Disabling LUKS encryption without backing up and restoring the whole partition

The problem: When installing Ubuntu, Debian or another flavour of Linux you chose to encrypt the hard disk, but now you want to get rid of the password prompt while booting. How do you do this without backing up, reformatting and restoring the entire partition?

The short answer first: There is no way to completely disable LUKS encryption - for that reason, many people in forums etc. recommend to back up the data, recreate partitions (or re-install the system) and then to restore the data.

However, it is possible to disable the password prompt while keeping the encryption in place. This effectively makes the encryption useless because we store the decryption password on the machine in clear text. Still, it is a much easier route than a full backup and restore. Furthermore, you can re-enable encryption easily again at a later time. The extra overhead of de/encrypting data on the drive is negligible these days.

The approach is as follows: Add a trivial password to the LUKS password slots, then instruct the boot process to create the password by printing it from a script during boot.

This is made slightly more complex by the fact that the script with the password must be added to the initramfs, the regular filesystem is not enough. Furthermore, what I only found out after many tries and a lot of headscratching is that empty pass phrases are not allowed in LUKS. This means that specifying /dev/null as the password file (or using /bin/true as the script) does not work. Finally, the setup "helpfully" disallows reading the passphrase from a file directly, we need to write a short script to output it.

Step-by-step instructions:

• Add a new trivial passphrase to the passwords that will unlock your partition(s). My suggestion is the old classic, "password". Using a more obscure password will not give you extra security:
  cryptsetup luksAddKey /dev/sda2
  The sda2 may be different on your system - look at /etc/crypttab to find the correct value for your machine. (If LVM is used, /etc/crypttab may list "sda2_crypt" or similar instead of just "sda2".)
• Ensure that a file containing the passphrase will be available at boot time, by hooking into the initramfs generation. Put into /etc/initramfs-tools/hooks/local-noluksprompt the following lines:
  mkdir -p ${DESTDIR}/root/bin
cat >${DESTDIR}/root/bin/luks-password <<END
#!/bin/sh --
echo -n password
END
chmod +x ${DESTDIR}/root/bin/luks-password
echo "Added cleartext password -- http://atterer.org/linux-remove-disable-luks-encryption-password-on-disk-partition-crypttab-initrd"
• Make the file executable with
  chmod +x /etc/initramfs-tools/hooks/local-noluksprompt
• Specify in /etc/crypttab that LUKS should get the passphrase during boot by executing the "luks-password" script. Do this by changing the last word luks into luks,keyscript=/root/bin/luks-password, so the line in crypttab will look similar to the following:
  sda2_crypt UUID=123456789 none luks,keyscript=/root/bin/luks-password
  Note: More recent versions of update-initramfs seem to be clever enough to make a copy of the script in the initramfs's /lib/cryptsetup/scripts/luks-password, so the extra hook script may not be required.
• Finally, ensure that the initramfs filesystem of your currently running kernel is regenerated - if everything works, the "Added cleartext password" line should be output during the following command:
  update-initramfs -u -k`uname -r`
  I recommend against using the -kall option to update all initramfs-es - that way, if something goes wrong with this procedure you can still unlock the HD by booting an older kernel.

That's all! Should you want to disable this setup at a later point and go back to your previously set password, which still exists in one of the other LUKS password slots, just remove the trivial "password" entry:
cryptsetup luksRemoveKey /dev/sda2