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Encryption in Android Using PubNub Kotlin SDK

9 min read • Chandler Mayo on May 12, 2021
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Overview

Encryption or more precisely End-To-End Encryption is a system of communication where only specified / communicating users can read the messages. To achieve this, the messages that are sent are encrypted on the user device and can only be decrypted on the end-user device. It’s important to encrypt your sensitive data due to existing law, like HIPAA (The Health Insurance Portability and Accountability Act) for example.

Due to the fact that each use case is different, we should consciously decide for ourselves which data to encrypt and which not. Thanks to this, we will be sure that our implementations are safe. We should also create our own data encryption service, or use generally available which we can easily validate (open source). Let's try not to use libraries whose code we can't see - we won't know what's going on with our data underneath.

Sensitive Data

Before we begin, you need to answer an important question - which data in your application are sensitive. Why? The design philosophy is to encrypt the important personal data in the message body but still put metadata (or other non-private data) in the meta field to take full advantage of PubNub. Let's take a look at an example:

In the example above, we used metadata with user ID, that is not encrypted. Thanks to this, we can use them, for example, when filtering messages to prevent receiving your own messages.

Setting a filter applies to all channels that you will subscribe to from that particular client. This client filter excludes messages that have this subscriber's UUID set at the sender's UUID.

Goals

  1. Implement Encryption layer above PubNub SDK which allows to encrypt and decrypt messages.

  2. Encrypt outgoing and decrypt incoming messages.

  3. Force encryption for SQL databases to store secured messages.

  4. Implement server-side key repository to allow key changes.

Implementations

The minimum Android API is 23 (Android Marshmallow). As a support library we’ll use

Looking for a solution for API 16+? Please check Legacy Implementation.

Repository Interface Definition

First of all, let’s create a KeyRepository interface and define its methods. Which abstract methods should we declare? Let’s think about a functionality we need to implement:

  • add key,

  • remove key,

  • get single key,

  • get all keys,

  • delete all keys.

So it’s like a CRUD, but without an update - we don’t want to modify a key.

SecretKey - A secret (symmetric) key. Keys that implement this interface return the string RAW as their encoding format (see getFormat), and return the raw key bytes as the result of a getEncoded method call.

https://developer.android.com/reference/javax/crypto/SecretKey?hl=en

As you can see we used a filterName in a getKeys and clear definition. It’ll be used to filter keys by name later.

To be sure we are passing a valid algorithm the typealias was created.

The interface is done. What kind of implementation should we make on Android? Right now there are two possibilities, based on storage:

  • KeyStore, 

  • EncryptedSharedPreferences.

The main difference between those two is that you’re not able to get a key material from KeyStore once it is stored. It means that all the encryption logic is done on the Android side, but you cannot get a byte array of your key. 

Why is it important? Because some implementations, like SQLCipher, need a material of the key. We’ll see it later.

Keystore Implementation

Before we start implementing a KeyStore repository we need to check which algorithms are available on Android - SupportedAlgorithms

Available Algorithms

Algorithms contain algorithm name, block mode and padding splitted by slash character (for example AES/CBC/NoPadding, see KeyProperties).

Algorithm names:

Mode is the set of block modes with which the key can be used when encrypting/decrypting. Attempts to use the key with any other block modes will be rejected. Possible values:

Padding is the set of padding schemes with which the key can be used when encrypting/decrypting. Attempts to use the key with any other padding scheme will be rejected. Possible values:

Interface Implementation

At the beginning we need to declare a class, which implements the KeyRepository interface. Since our minimum version is API 23 we need to annotate the class with @RequiresApi. To easily operate on keystore we will initialize an object with AndroidKeyStore instance.

Next step will be to override the add method from the interface. We will create a SecretKey instance from a passed algorithm and byte array, and store it in a key store. To do it we need to create a KeyProtection object and define key purpose, block modes and encryption paddings. As a result we will return the previously created SecretKey.

Both delete and clear methods are quite easy to implement. For a delete we will just call deleteEntry with a key name. 

For removing all keys whose names match the passed filter we need to iterate through stored keys, filter it and remove it using the previously created delete method.

Getting a key is done by calling getKey with its name on the key store. The result is casted securely to the SecretKey instance.

Getting a key list which names matching a passed filter we will iterate over aliases and map result names to keys - getKey method will be used.

And that's all. We’ve got the implementation based on the Android key store.

Encrypted Shared Preferences Implementation

The main difference between storing a key in KeyStore or in the Shared Preferences is that with Preferences we are able to get a key material. It’ll be useful when trying to use SQLCipher extensions to secure a local database.

As in the previous example, let’s create a class which implements the KeyRepository interface. To use SharedPreferences we’ll need to have an application Context, so we’ll declare it as a class parameter. We also need to add preferences file name and prepare a variable to store an instance.

Note: EncryptedSharedPreferences takes a long time to initialize/open, so we’ll do it only once and keep an instance.

But how to make encrypted preferences? The Google latest security library will help us with this, so let’s import it:

To initialize the preferences we need to pass context, previously declared PREFERENCES_NAME, key alias, which will be used to encrypt a file and encryption schemes. 

We used MasterKey.Builder which will take the default key alias. It will create a new key or take current one, If the key already exists. Preference keys will be encrypted deterministically with AES256-SIV-CMAC, values with AES256-GCM.

Before we start overriding interface methods we will create some helpers to store and get keys. The first step will be to declare an Gson instance, create a data class to store keys and extension to map an object to a Key.

Let’s create also a helper for a Editor:

Now we can start overriding methods. Adding a new key will be quite easy - we need to map the parameters to Key data class and store it in preferences as a JSON String.

Removing a single key or keys which names matching a predicate is easy too:

Finally, getting a key needs to be overridden. We’ll get a stored String from a preferences and try to map it to Key with the previously created extension. Obtaining a list of keys which names matching a filter is done by iterating over all entities in preferences and mapping its values to keys. The list will be sorted descending by timestamp.

Encryption Service Implementation

Good job! We have implemented both repositories. Now it’s time for an Encryption Service. It will be responsible for encryption and decryption messages. We can define the interface:

Let’s create an implementation now. We’ll need to pass a previously created KeyRepository and algorithm parameters for encryption.

We recommended using following settings for Cipher transformations in KeyStore:

  • AES / CBC / NoPadding,

  • AES / CBC / PKCS7Padding,

  • AES / GCM / NoPadding

Now we will create some helper variables to storing algorithm and extracting initialization vector:

Remember, that we need to store at least one key to make it work. In our example we will use a hardcoded key, in a production it should be changed to some API authorization call.

It’s time to implement some helper methods which will allow us to get or store the key. We will use a passed KeyRepository for it.

Now you can see why we used the filter in the getKeys method - we can store keys from multiple services in our repository. We implemented the isGCM function to check what type of parameters we should pass to decryption.

Time to prepare encrypt functionality. We will take the algorithm instance, initialize it with the latest key and store the initialization vector bytes. Now, as a result we will return Base64 encoded String - initialization vector and encoded message splitted by our custom separator.

Let’s implement a decryption part. But what about the key changes? Imagine the situation when your key is revealed and you want to change it. What about previously encrypted messages? To decrypt them we will iterate through the keys (from the newest one to the oldest) and try to obtain a message. If the message cannot be decrypted with any key we will throw an exception.

And the last part - decryption with passed SecretKey. Remember that our encrypted message contains the initialization data and encrypted message, splitted by separator. We need to split those two data and decode the Base64 string into a byte array.

As you can see, we decoded an IV and a message to a byte array, get the algorithm instance and initialize it with parameters (which depends on algorithm block mode). As a result we are returning the String message.

Encrypting Messages in Pubnub

For encrypting and decrypting messages we will create a few extensions. First of all, we need to map our message object to JSON and back. To achieve this let’s create toJson and fromJson extensions:

Now to encrypt a message object we will need to pass a JSON into EncryptionService method::

For decrypt we need to map an decrypted String (JSON) into object, so we will use our fromJson extension:

Room Encryption Implementation

We have our messages encryption finished. But what about secure storage? On Android we are using SQL databases, so it should be possible to encrypt it with a key. There is a lot of information about it and implementations on the internet. We will try to show you the easiest way to achieve it. 

First of all - Google not supporting it directly. But thankfully SQLCipher has a SupportFactory which will help us with encryption. At the beginning we need to add the following imports.

Let’s create a factory which will use our KeyRepository and define a stored key name.

Warning

The repository passed as a parameter needs to be able to get a key material.

The main functionality will create a SupportFactory with the selected key. 

But how to get a key? We can get it from a repository if it exists or create and store the new one. As a result we will return a byte of array - key material.

That's the end of our implementation. How to use it with Room? Just add the instance of factory into openHelperFactory builder.

Now your database is secure with a user key.

Legacy Implementations

If you need to use encryption on lowest API devices please take a look at the following examples. Be careful - you cannot use Google’s security-crypto library, because it requires API 21+.

Keystore Implementation

The only difference between the new API and legacy implementation is that we cannot use the "AndroidKeyStore" instance - we’ll use default one instead. To do it, let’s add the open modifier into KeyStoreRepositoryImpl and create a new implementation, which uses the default keystore type.

That was simple! But we cannot use our previous key protection - it requires API 23. Let’s override the add function and use a compatible ProtectionParameter.

Now the implementation is ready, but to use it we need to change the EncryptionServiceImpl a little bit.

Shared Preferences Implementation

The main problem is that we cannot use Google’s cryptography library. We need to choose a different one, so we’ll use secure-preferences-lib. 

Please add a following import into build.gradle:

Next step is to add the open modifier into KeyPreferenceRepositoryImpl and create a new implementation, which extends it. We’ll define a file name for preferences too.

As you can see, the previous implementation is based on SharedPreferences. In the current solution, we’ll reuse it and replace the preferences object. To achieve this, let’s override initPreferences.

Be careful. Please check how SecurePreferences works. With our example we’re not using a password to secure it.

That’s all. Please check the next step.

Encryption Service Implementation

That’s the last step in our legacy implementation. We need to replace all the KeyProperties usage, which are added in API 23, starting with constructor:

We’ll need to rewrite the isGCM function too. We’ll replace KeyProperties with raw string and add sdk version check - GCMParameterSpec was added in API 21, so we’ll use IvParameterSpec for lower APIs.

That’s all. We’ve got encryption legacy implementations, which works from API 16+. Good job!

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