diff --git a/shadowsocks/obfsplugin/verify.py b/shadowsocks/obfsplugin/verify.py
index 770ce29..1c939ae 100644
--- a/shadowsocks/obfsplugin/verify.py
+++ b/shadowsocks/obfsplugin/verify.py
@@ -295,7 +295,7 @@ class verify_sha1(verify_base):
         ret = b''
         if not self.has_sent_header:
             datalen = self.get_head_size(buf, 30)
-            ret += self.pack_auth_data(buf[datalen:])
+            ret += self.pack_auth_data(buf[:datalen])
             buf = buf[datalen:]
             self.has_sent_header = True
         while len(buf) > self.unit_len: