encryption_with_preference2.kt

fun savePrivateString(key: String, value: String) {
  prefs.edit().putString(key, encrypt(key, value)).apply()
}

fun getPrivateString(key: String): String {
  val encrypted = prefs.getString(key, "")
  return decrypt(key, encrypted)
}

private fun encrypt(alias: String, plain: String): String {
  val cipher = initializeCipher(alias, Cipher.ENCRYPT_MODE)
  val encrypted = cipher.doFinal(plain.toByteArray()) ?: byteArrayOf()  // TODO catch exception
  return Base64.encodeToString(encrypted, Base64.DEFAULT)
}

private fun decrypt(alias: String, encrypted: String): String {
  val cipher = initializeCipher(alias, Cipher.DECRYPT_MODE)
  val decrypted = cipher.doFinal(plain.toByteArray()) ?: byteArrayOf()  // TODO catch exception
  return String(decrypted)
}

private fun initializeCipher(alias: String, mode: Int) = Cipher.getInstance("$ALGORITHM/$MODE/$PADDING").apply {
  val iv = BuildConfig.CIPHER_IV  // InitializationVector. 16bytes string.
  val secretKeySpec = buildSecretKey(alias)
  val ivSpec = IvParameterSpec(iv.toByteArray())
  init(mode, secretKeySpec, ivSpec)
}

private fun buildSecretKey(alias: String): SecretKey {
  val keystore = KeyStore.getInstance("AndroidKeyStore").apply { load(null) }
  return if (keystore.containsAlias(alias)) {  // use saved secretKey 
		val entry = (keystore.getEntry(keyAlias, null) as KeyStore.SecretKeyEntry)
	  entry.secretKey
	} else {  // generate new secretKey and save it into AndroidKeyStore
	  val keyGenerator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES,  "AndroidKeyStore")
	  keyGenerator.init(KeyGenParameterSpec.Builder(alias, KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT)
			    .setBlockModes(KeyProperties.BLOCK_MODE_CBC)
			    .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_PKCS7)
					.build())
		return keyGenerator.generateKey()
	}
}