[dm-crypt] Attaching loopback device failed

Hello,

I have both luks headerand key in files

# ls
LUKS-HEADER  key

the directory is read-only (chattr -R +i .)

When I try to luksOpen my paretition, I get following error:

  # cryptsetup --allow-discards luksOpen /dev/sda2 sda2 --header
LUKS-HEADER --key-file key
  Attaching loopback device failed (loop device with autoclear flag is
required).
  Device LUKS-HEADER doesn't exist or access denied.

when I change the current directory to read-write (chattr -R -i .) then
it works.

But why does cryptsetup need luks header and key to be writable ?

thanks,