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/stream.go

@@ -0,0 +1,72 @@
+package obfs
+
+import (
+	"net"
+
+	"github.com/Dreamacro/clash/common/pool"
+)
+
+// NewConn wraps a stream-oriented net.Conn with obfs decoding/encoding
+func NewConn(c net.Conn, o Obfs) net.Conn {
+	return &Conn{Conn: c, Obfs: o.initForConn()}
+}
+
+// Conn represents an obfs connection
+type Conn struct {
+	net.Conn
+	Obfs
+	buf    []byte
+	offset int
+}
+
+func (c *Conn) Read(b []byte) (int, error) {
+	if c.buf != nil {
+		n := copy(b, c.buf[c.offset:])
+		c.offset += n
+		if c.offset == len(c.buf) {
+			pool.Put(c.buf)
+			c.buf = nil
+		}
+		return n, nil
+	}
+
+	buf := pool.Get(pool.RelayBufferSize)
+	defer pool.Put(buf)
+	n, err := c.Conn.Read(buf)
+	if err != nil {
+		return 0, err
+	}
+	decoded, sendback, err := c.Decode(buf[:n])
+	// decoded may be part of buf
+	decodedData := pool.Get(len(decoded))
+	copy(decodedData, decoded)
+	if err != nil {
+		pool.Put(decodedData)
+		return 0, err
+	}
+	if sendback {
+		c.Write(nil)
+		pool.Put(decodedData)
+		return 0, nil
+	}
+	n = copy(b, decodedData)
+	if len(decodedData) > len(b) {
+		c.buf = decodedData
+		c.offset = n
+	} else {
+		pool.Put(decodedData)
+	}
+	return n, err
+}
+
+func (c *Conn) Write(b []byte) (int, error) {
+	encoded, err := c.Encode(b)
+	if err != nil {
+		return 0, err
+	}
+	_, err = c.Conn.Write(encoded)
+	if err != nil {
+		return 0, err
+	}
+	return len(b), nil
+}