init(Cipher.DECRYPT_MODE, getAESKeyFromKS(), new GCMParameterSpec(128,Base64.decode(publicIV, Base64.DEFAULT)));

            } catch(Exception e){
                e.printStackTrace();
            }
        } else{
            c = Cipher.getInstance(AES_MODE_M);
            try{
                c.init(Cipher.DECRYPT_MODE, getSecretKey(context), new GCMParameterSpec(128,Base64.decode(publicIV, Base64.DEFAULT)));
            } catch (Exception e){
                e.printStackTrace();
            }
        }

        byte[] decodedValue = Base64.decode(encrypted.getBytes("UTF-8"), Base64.DEFAULT);
        byte[] decryptedVal = c.doFinal(decodedValue);
        return new String(decryptedVal);
    }

    public void generateRandomIV(Context ctx){
        SharedPreferences pref = ctx.getSharedPreferences(SHARED_PREFENCE_NAME, Context.MODE_PRIVATE);
        String publicIV = pref.getString(PUBLIC_IV, null);

        if(publicIV == null){
            SecureRandom random = new SecureRandom();
            byte[] generated = random.generateSeed(12);
            String generatedIVstr = Base64.encodeToString(generated, Base64.DEFAULT);
            SharedPreferences.Editor edit = pref.edit();
            edit.putString(PUBLIC_IV_PERSONAL, generatedIVstr);
            edit.apply();
        }
    }

    private String getStringFromSharedPrefs(String key, Context ctx){
        SharedPreferences prefs = ctx.getSharedPreferences(MyConstants.APP_SHAREDPREFS, 0);
        return prefs.getString(key, null);
    }
}

NOTE: This is only for API 18 and above

Answer 2

Regarding your question about security on rooted device, I would recommend you the following paper:

Analysis of Secure Key Storage Solutions on Android