Decryptor.kt

class Decryptor {
    private var keyStore: KeyStore? = null

    init {
        initKeyStore()
    }

    @Throws(
        KeyStoreException::class,
        CertificateException::class,
        NoSuchAlgorithmException::class,
        IOException::class
    )
    private fun initKeyStore() {
        keyStore = KeyStore.getInstance(ANDROID_KEY_STORE)
        keyStore?.load(null)
    }

    @Throws(
        UnrecoverableEntryException::class,
        NoSuchAlgorithmException::class,
        KeyStoreException::class,
        NoSuchProviderException::class,
        NoSuchPaddingException::class,
        InvalidKeyException::class,
        IOException::class,
        BadPaddingException::class,
        IllegalBlockSizeException::class,
        InvalidAlgorithmParameterException::class
    )
    fun decryptData(alias: String, encryptedData: ByteArray?, encryptionIv: ByteArray?): String {
        val cipher: Cipher = Cipher.getInstance(TRANSFORMATION)
        val spec = GCMParameterSpec(128, encryptionIv)
        cipher.init(Cipher.DECRYPT_MODE, getSecretKey(alias), spec)
        return String(cipher.doFinal(encryptedData))
    }

    @Throws(
        NoSuchAlgorithmException::class,
        UnrecoverableEntryException::class,
        KeyStoreException::class
    )
    private fun getSecretKey(alias: String): SecretKey {
        return (keyStore?.getEntry(alias, null) as KeyStore.SecretKeyEntry).getSecretKey()
    }

    companion object {
        private const val TRANSFORMATION = "AES/GCM/NoPadding"
        private const val ANDROID_KEY_STORE = "AndroidKeyStore"
    }
}