Scytale

One tool to manage key generation, key storing and encryption on different APIs of Android.

As you may know Android provides API to use keystore that is stored in system only from API 18. They introduced AndroidKeyStore provider that is responsible for managing this.

But as always there are underwater stones. Up to API 23 you are only able to create asymmetric keys using AndroidKeyStore provider. Also algorithms that you can use are limited. And what about devices below API 18?

I've create API that wraps default JCA API and AndroidKeyStore API and makes it easy to create, manage and use your keys on any Android API.

Sample

// Create and save key
Store store = new Store(getApplicationContext());
if (!store.hasKey("test")) {
   SecretKey key = store.generateSymmetricKey("test", null);
}
...

// Get key
SecretKey key = store.getSymmetricKey("test", null);

// Encrypt/Decrypt data
Crypto crypto = new Crypto(Options.TRANSFORMATION_SYMMETRIC);
String text = "Sample text";

String encryptedData = crypto.encrypt(text, key);
Log.i("Scytale", "Encrypted data: " + encryptedData);

String decryptedData = crypto.decrypt(encryptedData, key);
Log.i("Scytale", "Decrypted data: " + decryptedData);

How it works?

Depending on what key you need and what Android you are using, API will create keystore file in application inner cache or will use AndroidKeyStore to hold keys. Key generation will be also made with different API. The tables below show what will be used in different cases.

In case you want to generate and save Asymmetric key

API	Application Keystore	AndroidKeyStore
< 18	+
>= 18		+

In case you want to generate and save Symmetric key

API	Application Keystore	AndroidKeyStore
< 23	+
>= 23		+

After calling one of generateKey methods, key will be automatically stored in keystore.

To store asymmetric PrivateKey we need to provide X509Certificate. And of course there is no default API to do that.

On 18+ devices it's pretty easy, Google did it for us.

For pre 18 there is one 3rd party library that can create self signed X509Certificate. It is called Bouncy Castle and is available on Maven as well. But after some research I found that Google copied this library to their API but made it private. Why? Don't ask me...

So I decided to make it like this:

• API will try to get Google Bouncy Castle using reflection (I've checked it on few APIs and it seems to work well)
• If Google version is missing, API will try to get 3rd party Bouncy Castle library. It will use reflection as well. This has two advantages:
• You can add this API for 18+ devices without any additional libraries.
• You can run this API on pre 18 devices without any additional libraries as well. And in case some device will miss Google hidden API you will receive an error and then include Bouncy Castle to project. This is pretty cool if you are getting error on 15 API but your min project API is 16, and there is no errors on it.

In general it creates simple interface to work with Keystore using API provided by Java and different versions of Android.

Extended Usage

Instead of using generateAsymmetricKey(@NonNull String alias, char[] password) method you can use generateAsymmetricKey(@NonNull KeyProps keyProps) one, and define key with specific options.

// Create store with specific name and password
Store store = new Store(context, STORE_NAME, STORE_PASSWORD);

final String alias = "alias";
final char[] password = "password".toCharArray();
final int keysize = 512;

final Calendar start = Calendar.getInstance();
final Calendar end = Calendar.getInstance();
end.add(Calendar.YEAR, 1);

// Create a key store params, some of them are specific per platform
// Check KeyProps doc for more info
KeyProps keyProps = new KeyProps.Builder()
   .setAlias(alias)
   .setPassword(password)
   .setKeySize(keysize)
   .setKeyType("RSA")
   .setSerialNumber(BigInteger.ONE)
   .setSubject(new X500Principal("CN=" + alias + " CA Certificate"))
   .setStartDate(start.getTime())
   .setEndDate(end.getTime())
   .setBlockModes("ECB")
   .setEncryptionPaddings("PKCS1Padding")
   .setSignatureAlgorithm("SHA256WithRSAEncryption")
   .build();

// Generate KeyPair depending on KeyProps
KeyPair keyPair = store.generateAsymmetricKey(keyProps);

// Encrypt/Dencrypt data using buffer with or without Initialisation Vectors
// This additional level of safety is required on 23 API level for
// some algorithms. Specify encryption/decryption block size to use buffer for
// large data when using block based algorithms (such as RSA)

final int encryptionBlockSize = keysize / 8 - 11; // as specified for RSA/ECB/PKCS1Padding keys
final int decryptionBlockSize = keysize / 8; // as specified for RSA/ECB/PKCS1Padding keys

Crypto crypto = new Crypto("RSA/ECB/PKCS1Padding", encryptionBlockSize, decryptionBlockSize);

String text = "Sample text";
String encryptedData = crypto.encrypt(text, key, false);
String decryptedData = crypto.decrypt(encryptedData, key, false);

Download

Add dependency to your app build.gradle file:

compile 'com.yakivmospan:scytale:1.0.1'

Minimum supported API version is 8.

License

Copyright 2016 Yakiv Mospan

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.