LUKS: How can I add more password slots (or remove/change a password)

Solution 1:

I assume that the partition is /dev/sda3
If you don't know the name you can list all partitions with lsblk -f.

Add a new password slot:
sudo cryptsetup luksAddKey /dev/sda3

• The next free password slot will be used
• First it will ask for a existing valid password, after that you have to input the new password.
  The old password will stay valid, as well.

If you want to add a keyfile (instead of a password) it would be
sudo cryptsetup luksAddKey /dev/sda3 MyKeyFile.txt

Remove a password slot:

• Possibility 1: sudo cryptsetup luksRemoveKey /dev/sda3
  => You have to enter the password which you want to delete (it will automatically find the correct password slot)
• Possibility 2: sudo cryptsetup luksKillSlot /dev/sda3 2
  => This will delete password slot 2 (you have to enter the password of any other password slot, but not of slot 2
  => This works even if you don't know the password of slot 2
• Possibility 3: sudo cryptsetup luksRemoveKey --key-file MyKeyFile.txt /dev/sda3
  => Removes the key file MyKeyFile.txt if it is valid

See how many slots are active:
sudo cryptsetup luksDump /dev/sda3
This will output all settings of luks for this partition.

Test if a password is valid for the partition:
sudo cryptsetup open --verbose --test-passphrase /dev/sda3
Now you can enter a password and it will tell you if it was wrong or to which password slot it belongs.
If you want to test if a KeyFile is valid it works like this:
sudo cryptsetup open --verbose --test-passphrase --key-file MyKeyFile.txt /dev/sda3

Backup the header of a luks partition:
sudo cryptsetup luksHeaderBackup /dev/sda3 --header-backup-file MyLuksHeader.dat
sudo chmod +r MyLuksHeader.dat
It will create a file of 16MB.
This is very important to save. If the header is damaged all data is lost without a backup!

List all encrypted file systems:
sudo cat /etc/crypttab

If you need more help you can type man cryptsetup.