Goal: have an encrypted loop device containing a btrfs filesystem. Be able to extend this filesystem.

Create a file to be used as the encrypted container

dd if=/dev/zero of=/data/dsk1 bs=1M count=500

Assign a loop device

losetup /dev/loop1 /data/dsk1

Crypt the device :

cryptsetup -c aes --verify-passphrase create dsk1 /dev/loop1

Format filesystem encrypted in the loop device

mkfs.btrfs /dev/mapper/dsk1

Now we can mount it

mount -t btrfs /dev/mapper/dsk1 /mnt/tmp

umount /mnt/tmp
cryptsetup remove dsk1
losetup -d /dev/loop1 

losetup /dev/loop1 /data/dsk1
cryptsetup create dsk1 /dev/loop1
mount /dev/mapper/dsk1 /mnt/tmp

Get size :

df -h|grep dsk1
/dev/mapper/dsk1      500M   56K  408M   1% /mnt/tmp

Make the file bigger

dd if=/dev/zero of=/data/dsk1 bs=1M count=300 conv=notrunc oflag=append

Resize the btrfs file system

btrfs filesystem resize max /mnt/tmp

Here we are :

df -h|grep dsk1
/dev/mapper/dsk1      800M   56K  708M   1% /mnt/tmp

Encrypted filesystem resized mounted.