diff --git a/shadowsocks/obfsplugin/verify_simple.py b/shadowsocks/obfsplugin/verify_simple.py
index 6b90a5a..517de6d 100644
--- a/shadowsocks/obfsplugin/verify_simple.py
+++ b/shadowsocks/obfsplugin/verify_simple.py
@@ -110,6 +110,17 @@ class verify_base(plain.plain):
             return self.sub_obfs.server_decode(buf)
         return (buf, True, False)
 
+    def get_head_size(self, buf, def_value):
+        if len(buf) < 2:
+            return def_value
+        if ord(buf[0]) == 1:
+            return 7
+        if ord(buf[0]) == 4:
+            return 19
+        if ord(buf[0]) == 3:
+            return 4 + ord(buf[1])
+        return def_value
+
 class verify_simple(verify_base):
     def __init__(self, method):
         super(verify_simple, self).__init__(method)
@@ -422,7 +433,8 @@ class auth_simple(verify_base):
     def client_pre_encrypt(self, buf):
         ret = b''
         if not self.has_sent_header:
-            datalen = min(len(buf), common.ord(os.urandom(1)[0]) % 32 + 4)
+            head_size = self.get_head_size(buf, 30)
+            datalen = min(len(buf), random.randint(0, 31) + head_size)
             ret += self.pack_data(self.auth_data() + buf[:datalen])
             buf = buf[datalen:]
             self.has_sent_header = True