Linux: Full Disk Encryption

Overview

The idea is to encrypt a partition with root filesystem using LUKS and store the keys in the TPM.
During boot user does not have to enter a decryption password, partition will be automatically decrypted using the keys from TPM.
It's a open-source alternative to Windows BitLocker.

• LUKS (Linux Unified Key Setup) - is a full volume encryption feature, the standard for Linux hard disk encryption
• TPM (Trusted Platform Module) - is dedicated micro-controller designed to secure hardware through integrated cryptographic keys

List of tested devices

The guide was tested on a system with the specs listed below, but should be easily adaptable.

Device: fitlet2
OS: Debian GNU/Linux buster
ISO: debian-buster-DI-rc2-amd64-netinst.iso
Kernel: 4.19.0-5-amd64
BIOS: 09/17/2018 American Megatrends Inc. FLT2.0.46.01.00
TPM: Firmware based TPM 2.0 implementation (fTPM)
Clevis: 11-2

Device: fitlet2
OS: Debian GNU/Linux bullseye
ISO: debian-bullseye-DI-rc2-amd64-netinst.iso
Kernel: 5.6.0-1-amd64
BIOS: 09/17/2018 American Megatrends Inc. FLT2.0.46.01.00
TPM: Firmware based TPM 2.0 implementation (fTPM)
Clevis: 13-1

Installation

• Follow the standard installation procedure - choose timezone, hostname, username, password etc.
• In the "Partition disks" dialog you can choose automatic or manual partitioning scheme
  • Automatic - select "Guided - use entire disk and set up encrypted LVM"
    • Partitions scheme will be defined automatically
    • You will be asked to enter passphrase for encryption
  • Manual (advanced) , for example:
    • #1, size 200.0 MB, use as "EFI System Partition"
    • #2, size 300.0 MB, use as "EXT4 journaling file system", mount point /boot
    • #3, size 3.0 GB, use as "physical volume for encryption"
    • Go to the "Configuring encrypted volumes" and select partition #3 as device to be encrypted
    • You will be asked to enter passphrase for encryption
    • In the "Partition disks" find new appeared encrypted volume #1, use it as "EXT4 journaling file system", mount point /
• Continue with package manager, software selection etc.
• Disconnect the installation media and reboot
• You will be asked for passphrase of encrypted disk, enter it manually, then boot process will continue
• Login using previously selected username and password.

• NOTE: The PATH definition in the /etc/profile should be fixed:

# /etc/profile: system-wide .profile file for the Bourne shell (sh(1))
# and Bourne compatible shells (bash(1), ksh(1), ash(1), ...).

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
export PATH
...

Automated Decryption

• Clevis is a framework for automated decryption of data or automated unlocking of LUKS volumes.

$ su
Password:
$ apt install clevis-tpm2 clevis-luks clevis-dracut

• NOTE: current version of clevis-dracut (11-2) requires patch to work correctly in Debian:

--- /usr/lib/dracut/modules.d/60clevis/module-setup.sh.orig	2019-03-01 12:37:24.000000000 +0200
+++ /usr/lib/dracut/modules.d/60clevis/module-setup.sh	2019-07-01 13:47:48.341990143 +0300
@@ -36,9 +36,7 @@
     inst_hook initqueue/settled 60 "$moddir/clevis-hook.sh"
 
     inst_multiple /etc/services \
-        clevis-decrypt-http \
-        clevis-decrypt-tang \
-        clevis-decrypt-sss \
+	cryptsetup \
         /usr/lib/x86_64-linux-gnu/clevis-luks-askpass \
         clevis-decrypt \
         luksmeta \
@@ -46,11 +44,16 @@
         mktemp \
         curl \
         jose \
-        socat
+        socat \
+	/usr/lib/x86_64-linux-gnu/libtss2-tcti-device.so.0 \
+	/usr/lib/x86_64-linux-gnu/libtss2-tcti-device.so.0.0.0 \
+	/usr/lib/x86_64-linux-gnu/libtss2-tcti-mssim.so.0 \
+	/usr/lib/x86_64-linux-gnu/libtss2-tcti-mssim.so.0.0.0
 
     for cmd in clevis-decrypt-tpm2 \
 	tpm2_createprimary \
 	tpm2_unseal \
+	tpm2_pcrlist \
 	tpm2_load; do
 
 	if ! find_binary "$cmd" &>/dev/null; then
@@ -62,6 +65,7 @@
 	inst_multiple clevis-decrypt-tpm2 \
 	    tpm2_createprimary \
 	    tpm2_unseal \
+	    tpm2_pcrlist \
 	    tpm2_load
     fi

• Download and apply the patch:

$ wget https://fit-iot.com/files/download/fitlet2/sw/tools/Linux-ftpm2.0/module-setup.sh.diff
$ patch -p1 /usr/lib/dracut/modules.d/60clevis/module-setup.sh module-setup.sh.diff

• Regenerate the iniramfs image with clevis modules to decrypt the root filesystem during early boot. You need to do this once:

$ dracut -f /boot/initrd.img-$(uname -r)

• Test the TPM encryption module, the following example encrypts the words “Hello World!” and writes them to test.txt. Give it a try!

$ echo Hello World! | clevis encrypt tpm2 '{}' > test.txt
$ cat test.txt
eyJhbGciOiJkaXIiLCJjbGV2aXMiOnsicGluIjoidHBtMiIsInRwbTIiOnsiaGFzaCI6InNoYTI
...
$ cat test.txt | clevis decrypt tpm2
Hello World!

• Bind master key generated by TPM to the LUKS volume. For this there is a specific set of Platform Configuration Registers (PCR) called PCR7.

# Review information about the cryptographic setup of encrypted partition:
$ cryptsetup luksDump /dev/sda3

$ clevis luks bind -d /dev/sda3 tpm2 '{"pcr_ids":"7"}'
Enter existing LUKS password: ******

# Review information about the cryptographic setup again, check if the new key has been written to the LUKS volume:
$ cryptsetup luksDump /dev/sda3

• Restart your machine, encrypted disk will be decrypted automatically and you will reach the login prompt

$ reboot

See also

• Original article: TPM Encryption in Fedora