diff --git a/utils/shortcuts.py b/utils/shortcuts.py
index 2f0e1c6..83b9567 100644
--- a/utils/shortcuts.py
+++ b/utils/shortcuts.py
@@ -1,4 +1,5 @@
 # coding=utf-8
+import os
 import hashlib
 import time
 import random
@@ -107,5 +108,6 @@ def paginate(request, query_set, object_serializer=None):
 
 
 def rand_str(length=32):
-    string = hashlib.md5(str(time.time()) + str(random.randrange(1, 987654321234567)) + str(random.randrange(1, 987654321234567))).hexdigest()
-    return string[0:length]
\ No newline at end of file
+    if length > 128:
+        raise ValueError("length must <= 128")
+    return hashlib.sha512(os.urandom(128)).hexdigest()[0:length]
\ No newline at end of file