How to determine whether a partition is a LUKS partition from its mountpoint?

I'm aware of being able to use the cryptsetup isLuks command to find out whether a partition is a LUKS container. I want to be able to use this command with the mountpoint of the LUKS container for the case that I cannot be sure that I know the partition's device name, how can I do this in one command?

For example, I have a Luks container at /dev/sda2, I open it with

$ cryptsetup luksOpen /dev/sda2 vault

Then I mount the container with

$ mount /dev/mapper/vault /mountpoint

At this point I would like to know whether /mountpoint is a Luks container

$ cryptsetup isLuks -v /dev/sda2
Command successful.
$ cryptsetup isLuks -v /mountpoint
Command failed with code 15: Block device required
$ cryptsetup isLuks -v /dev/mapper/vault
Command failed with code 22: Device /dev/mapper/vault is not a valid LUKS device.

Normally I'd use lsblk to get the block device of a mountpoint, but this returns the /dev/mapper/vault path.

└─sda2   8:8    0   9.3G  0 part
  └─vault 252:0    0   9.3G  0 crypt /mountpoint

If I could resolve /dev/mapper/vault to /dev/sda2 somehow, I would be able to use command substitution inside the cryptsetup isLuks command.

It would be great to find something more elegant than using awk or something to parse the output of mount, but if that's the only way then it works I suppose.

This should work. It's a little long, but making a bash function would simplify it:

cryptsetup isLuks -v `df /mountpoint | tail -n1 | awk '{print $1;}'`

I had the same problem to. Solved like this.

lsblk -sJp | jq -r --arg dsk "/dev/mapper/disk_name" '.blockdevices | .[] | select(.name == $dsk) | .children | .[0] | .name'

Return e.g. /dev/sda2