package server

import (
	"bytes"
	"encoding/binary"
	"errors"
	"fmt"
	"time"

	"github.com/cbeuw/Cloak/internal/common"

	log "github.com/sirupsen/logrus"
)

type ClientInfo struct {
	UID              []byte
	SessionId        uint32
	ProxyMethod      string
	EncryptionMethod byte
	Unordered        bool
	Transport        Transport
}

type authFragments struct {
	sharedSecret      [32]byte
	randPubKey        [32]byte
	ciphertextWithTag [64]byte
}

const (
	UNORDERED_FLAG = 0x01 // 0000 0001
)

var ErrTimestampOutOfWindow = errors.New("timestamp is outside of the accepting window")

// decryptClientInfo checks if a the authFragments are valid. It doesn't check if the UID is authorised
func decryptClientInfo(fragments authFragments, serverTime time.Time) (info ClientInfo, err error) {
	var plaintext []byte
	plaintext, err = common.AESGCMDecrypt(fragments.randPubKey[0:12], fragments.sharedSecret[:], fragments.ciphertextWithTag[:])
	if err != nil {
		return
	}

	info = ClientInfo{
		UID:              plaintext[0:16],
		SessionId:        0,
		ProxyMethod:      string(bytes.Trim(plaintext[16:28], "\x00")),
		EncryptionMethod: plaintext[28],
		Unordered:        plaintext[41]&UNORDERED_FLAG != 0,
	}

	timestamp := int64(binary.BigEndian.Uint64(plaintext[29:37]))
	clientTime := time.Unix(timestamp, 0)
	if !(clientTime.After(serverTime.Add(-timestampTolerance)) && clientTime.Before(serverTime.Add(timestampTolerance))) {
		err = fmt.Errorf("%v: received timestamp %v", ErrTimestampOutOfWindow, timestamp)
		return
	}
	info.SessionId = binary.BigEndian.Uint32(plaintext[37:41])
	return
}

var ErrReplay = errors.New("duplicate random")
var ErrBadProxyMethod = errors.New("invalid proxy method")
var ErrBadDecryption = errors.New("decryption/authentication faliure")

// AuthFirstPacket checks if the first packet of data is ClientHello or HTTP GET, and checks if it was from a Cloak client
// if it is from a Cloak client, it returns the ClientInfo with the decrypted fields. It doesn't check if the user
// is authorised. It also returns a finisher callback function to be called when the caller wishes to proceed with
// the handshake
func AuthFirstPacket(firstPacket []byte, transport Transport, sta *State) (info ClientInfo, finisher Responder, err error) {
	fragments, finisher, err := transport.processFirstPacket(firstPacket, sta.StaticPv)
	if err != nil {
		return
	}

	if sta.registerRandom(fragments.randPubKey) {
		err = ErrReplay
		return
	}

	info, err = decryptClientInfo(fragments, sta.WorldState.Now().UTC())
	if err != nil {
		log.Debug(err)
		err = fmt.Errorf("%w: %v", ErrBadDecryption, err)
		return
	}
	info.Transport = transport
	return
}