How to encrypt VM box via VBoxManage?

I've created VM box via Vagrantfile, however when I'm trying to encrypt the storage, I've got the following error:

$ VBoxManage encryptmedium aaa-bbb-ccc-ddd-eee --newpassword - --newpasswordid my_useame --cipher "AES-XTS256-PLAIN64"
Enter new password:
0%...
Progress state: VBOX_E_NOT_SUPPORTED
VBoxManage: error: Encrypt hard disk operation for this cipher is not implemented yet!

The same happening when not specifying any cipher.

Why this doesn't work? And how do I encrypt the VM in a proper way?

Additional info:

$ VBoxManage --version
5.0.14r105127

$ VBoxManage showvminfo secure | grep 'SATA.*UUID'
SATAController (0, 0): ~/VirtualBox VMs/secure/box-disk1.vmdk (UUID: aaa-bbb-ccc-ddd-eee)