func (peer *Peer) BeginSymmetricSession() error {
	device := peer.device
	handshake := &peer.handshake
	handshake.mutex.Lock()
	defer handshake.mutex.Unlock()
	var isInitiator bool
	var sendKey [chacha20poly1305.KeySize]byte
	var recvKey [chacha20poly1305.KeySize]byte
	if handshake.state == HandshakeResponseConsumed {
		KDF2(
			&sendKey,
			&recvKey,
			handshake.chainKey[:],
			nil,
		)
		isInitiator = true
	} else if handshake.state == HandshakeResponseCreated {
		KDF2(
			&recvKey,
			&sendKey,
			handshake.chainKey[:],
			nil,
		)
		isInitiator = false
	} else {
		return errors.New("invalid state for keypair derivation")
	}
	setZero(handshake.chainKey[:])
	setZero(handshake.hash[:])  
	setZero(handshake.localEphemeral[:])
	peer.handshake.state = HandshakeZeroed
	keypair := new(Keypair)
	keypair.send, _ = chacha20poly1305.New(sendKey[:])
	keypair.receive, _ = chacha20poly1305.New(recvKey[:])
	setZero(sendKey[:])
	setZero(recvKey[:])
	keypair.created = time.Now()
	keypair.sendNonce = 0
	keypair.replayFilter.Init()
	keypair.isInitiator = isInitiator
	keypair.localIndex = peer.handshake.localIndex
	keypair.remoteIndex = peer.handshake.remoteIndex
	device.indexTable.SwapIndexForKeypair(handshake.localIndex, keypair)
	handshake.localIndex = 0
	keypairs := &peer.keypairs
	keypairs.Lock()
	defer keypairs.Unlock()
	previous := keypairs.previous
	next := keypairs.next
	current := keypairs.current
	if isInitiator {
		if next != nil {
			keypairs.next = nil
			keypairs.previous = next
			device.DeleteKeypair(current)
		} else {
			keypairs.previous = current
		}
		device.DeleteKeypair(previous)
		keypairs.current = keypair
	} else {
		keypairs.next = keypair
		device.DeleteKeypair(next)
		keypairs.previous = nil
		device.DeleteKeypair(previous)
	}
	return nil
}
/* Derives a new keypair from the current handshake state
package device
import (
	"errors"
	"golang.org/x/crypto/blake2s"
	"golang.org/x/crypto/chacha20poly1305"
	"golang.org/x/crypto/poly1305"
	"golang.zx2c4.com/wireguard/tai64n"
	"sync"
	"time"
)
const (
	HandshakeZeroed = iota
	HandshakeInitiationCreated
	HandshakeInitiationConsumed
	HandshakeResponseCreated
	HandshakeResponseConsumed
)
const (
	NoiseConstruction = "Noise_IKpsk2_25519_ChaChaPoly_BLAKE2s"
	WGIdentifier      = "WireGuard v1 zx2c4 Jason@zx2c4.com"
	WGLabelMAC1       = "mac1----"
	WGLabelCookie     = "cookie--"
)
const (
	MessageInitiationType  = 1
	MessageResponseType    = 2
	MessageCookieReplyType = 3
	MessageTransportType   = 4
)
const (
	MessageInitiationSize      = 148                                            
	MessageResponseSize        = 92                                             
	MessageCookieReplySize     = 64                                             
	MessageTransportHeaderSize = 16                                             
	MessageTransportSize       = MessageTransportHeaderSize + poly1305.TagSize  
	MessageKeepaliveSize       = MessageTransportSize                           
	MessageHandshakeSize       = MessageInitiationSize                          
)
const (
	MessageTransportOffsetReceiver = 4
	MessageTransportOffsetCounter  = 8
	MessageTransportOffsetContent  = 16
)
type MessageInitiation struct {
	Type      uint32
	Sender    uint32
	Ephemeral NoisePublicKey
	Static    [NoisePublicKeySize + poly1305.TagSize]byte
	Timestamp [tai64n.TimestampSize + poly1305.TagSize]byte
	MAC1      [blake2s.Size128]byte
	MAC2      [blake2s.Size128]byte
}
type MessageResponse struct {
	Type      uint32
	Sender    uint32
	Receiver  uint32
	Ephemeral NoisePublicKey
	Empty     [poly1305.TagSize]byte
	MAC1      [blake2s.Size128]byte
	MAC2      [blake2s.Size128]byte
}
type MessageTransport struct {
	Type     uint32
	Receiver uint32
	Counter  uint64
	Content  []byte
}
type MessageCookieReply struct {
	Type     uint32
	Receiver uint32
	Nonce    [chacha20poly1305.NonceSizeX]byte
	Cookie   [blake2s.Size128 + poly1305.TagSize]byte
}
type Handshake struct {
	state                     int
	mutex                     sync.RWMutex
	hash                      [blake2s.Size]byte        
	chainKey                  [blake2s.Size]byte        
	presharedKey              NoiseSymmetricKey         
	localEphemeral            NoisePrivateKey           
	localIndex                uint32                    
	remoteIndex               uint32                    
	remoteStatic              NoisePublicKey            
	remoteEphemeral           NoisePublicKey            
	precomputedStaticStatic   [NoisePublicKeySize]byte  
	lastTimestamp             tai64n.Timestamp
	lastInitiationConsumption time.Time
	lastSentHandshake         time.Time
}
var (
	InitialChainKey [blake2s.Size]byte
	InitialHash     [blake2s.Size]byte
	ZeroNonce       [chacha20poly1305.NonceSize]byte
)
func mixKey(dst *[blake2s.Size]byte, c *[blake2s.Size]byte, data []byte) {
	KDF1(dst, c[:], data)
}
func mixHash(dst *[blake2s.Size]byte, h *[blake2s.Size]byte, data []byte) {
	hash, _ := blake2s.New256(nil)
	hash.Write(h[:])
	hash.Write(data)
	hash.Sum(dst[:0])
	hash.Reset()
}
func (h *Handshake) Clear() {
	setZero(h.localEphemeral[:])
	setZero(h.remoteEphemeral[:])
	setZero(h.chainKey[:])
	setZero(h.hash[:])
	h.localIndex = 0
	h.state = HandshakeZeroed
}
func (h *Handshake) mixHash(data []byte) {
	mixHash(&h.hash, &h.hash, data)
}
func (h *Handshake) mixKey(data []byte) {
	mixKey(&h.chainKey, &h.chainKey, data)
}
func init() {
	InitialChainKey = blake2s.Sum256([]byte(NoiseConstruction))
	mixHash(&InitialHash, &InitialChainKey, []byte(WGIdentifier))
}
func (device *Device) CreateMessageInitiation(peer *Peer) (*MessageInitiation, error) {
	device.staticIdentity.RLock()
	defer device.staticIdentity.RUnlock()
	handshake := &peer.handshake
	handshake.mutex.Lock()
	defer handshake.mutex.Unlock()
	if isZero(handshake.precomputedStaticStatic[:]) {
		return nil, errors.New("static shared secret is zero")
	}
	var err error
	handshake.hash = InitialHash
	handshake.chainKey = InitialChainKey
	handshake.localEphemeral, err = newPrivateKey()
	if err != nil {
		return nil, err
	}
	device.indexTable.Delete(handshake.localIndex)
	handshake.localIndex, err = device.indexTable.NewIndexForHandshake(peer, handshake)
	if err != nil {
		return nil, err
	}
	handshake.mixHash(handshake.remoteStatic[:])
	msg := MessageInitiation{
		Type:      MessageInitiationType,
		Ephemeral: handshake.localEphemeral.publicKey(),
		Sender:    handshake.localIndex,
	}
	handshake.mixKey(msg.Ephemeral[:])
	handshake.mixHash(msg.Ephemeral[:])
	func() {
		var key [chacha20poly1305.KeySize]byte
		ss := handshake.localEphemeral.sharedSecret(handshake.remoteStatic)
		KDF2(
			&handshake.chainKey,
			&key,
			handshake.chainKey[:],
			ss[:],
		)
		aead, _ := chacha20poly1305.New(key[:])
		aead.Seal(msg.Static[:0], ZeroNonce[:], device.staticIdentity.publicKey[:], handshake.hash[:])
	}()
	handshake.mixHash(msg.Static[:])
	timestamp := tai64n.Now()
	func() {
		var key [chacha20poly1305.KeySize]byte
		KDF2(
			&handshake.chainKey,
			&key,
			handshake.chainKey[:],
			handshake.precomputedStaticStatic[:],
		)
		aead, _ := chacha20poly1305.New(key[:])
		aead.Seal(msg.Timestamp[:0], ZeroNonce[:], timestamp[:], handshake.hash[:])
	}()
	handshake.mixHash(msg.Timestamp[:])
	handshake.state = HandshakeInitiationCreated
	return &msg, nil
}
func (device *Device) ConsumeMessageInitiation(msg *MessageInitiation) *Peer {
	var (
		hash     [blake2s.Size]byte
		chainKey [blake2s.Size]byte
	)
	if msg.Type != MessageInitiationType {
		return nil
	}
	device.staticIdentity.RLock()
	defer device.staticIdentity.RUnlock()
	mixHash(&hash, &InitialHash, device.staticIdentity.publicKey[:])
	mixHash(&hash, &hash, msg.Ephemeral[:])
	mixKey(&chainKey, &InitialChainKey, msg.Ephemeral[:])
	var err error
	var peerPK NoisePublicKey
	func() {
		var key [chacha20poly1305.KeySize]byte
		ss := device.staticIdentity.privateKey.sharedSecret(msg.Ephemeral)
		KDF2(&chainKey, &key, chainKey[:], ss[:])
		aead, _ := chacha20poly1305.New(key[:])
		_, err = aead.Open(peerPK[:0], ZeroNonce[:], msg.Static[:], hash[:])
	}()
	if err != nil {
		return nil
	}
	mixHash(&hash, &hash, msg.Static[:])
	peer := device.LookupPeer(peerPK)
	if peer == nil {
		return nil
	}
	handshake := &peer.handshake
	if isZero(handshake.precomputedStaticStatic[:]) {
		return nil
	}
	var timestamp tai64n.Timestamp
	var key [chacha20poly1305.KeySize]byte
	handshake.mutex.RLock()
	KDF2(
		&chainKey,
		&key,
		chainKey[:],
		handshake.precomputedStaticStatic[:],
	)
	aead, _ := chacha20poly1305.New(key[:])
	_, err = aead.Open(timestamp[:0], ZeroNonce[:], msg.Timestamp[:], hash[:])
	if err != nil {
		handshake.mutex.RUnlock()
		return nil
	}
	mixHash(&hash, &hash, msg.Timestamp[:])
	var ok bool
	ok = timestamp.After(handshake.lastTimestamp)
	ok = ok && time.Now().Sub(handshake.lastInitiationConsumption) > HandshakeInitationRate
	handshake.mutex.RUnlock()
	if !ok {
		return nil
	}
	handshake.mutex.Lock()
	handshake.hash = hash
	handshake.chainKey = chainKey
	handshake.remoteIndex = msg.Sender
	handshake.remoteEphemeral = msg.Ephemeral
	handshake.lastTimestamp = timestamp
	handshake.lastInitiationConsumption = time.Now()
	handshake.state = HandshakeInitiationConsumed
	handshake.mutex.Unlock()
	setZero(hash[:])
	setZero(chainKey[:])
	return peer
}
func (device *Device) CreateMessageResponse(peer *Peer) (*MessageResponse, error) {
	handshake := &peer.handshake
	handshake.mutex.Lock()
	defer handshake.mutex.Unlock()
	if handshake.state != HandshakeInitiationConsumed {
		return nil, errors.New("handshake initiation must be consumed first")
	}
	var err error
	device.indexTable.Delete(handshake.localIndex)
	handshake.localIndex, err = device.indexTable.NewIndexForHandshake(peer, handshake)
	if err != nil {
		return nil, err
	}
	var msg MessageResponse
	msg.Type = MessageResponseType
	msg.Sender = handshake.localIndex
	msg.Receiver = handshake.remoteIndex
	handshake.localEphemeral, err = newPrivateKey()
	if err != nil {
		return nil, err
	}
	msg.Ephemeral = handshake.localEphemeral.publicKey()
	handshake.mixHash(msg.Ephemeral[:])
	handshake.mixKey(msg.Ephemeral[:])
	func() {
		ss := handshake.localEphemeral.sharedSecret(handshake.remoteEphemeral)
		handshake.mixKey(ss[:])
		ss = handshake.localEphemeral.sharedSecret(handshake.remoteStatic)
		handshake.mixKey(ss[:])
	}()
	var tau [blake2s.Size]byte
	var key [chacha20poly1305.KeySize]byte
	KDF3(
		&handshake.chainKey,
		&tau,
		&key,
		handshake.chainKey[:],
		handshake.presharedKey[:],
	)
	handshake.mixHash(tau[:])
	func() {
		aead, _ := chacha20poly1305.New(key[:])
		aead.Seal(msg.Empty[:0], ZeroNonce[:], nil, handshake.hash[:])
		handshake.mixHash(msg.Empty[:])
	}()
	handshake.state = HandshakeResponseCreated
	return &msg, nil
}
func (device *Device) ConsumeMessageResponse(msg *MessageResponse) *Peer {
	if msg.Type != MessageResponseType {
		return nil
	}
	lookup := device.indexTable.Lookup(msg.Receiver)
	handshake := lookup.handshake
	if handshake == nil {
		return nil
	}
	var (
		hash     [blake2s.Size]byte
		chainKey [blake2s.Size]byte
	)
	ok := func() bool {
		handshake.mutex.RLock()
		defer handshake.mutex.RUnlock()
		if handshake.state != HandshakeInitiationCreated {
			return false
		}
		device.staticIdentity.RLock()
		defer device.staticIdentity.RUnlock()
		mixHash(&hash, &handshake.hash, msg.Ephemeral[:])
		mixKey(&chainKey, &handshake.chainKey, msg.Ephemeral[:])
		func() {
			ss := handshake.localEphemeral.sharedSecret(msg.Ephemeral)
			mixKey(&chainKey, &chainKey, ss[:])
			setZero(ss[:])
		}()
		func() {
			ss := device.staticIdentity.privateKey.sharedSecret(msg.Ephemeral)
			mixKey(&chainKey, &chainKey, ss[:])
			setZero(ss[:])
		}()
		var tau [blake2s.Size]byte
		var key [chacha20poly1305.KeySize]byte
		KDF3(
			&chainKey,
			&tau,
			&key,
			chainKey[:],
			handshake.presharedKey[:],
		)
		mixHash(&hash, &hash, tau[:])
		aead, _ := chacha20poly1305.New(key[:])
		_, err := aead.Open(nil, ZeroNonce[:], msg.Empty[:], hash[:])
		if err != nil {
			return false
		}
		mixHash(&hash, &hash, msg.Empty[:])
		return true
	}()
	if !ok {
		return nil
	}
	handshake.mutex.Lock()
	handshake.hash = hash
	handshake.chainKey = chainKey
	handshake.remoteIndex = msg.Sender
	handshake.state = HandshakeResponseConsumed
	handshake.mutex.Unlock()
	setZero(hash[:])
	setZero(chainKey[:])
	return lookup.peer
}