package main

import (
	"crypto/hmac"
	"crypto/hpke"
	"crypto/sha256"
	"encoding/binary"
	"encoding/hex"
	"log"
	"net"
	"os"
	"os/exec"
	"sync"
	"time"

	"baalvpn/shared"

	"github.com/songgao/water"
)

// TODO: parse some sort of config+key file
const (
	SERVER_IP  = "172.20.12.47"
	IFACE_NAME = "baalvpn"
)

var (
	iface        *water.Interface
	conn         *net.UDPConn
	serverAddr   *net.UDPAddr
	internalIP   net.IP
	privKey      hpke.PrivateKey
	multicastKey []byte
	challengeCh  = make(chan []byte, 1)
	authSecret   []byte

	establishMutex    sync.Mutex
	peerPubKeys       = shared.NewTMap[string, hpke.PublicKey]()
	peerSessionKeys   = shared.NewTMap[string, []byte]()
	peerEstablishAcks = shared.NewTMap[string, bool]()
)

func main() {
	if os.Geteuid() != 0 {
		panic("root permissions needed")
	}

	listenAddr, err := net.ResolveUDPAddr("udp", ":38000")
	if err != nil {
		panic(err)
	}
	conn, err = net.ListenUDP("udp", listenAddr)
	if err != nil {
		panic(err)
	}
	defer conn.Close()

	serverAddr, err = net.ResolveUDPAddr("udp", SERVER_IP+":38000")
	if err != nil {
		panic(err)
	}

	go receivePackets()
	register()

	<-make(chan int) // block forever
}

func register() {
	var err error
	privKey, err = hpke.MLKEM768X25519().GenerateKey()
	if err != nil {
		panic(err)
	}
	pubKey := privKey.PublicKey()

	// TODO: all this probably should be retried until we get RESP_REGISTER

	log.Println("requesting register challenge")
	send(shared.BuildPkt(shared.REQ_GET_CHALLENGE, pubKey.Bytes()))

	challenge := <-challengeCh
	log.Println("got register challenge")

	authSecret, err = hpke.Open(privKey, hpke.HKDFSHA256(), hpke.ChaCha20Poly1305(), []byte("baalvpn-challenge"), challenge)
	if err != nil {
		panic(err)
	}

	log.Println("registering")
	mac := hmac.New(sha256.New, authSecret)
	mac.Write(pubKey.Bytes())
	send(shared.BuildPkt(shared.REQ_REGISTER, mac.Sum(nil), pubKey.Bytes()))
}

func receivePackets() {
	buffer := make([]byte, 50000)
	for {
		n, addr, err := conn.ReadFromUDP(buffer[:])
		if err != nil {
			log.Println(err)
			continue
		}
		if n == 0 {
			continue
		}
		if addr.String() != serverAddr.String() {
			log.Println("non-server connection rejected")
			continue
		}

		req := make([]byte, n)
		copy(req, buffer[:n])
		go handleIncomingPkt(req)
	}
}

func handleIncomingPkt(pkt []byte) {
	defer func() {
		if r := recover(); r != nil {
			log.Printf("recovered from panic while handling a packet: %v\n", r)
		}
	}()

	if len(pkt) < 4 {
		log.Println("packet too short")
		return
	}
	version := binary.LittleEndian.Uint16(pkt[0:])
	if version != shared.PROTO_VERSION {
		log.Println("mismatched packet version")
		return
	}

	pktType := binary.LittleEndian.Uint16(pkt[2:])

	switch pktType {
	case shared.RESP_GET_CHALLENGE:
		challengeCh <- pkt[4:]
	case shared.RESP_REGISTER:
		internalIP = net.IP(pkt[4:20])

		var err error
		multicastKey, err = hpke.Open(privKey, hpke.HKDFSHA256(), hpke.ChaCha20Poly1305(), []byte("baalvpn-register"), pkt[20:])
		if err != nil {
			panic(err)
		}

		log.Println("got assigned IP by the server:", internalIP.String())
		setupInterface()
		go relayPackets()
	case shared.ENC_PKT:
		srcIP := net.IP(pkt[52:68]).String()

		sessionKey := getOrEstablishSessionKey(srcIP)

		decryptedPkt, err := DecryptSym(sessionKey, pkt[68:])
		if err != nil {
			panic(err)
		}

		log.Println("received pkt:", hex.EncodeToString(decryptedPkt))
		if _, err := iface.Write(decryptedPkt); err != nil {
			log.Println(err)
		}
	case shared.BROADCAST_PKT:
		decryptedPkt, err := DecryptSym(multicastKey, pkt[36:])
		if err != nil {
			panic(err)
		}
		log.Println("received broadcast:", hex.EncodeToString(decryptedPkt))
		if _, err := iface.Write(decryptedPkt); err != nil {
			log.Println(err)
		}
	case shared.RESP_GET_PUBKEY:
		respIP := net.IP(pkt[4:20]).String()
		pubKey, err := hpke.MLKEM768X25519().NewPublicKey(pkt[20:])
		if err != nil {
			panic(err)
		}
		peerPubKeys.Set(respIP, pubKey)
	case shared.REQ_ESTABLISH:
		srcIP := net.IP(pkt[52:68]).String()
		ciphertext := pkt[68:]

		r, err := hpke.NewRecipient(ciphertext, privKey, hpke.HKDFSHA256(), hpke.ExportOnly(), nil)
		if err != nil {
			panic(err)
		}
		sessionKey, err := r.Export("baalvpn-establish", 32)
		if err != nil {
			panic(err)
		}

		log.Println("received session key from", srcIP)
		peerSessionKeys.Set(srcIP, sessionKey)

		send(buildAuthPkt(shared.RESP_ESTABLISH, net.ParseIP(srcIP), internalIP))
	case shared.RESP_ESTABLISH:
		srcIP := net.IP(pkt[52:68]).String()
		log.Println("established session key with", srcIP)
		peerEstablishAcks.Set(srcIP, true)
	default:
		log.Println("unknown packet type")
	}
}

func getOrEstablishSessionKey(ip string) []byte {
	if key, ok := peerSessionKeys.GetOK(ip); ok {
		return key
	}

	establishMutex.Lock()
	defer establishMutex.Unlock()

	// check again after the other goroutine finished
	if key, ok := peerSessionKeys.GetOK(ip); ok {
		return key
	}

	// TODO: this definitely shouldnt block the main thread
	// TODO: this should try like 10 times tops since server doesnt respond at all if it doesnt have the pubkey
	for {
		// request pubkey every 50ms
		time.Sleep(50 * time.Millisecond)
		peerPubKey, ok := peerPubKeys.GetOK(ip)
		if !ok {
			send(shared.BuildPkt(shared.REQ_GET_PUBKEY, net.ParseIP(ip)))
			continue
		}

		log.Println("got pubkey of " + ip)

		ciphertext, sender, err := hpke.NewSender(peerPubKey, hpke.HKDFSHA256(), hpke.ExportOnly(), nil)
		if err != nil {
			panic(err)
		}
		sessionKey, err := sender.Export("baalvpn-establish", 32)
		if err != nil {
			panic(err)
		}

		send(buildAuthPkt(shared.REQ_ESTABLISH, net.ParseIP(ip), internalIP, ciphertext))

		for {
			// TODO: eww
			time.Sleep(50 * time.Millisecond)
			if peerEstablishAcks.Get(ip) {
				peerEstablishAcks.Delete(ip)
				break
			}
		}

		peerSessionKeys.Set(ip, sessionKey)
		return sessionKey
	}
}

func setupInterface() {
	config := water.Config{DeviceType: water.TUN}
	config.Name = IFACE_NAME

	var err error
	iface, err = water.New(config)
	if err != nil {
		panic(err)
	}

	if err := exec.Command("ip", "addr", "add", internalIP.String()+"/32", "dev", IFACE_NAME).Run(); err != nil {
		panic(err)
	}
	if err := exec.Command("ip", "link", "set", "dev", IFACE_NAME, "up").Run(); err != nil {
		panic(err)
	}
}

func relayPackets() {
	log.Println("Listening for packets...")

	for {
		pkt := make([]byte, 2000)
		n, err := iface.Read([]byte(pkt))
		if err != nil {
			panic(err)
		}
		pkt = pkt[:n]

		version := pkt[0] >> 4
		if version != 6 {
			continue
		}

		destIP := net.IP(pkt[24:40]).String()

		if pkt[24] == 0xff { // multicast
			encryptedPkt, err := EncryptSym(multicastKey, pkt)
			if err != nil {
				panic(err)
			}

			send(buildAuthPkt(shared.BROADCAST_PKT, encryptedPkt))
		} else {
			destSessionKey := getOrEstablishSessionKey(destIP)

			encryptedPkt, err := EncryptSym(destSessionKey, pkt)
			if err != nil {
				panic(err)
			}

			send(buildAuthPkt(shared.ENC_PKT, net.ParseIP(destIP), internalIP, encryptedPkt))
		}
	}
}

func buildAuthPkt(pktType uint16, parts ...[]byte) []byte {
	pkt := shared.BuildPkt(pktType, parts...)
	mac := hmac.New(sha256.New, authSecret)
	mac.Write(pkt[4:])
	return append(pkt[:4], append(mac.Sum(nil), pkt[4:]...)...)
}

func send(req []byte) {
	if _, err := conn.WriteToUDP(req, serverAddr); err != nil {
		panic(err)
	}
}