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/protocol/protocol.go

@@ -0,0 +1,61 @@
+package protocol
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"strings"
+	"sync"
+)
+
+var (
+	errAuthAES128HMACError         = errors.New("auth_aes128_* post decrypt hmac error")
+	errAuthAES128DataLengthError   = errors.New("auth_aes128_* post decrypt length mismatch")
+	errAuthSHA1v4CRC32Error        = errors.New("auth_sha1_v4 post decrypt data crc32 error")
+	errAuthSHA1v4DataLengthError   = errors.New("auth_sha1_v4 post decrypt data length error")
+	errAuthSHA1v4IncorrectChecksum = errors.New("auth_sha1_v4 post decrypt incorrect checksum")
+	errAuthChainDataLengthError    = errors.New("auth_chain_* post decrypt length mismatch")
+	errAuthChainHMACError          = errors.New("auth_chain_* post decrypt hmac error")
+)
+
+type authData struct {
+	clientID     []byte
+	connectionID uint32
+	mutex        sync.Mutex
+}
+
+type recvInfo struct {
+	recvID uint32
+	buffer *bytes.Buffer
+}
+
+type hmacMethod func(key []byte, data []byte) []byte
+type hashDigestMethod func(data []byte) []byte
+type rndMethod func(dataSize int, random *shift128PlusContext, lastHash []byte, dataSizeList, dataSizeList2 []int, overhead int) int
+
+// Protocol provides methods for decoding, encoding and iv setting
+type Protocol interface {
+	initForConn(iv []byte) Protocol
+	GetProtocolOverhead() int
+	SetOverhead(int)
+	Decode([]byte) ([]byte, int, error)
+	Encode([]byte) ([]byte, error)
+	DecodePacket([]byte) ([]byte, int, error)
+	EncodePacket([]byte) ([]byte, error)
+}
+
+type protocolCreator func(b *Base) Protocol
+
+var protocolList = make(map[string]protocolCreator)
+
+func register(name string, c protocolCreator) {
+	protocolList[name] = c
+}
+
+// PickProtocol returns a protocol of the given name
+func PickProtocol(name string, b *Base) (Protocol, error) {
+	if protocolCreator, ok := protocolList[strings.ToLower(name)]; ok {
+		return protocolCreator(b), nil
+	}
+	return nil, fmt.Errorf("Protocol %s not supported", name)
+}