http://bugs.gentoo.org/501370
http://bugzilla.xfce.org/show_bug.cgi?id=10679

--- a/scripts/xflock4
+++ b/scripts/xflock4
@@ -35,7 +35,8 @@
 # else run another access locking utility, if installed
 for lock_cmd in \
   "xlock -mode blank" \
-  "slock"
+  "slock" \
+  "alock -bg blank -auth pam"
   do
     set -- $lock_cmd
     if command -v -- $1 >/dev/null 2>&1; then