Android Decryptor (Helper class)

Decryptor.kt

class Decryptor(private val storage: SharedPreferences) {
    private val ANDROID_KEY_STORE = "AndroidKeyStore"
    private val TRANSFORMATION = "AES/GCM/NoPadding"

    private var keyStore: KeyStore? = null

    init {
        initKeyStore()
    }

    @Throws(Exception::class)
    private fun initKeyStore() {
        keyStore = KeyStore.getInstance(ANDROID_KEY_STORE)
        keyStore!!.load(null)
    }

    @Throws(Exception::class)
    fun decryptDataWithAES(alias: String): String {
        var alias = alias
        alias = alias + "_AES"

        val base64InitVector = storage.getString(alias + "_initVector", null)
        val base64Encryption = storage.getString(alias + "_encryption", null)

        if (TextUtils.isEmpty(base64InitVector) || TextUtils.isEmpty(base64Encryption))
            throw NullPointerException("Not found initialization vector or encryption data for specified alias")

        val encryptionIv = Base64.decode(base64InitVector, Base64.NO_WRAP)
        val encryptedData = Base64.decode(base64Encryption, Base64.NO_WRAP)

        Log.d("AndroidKeyStore", "encrypted data: " + Arrays.toString(encryptedData) + " encrypted iv: "
                + Arrays.toString(encryptionIv))
        val cipher = Cipher.getInstance(TRANSFORMATION)
        val spec = GCMParameterSpec(128, encryptionIv)
        cipher.init(Cipher.DECRYPT_MODE, getSecretKey(alias), spec)

        return String(cipher.doFinal(encryptedData), StandardCharsets.UTF_8)
    }

    fun decryptDataWithRSA(alias: String): String {
        var alias = alias
        alias = alias + "_RSA"
        val base64PublicKey = storage.getString(alias + "_publicKey", null)
        val base64Encryption = storage.getString(alias + "_encryption", null)

        val keyBytes = Base64.decode(base64PublicKey, Base64.NO_WRAP)
        val encryption = Base64.decode(base64Encryption, Base64.NO_WRAP)
        val spec = X509EncodedKeySpec(keyBytes)

        var decodedBytes = ByteArray(0)
        try {
            val keyFactory = KeyFactory.getInstance(KeyProperties.KEY_ALGORITHM_RSA)
            val key = keyFactory.generatePublic(spec)
            val cipher = Cipher.getInstance(KeyProperties.KEY_ALGORITHM_RSA)
            cipher.init(Cipher.DECRYPT_MODE, key)
            decodedBytes = cipher.doFinal(encryption)
        } catch (e: Exception) {
            e.printStackTrace()
        }

        return String(decodedBytes, StandardCharsets.UTF_8)
    }

    @Throws(Exception::class)
    private fun getSecretKey(alias: String): SecretKey {
        return (keyStore!!.getEntry(alias, null) as KeyStore.SecretKeyEntry).getSecretKey()
    }
}