device: optimize message encoding

Optimize message encoding by eliminating binary.Write (which internally
uses reflection) in favour of hand-rolled encoding.

This is companion to 9e7529c3d2.

Synthetic benchmark:

    var packetSink []byte
    func BenchmarkMessageInitiationMarshal(b *testing.B) {
        var msg MessageInitiation
        b.Run("binary.Write", func(b *testing.B) {
            b.ReportAllocs()
            for range b.N {
                var buf [MessageInitiationSize]byte
                writer := bytes.NewBuffer(buf[:0])
                _ = binary.Write(writer, binary.LittleEndian, msg)
                packetSink = writer.Bytes()
            }
        })
        b.Run("binary.Encode", func(b *testing.B) {
            b.ReportAllocs()
            for range b.N {
                packet := make([]byte, MessageInitiationSize)
                _, _ = binary.Encode(packet, binary.LittleEndian, msg)
                packetSink = packet
            }
        })
        b.Run("marshal", func(b *testing.B) {
            b.ReportAllocs()
            for range b.N {
                packet := make([]byte, MessageInitiationSize)
                _ = msg.marshal(packet)
                packetSink = packet
            }
        })
    }

Results:
                                             │      -      │
                                             │   sec/op    │
    MessageInitiationMarshal/binary.Write-8    1.337µ ± 0%
    MessageInitiationMarshal/binary.Encode-8   1.242µ ± 0%
    MessageInitiationMarshal/marshal-8         53.05n ± 1%

                                             │     -      │
                                             │    B/op    │
    MessageInitiationMarshal/binary.Write-8    368.0 ± 0%
    MessageInitiationMarshal/binary.Encode-8   160.0 ± 0%
    MessageInitiationMarshal/marshal-8         160.0 ± 0%

                                             │     -      │
                                             │ allocs/op  │
    MessageInitiationMarshal/binary.Write-8    3.000 ± 0%
    MessageInitiationMarshal/binary.Encode-8   1.000 ± 0%
    MessageInitiationMarshal/marshal-8         1.000 ± 0%

Signed-off-by: Alexander Yastrebov <yastrebov.alex@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/noise-protocol.go

@@ -134,6 +134,22 @@ func (msg *MessageInitiation) unmarshal(b []byte) error {
 	return nil
 }
 
+func (msg *MessageInitiation) marshal(b []byte) error {
+	if len(b) != MessageInitiationSize {
+		return errMessageLengthMismatch
+	}
+
+	binary.LittleEndian.PutUint32(b, msg.Type)
+	binary.LittleEndian.PutUint32(b[4:], msg.Sender)
+	copy(b[8:], msg.Ephemeral[:])
+	copy(b[8+len(msg.Ephemeral):], msg.Static[:])
+	copy(b[8+len(msg.Ephemeral)+len(msg.Static):], msg.Timestamp[:])
+	copy(b[8+len(msg.Ephemeral)+len(msg.Static)+len(msg.Timestamp):], msg.MAC1[:])
+	copy(b[8+len(msg.Ephemeral)+len(msg.Static)+len(msg.Timestamp)+len(msg.MAC1):], msg.MAC2[:])
+
+	return nil
+}
+
 func (msg *MessageResponse) unmarshal(b []byte) error {
 	if len(b) != MessageResponseSize {
 		return errMessageLengthMismatch
@@ -150,6 +166,22 @@ func (msg *MessageResponse) unmarshal(b []byte) error {
 	return nil
 }
 
+func (msg *MessageResponse) marshal(b []byte) error {
+	if len(b) != MessageResponseSize {
+		return errMessageLengthMismatch
+	}
+
+	binary.LittleEndian.PutUint32(b, msg.Type)
+	binary.LittleEndian.PutUint32(b[4:], msg.Sender)
+	binary.LittleEndian.PutUint32(b[8:], msg.Receiver)
+	copy(b[12:], msg.Ephemeral[:])
+	copy(b[12+len(msg.Ephemeral):], msg.Empty[:])
+	copy(b[12+len(msg.Ephemeral)+len(msg.Empty):], msg.MAC1[:])
+	copy(b[12+len(msg.Ephemeral)+len(msg.Empty)+len(msg.MAC1):], msg.MAC2[:])
+
+	return nil
+}
+
 func (msg *MessageCookieReply) unmarshal(b []byte) error {
 	if len(b) != MessageCookieReplySize {
 		return errMessageLengthMismatch
@@ -163,6 +195,19 @@ func (msg *MessageCookieReply) unmarshal(b []byte) error {
 	return nil
 }
 
+func (msg *MessageCookieReply) marshal(b []byte) error {
+	if len(b) != MessageCookieReplySize {
+		return errMessageLengthMismatch
+	}
+
+	binary.LittleEndian.PutUint32(b, msg.Type)
+	binary.LittleEndian.PutUint32(b[4:], msg.Receiver)
+	copy(b[8:], msg.Nonce[:])
+	copy(b[8+len(msg.Nonce):], msg.Cookie[:])
+
+	return nil
+}
+
 type Handshake struct {
 	state                     handshakeState
 	mutex                     sync.RWMutex