diff --git a/shadowsocks/obfsplugin/plain.py b/shadowsocks/obfsplugin/plain.py
index 8c6355c..593a23f 100644
--- a/shadowsocks/obfsplugin/plain.py
+++ b/shadowsocks/obfsplugin/plain.py
@@ -21,76 +21,81 @@ import os
 import sys
 import hashlib
 import logging
+import typing
+from ..obfs import (server_info as ServerInfo)
 
 from shadowsocks.common import ord
 
+
 def create_obfs(method):
     return plain(method)
 
-obfs_map = {
-        'plain': (create_obfs,),
-        'origin': (create_obfs,),
+
+obfs_map: typing.Dict[str, tuple] = {
+    'plain': (create_obfs,),
+    'origin': (create_obfs,),
 }
 
+
 class plain(object):
-    def __init__(self, method):
+    def __init__(self, method: str):
         self.method = method
-        self.server_info = None
+        self.server_info: ServerInfo = None
 
-    def init_data(self):
+    def init_data(self) -> bytes:
         return b''
 
-    def get_overhead(self, direction): # direction: true for c->s false for s->c
+    def get_overhead(self, direction) -> int:  # direction: true for c->s false for s->c
         return 0
 
-    def get_server_info(self):
+    def get_server_info(self) -> ServerInfo:
         return self.server_info
 
-    def set_server_info(self, server_info):
+    def set_server_info(self, server_info: ServerInfo):
         self.server_info = server_info
 
-    def client_pre_encrypt(self, buf):
+    def client_pre_encrypt(self, buf: bytes) -> bytes:
         return buf
 
-    def client_encode(self, buf):
+    def client_encode(self, buf: bytes) -> bytes:
         return buf
 
-    def client_decode(self, buf):
+    def client_decode(self, buf: bytes) -> typing.Tuple[bytes, bool]:
         # (buffer_to_recv, is_need_to_encode_and_send_back)
         return (buf, False)
 
-    def client_post_decrypt(self, buf):
+    def client_post_decrypt(self, buf: bytes) -> bytes:
         return buf
 
-    def server_pre_encrypt(self, buf):
+    def server_pre_encrypt(self, buf: bytes) -> bytes:
         return buf
 
-    def server_encode(self, buf):
+    def server_encode(self, buf: bytes) -> bytes:
         return buf
 
-    def server_decode(self, buf):
+    def server_decode(self, buf: bytes) -> typing.Tuple[bytes, bool, bool]:
         # (buffer_to_recv, is_need_decrypt, is_need_to_encode_and_send_back)
         return (buf, True, False)
 
-    def server_post_decrypt(self, buf):
+    def server_post_decrypt(self, buf: bytes) -> typing.Tuple[bytes, bool]:
         return (buf, False)
 
-    def client_udp_pre_encrypt(self, buf):
+    def client_udp_pre_encrypt(self, buf: bytes) -> bytes:
         return buf
 
-    def client_udp_post_decrypt(self, buf):
+    def client_udp_post_decrypt(self, buf: bytes) -> bytes:
         return buf
 
-    def server_udp_pre_encrypt(self, buf, uid):
+    def server_udp_pre_encrypt(self, buf: bytes, uid) -> bytes:
         return buf
 
-    def server_udp_post_decrypt(self, buf):
+    def server_udp_post_decrypt(self, buf: bytes) -> typing.Tuple[bytes, any]:
         return (buf, None)
 
     def dispose(self):
         pass
 
-    def get_head_size(self, buf, def_value):
+    def get_head_size(self, buf: bytes, def_value: int) -> int:
         if len(buf) < 2:
             return def_value
         head_type = ord(buf[0]) & 0x7
@@ -101,4 +106,3 @@ class plain(object):
         if head_type == 3:
             return 4 + ord(buf[1])
         return def_value
-