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/obfs.go

@@ -0,0 +1,37 @@
+package obfs
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+var (
+	errTLS12TicketAuthIncorrectMagicNumber = errors.New("tls1.2_ticket_auth incorrect magic number")
+	errTLS12TicketAuthTooShortData         = errors.New("tls1.2_ticket_auth too short data")
+	errTLS12TicketAuthHMACError            = errors.New("tls1.2_ticket_auth hmac verifying failed")
+)
+
+// Obfs provides methods for decoding and encoding
+type Obfs interface {
+	initForConn() Obfs
+	GetObfsOverhead() int
+	Decode(b []byte) ([]byte, bool, error)
+	Encode(b []byte) ([]byte, error)
+}
+
+type obfsCreator func(b *Base) Obfs
+
+var obfsList = make(map[string]obfsCreator)
+
+func register(name string, c obfsCreator) {
+	obfsList[name] = c
+}
+
+// PickObfs returns an obfs of the given name
+func PickObfs(name string, b *Base) (Obfs, error) {
+	if obfsCreator, ok := obfsList[strings.ToLower(name)]; ok {
+		return obfsCreator(b), nil
+	}
+	return nil, fmt.Errorf("Obfs %s not supported", name)
+}