wg-app-link/app/src/main/kotlin/com/example/wgapplink/ServerConfig.kt

package com.example.wgapplink

import android.content.Context
import android.net.Uri
import android.security.keystore.KeyGenParameterSpec
import android.security.keystore.KeyProperties
import android.util.Base64
import androidx.core.content.edit
import java.security.KeyStore
import javax.crypto.Cipher
import javax.crypto.KeyGenerator
import javax.crypto.SecretKey
import javax.crypto.spec.GCMParameterSpec

/**
 * Where a backend is and how to authenticate to it. Absent until the phone is enrolled, which
 * happens by scanning the QR the server prints to its terminal.
 */
data class ServerSettings(val host: String, val port: Int, val token: String) {
    val baseUrl: String
        get() = "https://$host:$port"
}

/**
 * Reads and writes one app's enrollment.
 *
 * Two things are per-app and both are passed in rather than derived, because getting either wrong
 * is silent rather than loud:
 * - [scheme] is what routes a scanned QR back to the right app (`aiapp`, `devupdater`). Two apps
 *   accepting the same scheme would each offer to handle the other's enrollment.
 * - [keyAlias] names the Android Keystore key the token is sealed under. It is **persisted**, so an
 *   app that changes it stops being able to unseal what it already stored, and the phone silently
 *   reads as not enrolled. Existing values must be carried over exactly when adopting this class.
 */
class ServerStore(private val scheme: String, private val keyAlias: String) {

    fun load(context: Context): ServerSettings? {
        val prefs = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE)
        val host = prefs.getString(KEY_HOST, null) ?: return null
        val port = prefs.getInt(KEY_PORT, 0)
        val sealed = prefs.getString(KEY_TOKEN, null) ?: return null
        val token = unseal(sealed) ?: return null
        if (port == 0 || token.isEmpty()) return null
        return ServerSettings(host, port, token)
    }

    fun save(context: Context, settings: ServerSettings) {
        context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE).edit {
            putString(KEY_HOST, settings.host)
            putInt(KEY_PORT, settings.port)
            putString(KEY_TOKEN, seal(settings.token))
        }
    }

    /**
     * Parses the enrollment URI the QR carries, e.g.
     * `aiapp://enroll?host=10.66.0.1&port=8443&token=...`.
     *
     * Null if any part is missing, so a malformed or foreign scan cannot clobber a working
     * enrollment with a half-filled one.
     */
    fun parseEnrollmentUri(uri: Uri): ServerSettings? {
        if (uri.scheme != scheme || uri.host != "enroll") return null
        val host = uri.getQueryParameter("host") ?: return null
        val port = uri.getQueryParameter("port")?.toIntOrNull() ?: return null
        val token = uri.getQueryParameter("token") ?: return null
        if (host.isEmpty() || token.isEmpty()) return null
        return ServerSettings(host, port, token)
    }

    // -----------------------------------------------------------------------
    // Token sealing. The token authenticates requests to a server whose API is
    // remote code execution, so it is stored AES-GCM-encrypted under an Android
    // Keystore key (hardware-backed where the device has one) rather than in
    // plain preferences. Hand-rolled rather than Jetpack's
    // EncryptedSharedPreferences, which is deprecated with no drop-in
    // successor -- Google's own guidance is now to use Keystore directly.

    /**
     * The key if there is one, without making one.
     *
     * The read side must never create. A sealed token with no key behind it means the key was lost
     * -- a device reset, or the app's data restored onto another device, where Keystore keys do not
     * travel -- and generating a fresh one there would leave a key nothing had ever sealed with and
     * still fail to decrypt. Absent is the honest answer, and the caller reads it as "not
     * enrolled".
     */
    private fun existingTokenKey(): SecretKey? {
        val keyStore = KeyStore.getInstance(KEYSTORE).apply { load(null) }
        return keyStore.getKey(keyAlias, null) as? SecretKey
    }

    private fun tokenKey(): SecretKey {
        existingTokenKey()?.let {
            return it
        }
        val generator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, KEYSTORE)
        generator.init(
            KeyGenParameterSpec.Builder(
                    keyAlias,
                    KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT,
                )
                .setBlockModes(KeyProperties.BLOCK_MODE_GCM)
                .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
                .build()
        )
        return generator.generateKey()
    }

    /** `iv:ciphertext`, both base64 -- the stored form of the token. */
    private fun seal(token: String): String {
        val cipher = Cipher.getInstance(TRANSFORMATION)
        cipher.init(Cipher.ENCRYPT_MODE, tokenKey())
        val ciphertext = cipher.doFinal(token.encodeToByteArray())
        return Base64.encodeToString(cipher.iv, Base64.NO_WRAP) +
            ":" +
            Base64.encodeToString(ciphertext, Base64.NO_WRAP)
    }

    /**
     * Null on any failure, including the lost-key case above. The caller treats that as "not
     * enrolled"; re-scanning the QR, or rotating the token server-side, is the recovery -- so
     * failing soft here is right rather than lenient.
     */
    private fun unseal(sealed: String): String? =
        try {
            val (ivB64, dataB64) =
                sealed.split(":", limit = 2).let {
                    if (it.size != 2) return null else it[0] to it[1]
                }
            val cipher = Cipher.getInstance(TRANSFORMATION)
            cipher.init(
                Cipher.DECRYPT_MODE,
                existingTokenKey() ?: return null,
                GCMParameterSpec(GCM_TAG_BITS, Base64.decode(ivB64, Base64.NO_WRAP)),
            )
            cipher.doFinal(Base64.decode(dataB64, Base64.NO_WRAP)).decodeToString()
        } catch (_: Exception) {
            null
        }
}

/**
 * Whether this app may reach a local network address at all.
 *
 * Android 17 made `ACCESS_LOCAL_NETWORK` mandatory for it, and a denial is invisible at the socket:
 * the OS drops the traffic, so a blocked app and an unreachable server produce the same connect
 * timeout. Without asking explicitly there is no way to tell those apart, and the failure shown
 * would blame the server or the tunnel for something neither is doing.
 *
 * **The permission is still required when the server is reached through WireGuard, and the
 * platform's own documentation says otherwise.** Android's Local Network Definition describes a
 * local network as one that "utilizes a broadcast-capable network interface, such as Wi-Fi or
 * Ethernet, but excludes cellular (WWAN) or VPN connections" — read straight, a tunnelled 10.66.0.1
 * is excluded and needs nothing. Measured on a real device on 2026-08-28: it is not excluded, and
 * without the permission the traffic is dropped. Do not remove the permission on the strength of
 * that paragraph.
 *
 * **Neither project can catch this in an emulator.** The API 36 images both are tested against do
 * not enforce the permission at all, so removing it passes every local test and fails only on a
 * phone. That asymmetry is the reason this note is here rather than in a commit message.
 */
fun localNetworkAllowed(context: Context): Boolean =
    android.os.Build.VERSION.SDK_INT < 37 ||
        context.checkSelfPermission("android.permission.ACCESS_LOCAL_NETWORK") ==
            android.content.pm.PackageManager.PERMISSION_GRANTED

// Both apps store under the same preferences name, which does not collide:
// SharedPreferences are per-application, so "server" means this app's server.
private const val PREFS_NAME = "server"
private const val KEY_HOST = "host"
private const val KEY_PORT = "port"
private const val KEY_TOKEN = "token"

private const val KEYSTORE = "AndroidKeyStore"
private const val TRANSFORMATION = "AES/GCM/NoPadding"
private const val GCM_TAG_BITS = 128