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

#Please edit
my_crypt_name=sda5_crypt

cryptsetup luksOpen /dev/sda5 ${my_crpyt_name}

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 sda5_crypt vg--whatever--root  vg--whatever--swap

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

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

# Chroot
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 ${my_root}/*
sudo umount /dev/mapper/${my_vg}
sudo vgchange -a n
sudo cryptsetup luksClose ${my_crpyt}