Has anyone tried to use loopback encrypted filesystems on SL 4 yet?

dd if=/dev/urandom of=/randomfile bs=1M count=10
losetup -E 1 /dev/loop0 /randomfile
mke2fs /dev/loop0
mount /dev/loop0 /mnt/efs
...
umount /mnt/efs
losetup -d /dev/loop0

All of that works.  From the man page, -E 1 selects the built-in XOR.  Now I
want to use a more practical (e.g. tougher) encryption.  I tried -e serpent,
etc. after insmod /lib/modules/2.6.9-5.0.5.ELsmp/kernel/crypto/serpent.ko
(running 2.6.9-5.0.5.ELsmp).  The module is loaded after insmod, but losetup
complains:

ioctl: LOOP_SET_STATUS: Invalid argument

with anything but -E 1 or -E 0 (no encryption).  That is, I tried by name (-e
serpent and -e des) and by number (-E #) up to 21 (arbitrary, I got tired).

Thanks for any pointers,
Dan W.