Cryptsetup: do I need to manually losetup an image?

Question

I'm new to dm-crypt and trying to create/use a encrypted image file. I followed the wiki page of dm-crypt and everything works.
But in the wiki: https://wiki.archlinux.org/index.php/Dm … oop_device , an image file is first setup as a loop device using losetup. And accidentally, I find out that cryptsetup seems to be able to handle the image file directly, i.e. it's ok to do "cryptsetup open crypt.img secret". So why the wiki page instructs us to manually losetup the image before running cryptsetup? Is there any benefit of doing so? Or will letting cryptsetup handling image file as device dangerous?
Thank you!

Answer 1

This is how it works.
First,you start with an image file at a path like "/home/ink/crypt.img".
Second,a loop device is attached to the file and you end up with an address like "/dev/loop0".
Third,cryptseup creates an encryption mapper in "/dev/mapper" with an address like "/dev/mapper/crypt.img"
Fourth, the file system in the encryption mapper at "/dev/mapper/luks.img" is mounted and the volume is accessed.
When done,all above steps are taken in reverse order.
First,the file system on the encryption mapper is unmounted.
Second,the encryption mapper is closed.
Third,the loop device is detached.
Fourth,you end up where you started,with a locked volume.
To check if the volume is locked,the place to  look for it is in "/dev/mapper".If its encryption mapper is not there,then the volume is not open.
Its not possible to have an unlocked volume that is not baked by a loop device so if the loop device is gone,then then volume is probably also
locked so you should be safe.
I do not use GNOME so i cant test for you but if the encryption mapper is gone and the loop device is also gone when you unmount the volume,then
your volume is probably safe as somebody else auto does all the mentioned reversed steps for you.