diff --git a/test/test_onion.py b/test/test_onion.py
index c631f5e08..a09983fac 100644
--- a/test/test_onion.py
+++ b/test/test_onion.py
@@ -174,8 +174,8 @@ class Onion(object):
 
         enckey = cls.tweak_sha(sec, b'\x00')[:16]
         hmac   = cls.tweak_sha(sec, b'\x01')
-        iv     = cls.tweak_sha(sec, b'\x02')[:16]
-        pad_iv = cls.tweak_sha(sec, b'\x02')[16:]
+        ivs    = cls.tweak_sha(sec, b'\x02')
+        iv, pad_iv = ivs[:16], ivs[16:]
 
         return enckey, hmac, iv, pad_iv