Unlocking LUKS Volumes with TPM2

Unlocking your LUKS volume with a Trusted Platform Module 2.0 (TPM2) provides a secure way to enable automatic decryption during boot, usually eliminating the need to type a passphrase unless the system state changes.

The most common and recommended way to achieve this on modern Linux systems, especially those using LUKS2 and systemd, is by using the systemd-cryptenroll tool.

Prerequisites:

1. TPM2 Chip: Your computer must have an active TPM2 chip. Most modern hardware does, but you may need to enable in UEFI settings.
   
2. LUKS2: Your encrypted volume must be using LUKS2 format.
   • You can check this with: cryptsetup luksDump /dev/your_device
   • If your block device is LUKS1 you may need to convert it. This is a high-risk operation, so back up your data first.
3. Packages: Ensure you have the necessary packages installed.
   • systemd-cryptenroll
   • tpm2-tss
4. Initramfs Support: Your system's initial ramdisk (initramfs) must be configured to include the necessary components to perform the unlock early in the boot process.
   • Initial ramdisk generated by tools like: dracut (Fedora/Arch) and mkinitcpio (Debian/Ubuntu)
   • tpm2-tss and sd-encrypt perform the unlock early in the boot process

Step-by-step Configuration

1. Identify your LUKS device.
   • Find the partition or block device that contains your LUKS volume.
   • You can use lsblk or fdisk -l
   • Example: /dev/nvme0n1p3
2. Enroll the TPM2 key.
   • The systemd-cryptenroll command adds a new random key to one of your LUKS key slots and seals it with the TPM2, binding it to a set of Platform Configuration Registers (PCRs).
   • The PCRs record a cryptographic hash of the boot-time state (firmware, bootloader, kernel, etc.).
   • If an attacker alters the boot chain, the PCR values change, and the key will not be released.
   • Run the enrollment command as root. Replace /dev/your_device with your actual device path. Bash sudo systemd-cryptenroll --tpm2-device=auto --tpm2-pcrs=0+7 /dev/your_device
   • --tpm2-device=auto: Automatically detects the TPM2 device.
   • --tpm2-pcrs=0+7: Specifies the PCRs to bind to.
     • PCR 0 typically covers the firmware/BIOS.
     • PCR 7 covers the Secure Boot state.
   • When prompted, enter an existing passphrase for your LUKS volume to authorize the new key slot.

3. Configure crypttab

   • Edit the /etc/crypttab file to tell the boot process to use the TPM2 device.

   • Find the line for your LUKS volume and append tpm2-device=auto to the options field (the fourth column).

     Before (Example): Bash luks-UUID-HERE UUID=... none luks
     After (Example): Bash luks-UUID-HERE UUID=... none luks,tpm2-device=auto

   • If your encrypted volume contains the root filesystem, you might need to add this option to the kernel command line in your bootloader configuration using a format like rd.luks.options=tpm2-device=auto.

     1. Open /etc/default/grub with a text editor as a superuser. (e.g., using nano or vim) Bash sudo nano /etc/default/grub
     2. Find the line that starts with GRUB_CMDLINE_LINUX_DEFAULT or GRUB_CMDLINE_LINUX.

     3. Append the new option inside the quotation marks, separated by a space from any existing parameters:

        Example (If you only use this option): Bash GRUB_CMDLINE_LINUX="rd.luks.options=tpm2-device=auto" Example (If other options already exist): Bash GRUB_CMDLINE_LINUX="quiet splash rd.luks.options=tpm2-device=auto" Note: Some distributions may require a separate option for the UUID, such as rd.luks.options=UUID-OF-YOUR-LUKS-PARTITION=tpm2-device=auto. Check your distribution's documentation for the exact syntax if the simpler option above doesn't work. I needed to use this syntax on Fedora 42.

     4. Save and close the /etc/default/grub file.

     5. Update the GRUB configuration.

        • The change you made in /etc/default/grub will not take effect until you regenerate the main GRUB configuration file, which is usually located at /boot/grub2/grub.cfg.
        • Run the appropriate command for your distribution:
          • For Debian/Ubuntu use update-grub: Bash sudo update-grub
          • For Fedora/Arch use grub2-mkconfig: Bash sudo grub2-mkconfig -o /boot/grub/grub.cfg

4. Regenerate the initramfs.

   • The boot unlocking happens in the early boot stage (initramfs/initrd), so you must rebuild it to include the new configuration and the necessary TPM modules.
     • For Fedora/RHEL/Arch use dracut command: Bash sudo dracut -f
       
     • For Debian/Ubuntu systems use mkinitcpio command: Bash sudo mkinitcpio -P --- ## Important Notes

• Backup a key: Always keep at least one regular passphrase or a recovery key for your LUKS volume as a backup. If the TPM fails, the UEFI is updated, or your boot configuration changes in a way that alters the PCR values, the TPM will not release the key.
  • To enroll a recovery key: sudo systemd-cryptenroll --recovery-key /dev/your_device

• Wiping the slot: If you update your firmware, kernel, or bootloader and the automatic unlock stops working, you will need to use your backup passphrase and then wipe and re-enroll the TPM key. ```Bash sudo systemd-cryptenroll --wipe-slot=tpm2 --tpm2-device=auto --tpm2-pcrs=0+7 /dev/your_device

  sudo dracut -f # or mkinitcpio -P ```

• Security: This method trades a bit of security for convenience. If an attacker can physically access your machine and modify the non-encrypted boot partition (but not the sealed PCRs), certain "Evil Maid" attacks might be possible.

  • Using a TPM PIN in addition to the PCRs can mitigate some of these risks. This can be done by using the flag --tpm2-with-pin=yes with the enrollment command.

    Example: Bash sudo systemd-cryptenroll --tpm2-device=auto --tpm2-pcrs=0+7 --tpm2-with-pin=yes /dev/your_device