Trusted Platform Module (TPM) is an international standard for a secure cryptoprocessor, which is a dedicated microprocessor designed to secure hardware by integrating cryptographic keys into devices.
TPM is naturally supported only on devices that have TPM hardware support. If your hardware has TPM support but it is not showing up, it might need to be enabled in the BIOS settings.
There are two very different TPM specifications: 2.0 and 1.2, which also use different software stacks. This article only describes TPM 2.0, for the older TPM 1.2 see /1.2.
TPM 2.0 allows direct access via
/dev/tpm0 (one client at a time), kernel-managed access via
/dev/tpmrm0, or managed access through the resource manager daemon. According to a systemd project member, using is no longer recommended. There are two choices of userspace tools, by Intel and AUR by IBM.
TPM 2.0 requires UEFI boot; BIOS or Legacy boot systems can only use TPM 1.2.
Some TPM chips can be switched between 2.0 and 1.2 through a firmware upgrade (which can be done only a limited number of times).
Many informative resources to learn how to configure and make use of TPM 2.0 services in daily applications are available from the tpm2-software community.
A TPM 2.0 chip has been a requirement for computers certified to run Windows 10 since 2016-07-28. Linux has support for TPM 2.0 since version 3.20 and should not require any other steps to be enabled on a default Arch install.
Two ways to verify whether TPM 2.0 is setup without specific software:
- checking the logs, e.g., by running
journalctl -k --grep=tpmas root
- read the value of
Data-at-rest encryption with LUKS
Using either method, an encrypted volume or volumes may be unlocked using keys stored in a TPM, either automatically at boot or manually at a later time. Using a TPM for this purpose ensures that your drives will not unlock unless certain conditions are met, such as your firmware not having been modified and Secure Boot not having been disabled (see #Accessing PCR registers).
- This means that access to data is protected in case only the encrypted disk is stolen, but not in case the whole PC gets stolen.
- Be aware that this method makes you more vulnerable to cold boot attacks, because even if your computer has been powered off for a long time (ensuring the memory is completely cleared), an attacker could simply turn it on and wait for the TPM to load the key automatically. This may be a concern for high-value targets.
For TPM sealed SSH keys, there are two options:
- ssh-tpm-agent — ssh-agent compatible agent using TPM backed keys.
- tpm2-pkcs11 — PKCS#11 interface for Trusted Platform Module 2.0 hardware.
- https://github.com/tpm2-software/tpm2-pkcs11 ||
- See SSH configuration and Using a TPM for SSH authentication (2020-01).
Other good examples of TPM 2.0 usage
- Configuring Secure Boot + TPM 2 (2018-06, Debian)
- Using the TPM - It's Not Rocket Science (Anymore) - Johannes Holland & Peter Huewe (2020-11, Youtube): examples for OpenSSL with
Accessing PCR registers
Platform Configuration Registers (PCR) contain hashes that can be read at any time but can only be written via the extend operation, which depends on the previous hash value, thus making a sort of blockchain. They are intended to be used for platform hardware and software integrity checking between boots (e.g. protection against Evil Maid attack). They can be used to unlock encryption keys and proving that the correct OS was booted.
The TCG PC Client Specific Platform Firmware Profile Specification defines the registers in use, and The Linux TPM PCR Registry assigns Linux system components using them.
The registers are:
|Core System Firmware executable code (aka Firmware)
|May change if you upgrade your UEFI
|Core System Firmware data (aka UEFI settings)
|Extended or pluggable executable code
|Extended or pluggable firmware data
|Set during Boot Device Select UEFI boot phase
|Boot Manager Code and Boot Attempts
|Measures the boot manager and the devices that the firmware tried to boot from
|Boot Manager Configuration and Data
|Can measure configuration of boot loaders; includes the GPT Partition Table
|Resume from S4 and S5 Power State Events
|Secure Boot State
|Contains the full contents of PK/KEK/db, as well as the specific certificates used to validate each boot application
|Hash of the kernel command line
|Supported by grub and systemd-boot
|Hash of the initrd and EFI Load Options
|Linux measures the initrd and EFI Load Options, essentially the kernel cmdline options.
|Reserved for Future Use
|Hash of the Unified kernel image
|Overridden kernel command line, Credentials
|shim's MokList, MokListX, and MokSBState.
|May be used and reset at any time. May be absent from an official firmware release.
|The OS can set and reset this PCR.
- Use case defined by the OS and might change between various Linux distros and Windows devices.
On Windows, BitLocker uses PCR8-11 (Legacy) or PCR11-14 (UEFI) for its own purposes. Documentation from tianocore.
facilitates this check with a human observer and dedicated trusted device.
The current PCR values can be listed with:
$ systemd-analyze pcrs
Or, alternatively withfrom :
TPM2 LUKS2 unlocking still asking for password
If you followed the instruction described above for automatically unlocking luks2 devices with enrolled keys in a TPM2 hardware module, but still receive a prompt to input a password during the initramfs boot stage. You may need to early load the kernel module (you can obtain its name with
systemd-cryptenroll --tpm2-device=list) that is responsible for handling your specific TPM2 module.