howto mount an encrypted partition as user?

[h.engemann at basicartstudios.de]

Hi,
i sat up an encrypted partition and now I wanna use it as a normal user.
I use this line in /etc/fstab

/dev/sdc1               /mnt/crypto             ext3 
loop=/dev/loop0,encryption=serpent,noauto,user,rw 0 0

i can mount it with root previleges with this line:
mount -o user,rw /dev/loop0 crypto/
but the users cant write there.

when i try  mount /mnt/crypto it asks me for a password and and gives me 
an error after that:

ioctl: LOOP_SET_FD:  Ressource is busy (translated from german)

The main goal would be that i can select the encrypted drive in 
nautilus, enter the password and use it like any other drive too.
I never done this before.

thx
h.e.