SystemServices.kt

val message = "Very large message, bigger then 250 symblos..."

// Simple Shared Preferences wrapper, will be used to save wrapped key
val storage = Storage(context)

// Creates Android Key Store and provides manage functions
val keyStoreWrapper = KeyStoreWrapper(context)

// Running M and later, use one symmetric key
if (SystemServices.hasMarshmallow()) {

  // Create and Save asymmetric key
  keyStoreWrapper.createAndroidKeyStoreSymmetricKey("MASTER_KEY")
    
  // Get key from keyStore
  val masterKey = keyStoreWrapper.getAndroidKeyStoreSymmetricKey("MASTER_KEY")

  // Creates Cipher with symmetric transformation and provides encrypt and decrypt functions
  val cipher = CipherWrapper("AES/CBC/PKCS7Padding")
    
  // Encrypt message  
  val encryptedMessage = cipher.encrypt(message, masterKey)
  
  // Decrypt message  
  val decryptedMessage = cipher.decrypt(encryptedMessage, masterKey)

  // Ooops, InvalidKeyException: no IV set when one expected
} 

// Before M, use two, asymmetric and symmetric, keys
else{

  // Creates Cipher with asymmetric transformation and provides wrap and unwrap functions
  val cipherForWrapping = CipherWrapper("RSA/ECB/PKCS1Padding")
    
  // Creates Cipher with symmetric transformation and provides encrypt and decrypt functions
  val cipherForEncryption = CipherWrapper("AES/CBC/PKCS7Padding")

  // ---------------- Create Keys
    
  // Create AES BC provider key  
  val symmetricKey = keyStoreWrapper.generateDefaultSymmetricKey()
  
  // Create RSA AndroidKeyStore Provider key and save it into keystore
  val masterKey = keyStoreWrapper.createAndroidKeyStoreAsymmetricKey(MASTER_KEY)

  // Wrap AES Secret key with RSA Public key 
  val encryptedSymmetricKey = cipherForWrapping.wrapKey(symmetricKey, masterKey.public)
    
  // And save it to Shared Preferences  
  storage.saveEncryptionKey(encryptedSymmetricKey)
    
  //----------------- Encrypt / Decrypt with keys
  
  // Get RSA master key from Android Key Store
  masterKey = keyStoreWrapper.getAndroidKeyStoreAsymmetricKeyPair("MASTER_KEY")

  // Get AES wrapped raw data from preferences
  val encryptionKey = storage.getEncryptionKey()

  // Unwrap AES key data with RSA Private key
  symmetricKey = cipherForWrapping.unWrapKey(encryptionKey, ALGORITHM_AES, Cipher.SECRET_KEY, masterKey?.private) as SecretKey
  
  // Encrypt message with AES Secret key
  val encryptedMessage = cipherForEncryption.encrypt(message, symmetricKey)
  
  // Encrypt message with AES Secret key
  val decryptedMessage = cipherForEncryption.decrypt(encryptedMessage, symmetricKey)

  // Ooops, InvalidKeyException: no IV set when one expected
}