Feature: add ssr support (#805)

* Refactor ssr stream cipher to expose iv and key

References:
https://github.com/Dreamacro/go-shadowsocks2
https://github.com/sh4d0wfiend/go-shadowsocksr2

* Implement ssr obfs

Reference:
https://github.com/mzz2017/shadowsocksR

* Implement ssr protocol

References:
https://github.com/mzz2017/shadowsocksR
https://github.com/shadowsocksRb/shadowsocksr-libev
https://github.com/shadowsocksr-backup/shadowsocksr

component/ssr/obfs/random_head.go

@@ -0,0 +1,75 @@
+package obfs
+
+import (
+	"encoding/binary"
+	"hash/crc32"
+	"math/rand"
+)
+
+type randomHead struct {
+	*Base
+	firstRequest  bool
+	firstResponse bool
+	headerSent    bool
+	buffer        []byte
+}
+
+func init() {
+	register("random_head", newRandomHead)
+}
+
+func newRandomHead(b *Base) Obfs {
+	return &randomHead{Base: b}
+}
+
+func (r *randomHead) initForConn() Obfs {
+	return &randomHead{
+		Base:          r.Base,
+		firstRequest:  true,
+		firstResponse: true,
+	}
+}
+
+func (r *randomHead) GetObfsOverhead() int {
+	return 0
+}
+
+func (r *randomHead) Encode(b []byte) (encoded []byte, err error) {
+	if !r.firstRequest {
+		return b, nil
+	}
+
+	bSize := len(b)
+	if r.headerSent {
+		if bSize > 0 {
+			d := make([]byte, len(r.buffer)+bSize)
+			copy(d, r.buffer)
+			copy(d[len(r.buffer):], b)
+			r.buffer = d
+		} else {
+			encoded = r.buffer
+			r.buffer = nil
+			r.firstRequest = false
+		}
+	} else {
+		size := rand.Intn(96) + 8
+		encoded = make([]byte, size)
+		rand.Read(encoded)
+		crc := (0xFFFFFFFF - crc32.ChecksumIEEE(encoded[:size-4])) & 0xFFFFFFFF
+		binary.LittleEndian.PutUint32(encoded[size-4:], crc)
+
+		d := make([]byte, bSize)
+		copy(d, b)
+		r.buffer = d
+	}
+	r.headerSent = true
+	return encoded, nil
+}
+
+func (r *randomHead) Decode(b []byte) ([]byte, bool, error) {
+	if r.firstResponse {
+		r.firstResponse = false
+		return b, true, nil
+	}
+	return b, false, nil
+}