LVM

Install necessary apps

E.g. on a rescue disc ...

apt-get install cryptsetup

Open encryted lvm partition

Be arefulat this step!

If this is the root device, you will need to USE EXACTLY THE SAME NAME FOR THE CRYPT (here: "sda5_crypt") like in your Ubuntu environment.

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

If you are unsure, uncrypt your device, mount it and have a look into the /etc/crypttab for the name like this:

cryptsetup luksOpen /dev/sda5 test_crypt
mount /dev/mapper/vg-somename-root /mnt/test

nano /mnt/test/etc/crypttab
> sda5_crypt UUID=def346a0-6e33-4523-b99c-d7777b980b34 none luks,discard

umount /mnt/test
crpytsetup luksClose test_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

# Please edit
my_root=/mnt/whatever_root
my_vg=vg--whatever--root
my_boot=/dev/sda1

# Mount /root and /boot
mkdir ${my_root}
mount /dev/mapper/${my_vg} ${my_root}
mount ${my_boot} ${my_root}/boot

# Mount 
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