LVM

Install necessary apps

E.g. on a rescue disc ...

apt-get install cryptsetup

Open encryted lvm partition

Be careful: if this is the root device, you will need to use EXACTLY the same name for the crpyt (here: sda5_crypt) like in your Ubuntu enviroment. If not, you will not be able to boot your device, because the system will be configured using the wrong name for your mapper:

cryptsetup luksOpen /dev/sda5 sda5_crypt

Mount existing volume groups

sudo apt-get install lvm2
sudo modprobe dm-mod

sudo vgchange -a y
> 2 logical volume(s) in volume group "vg-whatever" now active

ls /dev/mapper
> control vg--whatever--root  vg--whatever--swap

my_root=/mnt/whatever_root
mkdir ${my_root}
mount /dev/mapper/vg--whatever--root ${my_root}
mount /dev/sdax ${my_root}/boot

mount -o bind /dev ${my_root}/dev; \
mount -o bind /run ${my_root}/run; \
mount -t proc /proc ${my_root}/proc; \
mount -t sysfs /sys ${my_root}/sys

chroot ${my_root}

Umount existing volume groups and close encrypted container

sudo umount ${my_root}/*
sudo umount /dev/mapper/whatever
sudo vgchange -a n
sudo cryptsetup luksClose my-crypt