Encrypt a volume and destroy it by its key

short · 35 min · Objective 3.5

Before you start — what this lab needs

A disposable virtual machine. You will create and destroy an encrypted volume on a loop file, which keeps it off real storage, but the cryptsetup commands are destructive if aimed wrongly:

dd if=/dev/zero of=/tmp/crypt.img bs=1M count=128

Task

Create a LUKS-encrypted volume, add a second passphrase, back up the header, and then destroy the data instantly by erasing the key rather than overwriting the disk. Along the way, see why the header backup is not optional.

Steps

Format the loop file as LUKS2 with a passphrase: cryptsetup luksFormat --type luks2 /tmp/crypt.img. Note that LUKS2 uses Argon2, which is memory-hard.
Open it, create a filesystem inside, mount it, and write a recognisable file.
Add a second passphrase to a new key slot with cryptsetup luksAddKey and confirm both open the volume. Read cryptsetup luksDump and find the two populated slots.
Back up the header: cryptsetup luksHeaderBackup /tmp/crypt.img --header-backup-file /root/crypt.header. This is the step people skip.
Simulate header damage: overwrite the first megabyte of the image with dd if=/dev/zero of=/tmp/crypt.img bs=1M count=1 conv=notrunc. Confirm the volume can no longer be opened -- the data is intact but unreachable.
Restore the header from your backup and confirm the volume opens again. State what would have happened without the backup.
Now destroy it properly: cryptsetup luksErase /tmp/crypt.img, which destroys the key slots. Confirm the data is unrecoverable instantly, regardless of medium, and without overwriting 128MB.

Verify

cryptsetup luksDump /tmp/crypt.img | grep -c 'luks2'          # while it exists
cryptsetup open /tmp/crypt.img lab <<<"wrongpass" 2>&1 | grep -qi 'no key' && echo "wrong pass refused"
# after luksErase:
cryptsetup open /tmp/crypt.img lab 2>&1 | grep -qi 'device.*not.*valid\|no.*luks' && echo "keys destroyed"

The final check confirms cryptographic erasure: the ciphertext is still on the disk, but with the key slots gone it is permanently unreadable. That is why encrypting a disk from first use makes its eventual decommissioning a one-command job.

Notes

Step 4 is the one that matters most in practice. The master key lives in the header, encrypted under your passphrase. Corrupt those first few megabytes -- a stray dd, a partition tool writing a new table -- and every byte on the volume is gone, passphrase or not. luksHeaderBackup takes two seconds.