diff --git a/shell/hardware/keydialog.py b/shell/hardware/keydialog.py
index 6c66006..6b5976c 100644
--- a/shell/hardware/keydialog.py
+++ b/shell/hardware/keydialog.py
@@ -67,7 +67,7 @@ def string_is_ascii(string):
 def string_to_hex(passphrase):
     key = ''
     for c in passphrase:
-        key += hex(ord(c))[2:]
+        key += '%02x' % ord(c)
     return key
 
 def hash_passphrase(passphrase):