package ssh

import (
	"crypto"
	"crypto/rand"
	"encoding/binary"
	"encoding/json"
	"fmt"
	"io"
	"math/big"

	ztoolsKeys "github.com/zmap/zgrab2/tools/keys"
)

const (
	kexAlgoDHGEXSHA1   = "diffie-hellman-group-exchange-sha1"
	kexAlgoDHGEXSHA256 = "diffie-hellman-group-exchange-sha256"
)

// Messages
type kexDHGexGroupMsg struct {
	P *big.Int `sshtype:"31"`
	G *big.Int
}

type kexDHGexInitMsg struct {
	X *big.Int `sshtype:"32"`
}

type kexDHGexReplyMsg struct {
	HostKey   []byte `sshtype:"33"`
	Y         *big.Int
	Signature []byte
}

type kexDHGexRequestMsg struct {
	MinBits      uint32 `sshtype:"34"`
	PreferedBits uint32
	MaxBits      uint32
}

type gexJsonLog struct {
	Parameters      *ztoolsKeys.DHParams  `json:"dh_params,omitempty"`
	ServerSignature *JsonSignature        `json:"server_signature,omitempty"`
	ServerHostKey   *ServerHostKeyJsonLog `json:"server_host_key,omitempty"`
}

type dhGEXSHA struct {
	g, p     *big.Int
	hashFunc crypto.Hash
	JsonLog  *gexJsonLog
}

func (gex *dhGEXSHA) GetNew(keyType string) kexAlgorithm {
	switch keyType {
	case kexAlgoDHGEXSHA1:
		ret := new(dhGEXSHA)
		ret.hashFunc = crypto.SHA1
		ret.JsonLog = new(gexJsonLog)
		ret.JsonLog.Parameters = new(ztoolsKeys.DHParams)
		return ret

	case kexAlgoDHGEXSHA256:
		ret := new(dhGEXSHA)
		ret.hashFunc = crypto.SHA256
		ret.JsonLog = new(gexJsonLog)
		ret.JsonLog.Parameters = new(ztoolsKeys.DHParams)
		return ret

	default:
		panic("Unimplemented GEX selected")
	}
}

func (gex *dhGEXSHA) MarshalJSON() ([]byte, error) {
	return json.Marshal(gex.JsonLog)
}

func (gex *dhGEXSHA) diffieHellman(theirPublic, myPrivate *big.Int) (*big.Int, error) {
	if theirPublic.Sign() <= 0 || theirPublic.Cmp(gex.p) >= 0 {
		return nil, fmt.Errorf("ssh: DH parameter out of bounds")
	}
	return new(big.Int).Exp(theirPublic, myPrivate, gex.p), nil
}

func (gex *dhGEXSHA) Client(c packetConn, randSource io.Reader, magics *handshakeMagics, config *Config) (*kexResult, error) {
	// Send GexRequest
	kexDHGexRequest := kexDHGexRequestMsg{
		MinBits:      uint32(config.GexMinBits),
		PreferedBits: uint32(config.GexPreferredBits),
		MaxBits:      uint32(config.GexMaxBits),
	}
	if err := c.writePacket(Marshal(&kexDHGexRequest)); err != nil {
		return nil, err
	}

	// *Receive GexGroup*
	packet, err := c.readPacket()
	if err != nil {
		return nil, err
	}

	var kexDHGexGroup kexDHGexGroupMsg
	if err = Unmarshal(packet, &kexDHGexGroup); err != nil {
		return nil, err
	}

	if gex.JsonLog != nil {
		gex.JsonLog.Parameters.Prime = kexDHGexGroup.P
		gex.JsonLog.Parameters.Generator = kexDHGexGroup.G
	}

	// reject if p's bit length < pkgConfig.GexMinBits or > pkgConfig.GexMaxBits
	if kexDHGexGroup.P.BitLen() < int(config.GexMinBits) || kexDHGexGroup.P.BitLen() > int(config.GexMaxBits) {
		return nil, fmt.Errorf("Server-generated gex p (dont't ask) is out of range (%d bits)", kexDHGexGroup.P.BitLen())
	}

	gex.p = kexDHGexGroup.P
	gex.g = kexDHGexGroup.G

	// *Send GexInit
	x, err := rand.Int(randSource, gex.p)
	if err != nil {
		return nil, err
	}
	X := new(big.Int).Exp(gex.g, x, gex.p)
	kexDHGexInit := kexDHGexInitMsg{
		X: X,
	}

	if gex.JsonLog != nil && config.Verbose {
		gex.JsonLog.Parameters.ClientPrivate = x
		gex.JsonLog.Parameters.ClientPublic = X
	}

	if err := c.writePacket(Marshal(&kexDHGexInit)); err != nil {
		return nil, err
	}

	// Receive GexReply
	packet, err = c.readPacket()
	if err != nil {
		return nil, err
	}

	var kexDHGexReply kexDHGexReplyMsg
	if err = Unmarshal(packet, &kexDHGexReply); err != nil {
		return nil, err
	}

	if gex.JsonLog != nil {
		gex.JsonLog.Parameters.ServerPublic = kexDHGexReply.Y
		gex.JsonLog.ServerSignature = new(JsonSignature)
		gex.JsonLog.ServerSignature.Raw = kexDHGexReply.Signature
		gex.JsonLog.ServerSignature.Parsed, _, _ = parseSignatureBody(kexDHGexReply.Signature)
		gex.JsonLog.ServerHostKey = LogServerHostKey(kexDHGexReply.HostKey)
	}

	kInt, err := gex.diffieHellman(kexDHGexReply.Y, x)
	if err != nil {
		return nil, err
	}

	h := gex.hashFunc.New()
	magics.write(h)
	writeString(h, kexDHGexReply.HostKey)
	binary.Write(h, binary.BigEndian, uint32(config.GexMinBits))
	binary.Write(h, binary.BigEndian, uint32(config.GexPreferredBits))
	binary.Write(h, binary.BigEndian, uint32(config.GexMaxBits))
	writeInt(h, gex.p)
	writeInt(h, gex.g)
	writeInt(h, X)
	writeInt(h, kexDHGexReply.Y)
	K := make([]byte, intLength(kInt))
	marshalInt(K, kInt)
	h.Write(K)

	return &kexResult{
		H:         h.Sum(nil),
		K:         K,
		HostKey:   kexDHGexReply.HostKey,
		Signature: kexDHGexReply.Signature,
		Hash:      gex.hashFunc,
	}, nil
}

func (gex *dhGEXSHA) Server(c packetConn, randSource io.Reader, magics *handshakeMagics, priv Signer, config *Config) (result *kexResult, err error) {
	// *Receive GexRequest*
	packet, err := c.readPacket()
	if err != nil {
		return
	}
	var kexDHGexRequest kexDHGexRequestMsg
	if err = Unmarshal(packet, &kexDHGexRequest); err != nil {
		return
	}

	// smoosh the user's preferred size into our own limits
	if kexDHGexRequest.PreferedBits > uint32(config.GexMaxBits) {
		kexDHGexRequest.PreferedBits = uint32(config.GexMaxBits)
	}
	if kexDHGexRequest.PreferedBits < uint32(config.GexMinBits) {
		kexDHGexRequest.PreferedBits = uint32(config.GexMinBits)
	}
	// fix min/max if they're inconsistent.  technically, we could just pout
	// and hang up, but there's no harm in giving them the benefit of the
	// doubt and just picking a bitsize for them.
	if kexDHGexRequest.MinBits > kexDHGexRequest.PreferedBits {
		kexDHGexRequest.MinBits = kexDHGexRequest.PreferedBits
	}
	if kexDHGexRequest.MaxBits < kexDHGexRequest.PreferedBits {
		kexDHGexRequest.MaxBits = kexDHGexRequest.PreferedBits
	}

	// *Send GexGroup*
	// generate prime
	// TODO: Not implemented yet, should load primes from /etc/ssh/moduli
	gex.p, _ = new(big.Int).SetString("D1391174233D315398FE2830AC6B2B66BCCD01B0A634899F339B7879F1DB85712E9DC4E4B1C6C8355570C1D2DCB53493DF18175A9C53D1128B592B4C72D97136F5542FEB981CBFE8012FDD30361F288A42BD5EBB08BAB0A5640E1AC48763B2ABD1945FEE36B2D55E1D50A1C86CED9DD141C4E7BE2D32D9B562A0F8E2E927020E91F58B57EB9ACDDA106A59302D7E92AD5F6E851A45FA1CFE86029A0F727F65A8F475F33572E2FDAB6073F0C21B8B54C3823DB2EF068927E5D747498F96361507", 16)
	gex.g = big.NewInt(5)
	kexDHGexGroup := kexDHGexGroupMsg{
		P: gex.p,
		G: gex.g,
	}
	if err := c.writePacket(Marshal(&kexDHGexGroup)); err != nil {
		return nil, err
	}

	// *Receive GexInit
	packet, err = c.readPacket()
	if err != nil {
		return
	}
	var kexDHGexInit kexDHGexInitMsg
	if err = Unmarshal(packet, &kexDHGexInit); err != nil {
		return
	}

	y, err := rand.Int(randSource, gex.p)
	if err != nil {
		return
	}

	Y := new(big.Int).Exp(gex.g, y, gex.p)
	kInt, err := gex.diffieHellman(kexDHGexInit.X, y)
	if err != nil {
		return nil, err
	}

	hostKeyBytes := priv.PublicKey().Marshal()

	h := gex.hashFunc.New()
	magics.write(h)
	writeString(h, hostKeyBytes)
	binary.Write(h, binary.BigEndian, uint32(config.GexMinBits))
	binary.Write(h, binary.BigEndian, uint32(config.GexPreferredBits))
	binary.Write(h, binary.BigEndian, uint32(config.GexMaxBits))
	writeInt(h, gex.p)
	writeInt(h, gex.g)
	writeInt(h, kexDHGexInit.X)
	writeInt(h, Y)

	K := make([]byte, intLength(kInt))
	marshalInt(K, kInt)
	h.Write(K)

	H := h.Sum(nil)

	// H is already a hash, but the hostkey signing will apply its
	// own key-specific hash algorithm.
	sig, err := signAndMarshal(priv, randSource, H)
	if err != nil {
		return nil, err
	}

	kexDHGexReply := kexDHGexReplyMsg{
		HostKey:   hostKeyBytes,
		Y:         Y,
		Signature: sig,
	}
	packet = Marshal(&kexDHGexReply)

	err = c.writePacket(packet)

	return &kexResult{
		H:         H,
		K:         K,
		HostKey:   hostKeyBytes,
		Signature: sig,
		Hash:      gex.hashFunc,
	}, nil
}