The real PAN
never touches the phone.

class LukStore(
    private val context: Context,
    private val wrapKeyAlias: String
) {
    private val lukDir = File(context.filesDir, "luks").also { it.mkdirs() }

    fun storeLuk(index: Int, lukBytes: ByteArray) {
        val ks = KeyStore.getInstance("AndroidKeyStore")
        ks.load(null)
        val key = ks.getKey(wrapKeyAlias, null) as SecretKey

        val cipher = Cipher.getInstance("AES/GCM/NoPadding")
        cipher.init(Cipher.ENCRYPT_MODE, key)

        val encrypted = cipher.doFinal(lukBytes)
        val iv = cipher.iv

        // Store IV + ciphertext together
        val file = File(lukDir, "luk_$index.enc")
        file.writeBytes(iv + encrypted)
    }

    fun popNextLuk(): ByteArray? {
        val files = lukDir.listFiles()
            ?.filter { it.name.startsWith("luk_") }
            ?.sortedBy { it.name }
            ?: return null

        val file = files.firstOrNull() ?: return null
        val raw = file.readBytes()

        // First 12 bytes = GCM IV, rest = ciphertext
        val iv = raw.copyOfRange(0, 12)
        val ciphertext = raw.copyOfRange(12, raw.size)

        val ks = KeyStore.getInstance("AndroidKeyStore")
        ks.load(null)
        val key = ks.getKey(wrapKeyAlias, null) as SecretKey

        val cipher = Cipher.getInstance("AES/GCM/NoPadding")
        cipher.init(Cipher.DECRYPT_MODE, key, GCMParameterSpec(128, iv))
        val luk = cipher.doFinal(ciphertext)

        // Delete after retrieval — LUK is single-use
        file.delete()
        return luk
    }

    fun remaining(): Int =
        lukDir.listFiles()?.count { it.name.startsWith("luk_") } ?: 0

    fun needsReplenishment(threshold: Int = 5): Boolean =
        remaining() < threshold
}