Unified Extensible Firmware Interface/Secure Boot

From ArchWiki

Secure Boot is a security feature found in the UEFI standard, designed to add a layer of protection to the pre-boot process: by maintaining a cryptographically signed list of binaries authorized or forbidden to run at boot, it helps in improving the confidence that the machine core boot components (boot manager, kernel, initramfs) have not been tampered with.

As such it can be seen as a continuation or complement to the efforts in securing one's computing environment, reducing the attack surface that other software security solutions such as system encryption cannot easily cover, while being totally distinct and not dependent on them. Secure Boot just stands on its own as a component of current security practices, with its own set of pros and cons.

Note: For a deeper overview about Secure Boot in Linux, see Rodsbooks' Secure Boot article and other online resources. This article focuses on how to set up Secure Boot in Arch Linux.

Checking Secure Boot status

Before booting the OS

At this point, one has to look at the firmware setup. If the machine was booted and is running, in most cases it will have to be rebooted.

You may access the firmware configuration by pressing a special key during the boot process. The key to use depends on the firmware. It is usually one of Esc, F2, Del or possibly another Fn key. Sometimes the right key is displayed for a short while at the beginning of the boot process. The motherboard manual usually records it. You might want to press the key, and keep pressing it, immediately following powering on the machine, even before the screen actually displays anything.

After entering the firmware setup, be careful not to change any settings without prior intention. Usually there are navigation instructions, and short help for the settings, at the bottom of each setup screen. The setup itself might be composed of several pages. You will have to navigate to the correct place. The interesting setting might be simply denoted by secure boot, which can be set on or off.

After booting the OS

An easy way to check Secure Boot status on systems using systemd is to use systemd-boot:

Note: There is no need to be using systemd-boot as your boot manager for this command to work, it is more akin to the others *ctl systemd utilities (localectl, timedatectl...) and will not touch your configuration.
$ bootctl
      Firmware: UEFI 2.80 (American Megatrends 5.26)
 Firmware Arch: x64
   Secure Boot: enabled (user)
  TPM2 Support: yes
  Measured UKI: yes
  Boot into FW: supported

Here we see that Secure Boot is enabled and enforced (in user mode); other values are disabled (setup) for Setup Mode, disabled (disabled) if Secure Boot is disabled and disabled (unsupported) if the firmware does not feature Secure Boot.

Another way to check whether the machine was booted with Secure Boot is to use this command:

$ od --address-radix=n --format=u1 /sys/firmware/efi/efivars/SecureBoot-8be4df61-93ca-11d2-aa0d-00e098032b8c

If Secure Boot is enabled, this command returns 1 as the final integer in a list of five, for example:

6  0  0  0  1

Note, however, that the kernel may be unaware of Secure Boot (even if it is enabled in the firmware) if an insufficiently capable boot loader is used. This can be verified by checking the kernel messages shortly after the system starts up:

# dmesg | grep -i secure
[    0.013442] Secure boot disabled
[    0.013442] Secure boot could not be determined

The kernel messages will otherwise read Secure boot enabled.

Booting an installation medium

The official installation image does not support Secure Boot (FS#53864). Secure Boot support was initially added in archlinux-2013.07.01-dual.iso and later removed in archlinux-2016.06.01-dual.iso. At that time prebootloader was replaced with efitools, even though the latter uses unsigned EFI binaries. There has been no support for Secure Boot in the official installation medium ever since.

In order to boot an installation medium in a Secure Boot system, you will need to either disable Secure Boot or modify the image in order to add a signed boot loader.

Archboot images provide a way to use secure boot on installation media.

Disabling Secure Boot

The Secure Boot feature can be disabled via the UEFI firmware interface. How to access the firmware configuration is described in #Before booting the OS.

If using a hotkey did not work and you can boot Windows, you can force a reboot into the firmware configuration in the following way (for Windows 10): Settings > Update & Security > Recovery > Advanced startup (Restart now) > Troubleshoot > Advanced options > UEFI Firmware settings > restart.

Note that some motherboards (this is the case in a Packard Bell laptop and recent Xiaomi laptops) only allow to disable secure boot if you have set an administrator password (that can be removed afterwards). See also Rod Smith's Disabling Secure Boot.

Repacking the installation image

See #ISO repacking.

Editing the installation medium

If you are using a USB flash installation medium, then it is possible to manually edit the EFI system partition on the medium to add support for Secure Boot.

Plug in the USB drive, then mount the partition:

# mount /dev/disk/by-label/ARCHISO_EFI /mnt

Then follow the instructions in #Using a signed boot loader to install any signed boot loader. For example, to install #PreLoader:

# mv /mnt/EFI/BOOT/BOOTx64.EFI /mnt/EFI/BOOT/loader.efi
# cp /usr/share/preloader-signed/PreLoader.efi /mnt/EFI/BOOT/BOOTx64.EFI
# cp /usr/share/preloader-signed/HashTool.efi /mnt/EFI/BOOT/HashTool.efi

Implementing Secure Boot

There are certain conditions making for an ideal setup of Secure boot:

  1. UEFI considered mostly trusted (despite having some well known criticisms and vulnerabilities[1]) and necessarily protected by a strong password
  2. Default manufacturer/third party keys are not in use, as they have been shown to weaken the security model of Secure Boot by a great margin[2]
  3. UEFI directly loads a user-signed EFISTUB-compatible unified kernel image (no boot manager), including microcode (if applicable) and initramfs so as to maintain throughout the boot process the chain of trust established by Secure Boot and reduce the attack surface
  4. Use of full drive encryption, so that the tools and files involved in the kernel image creation and signing process cannot be accessed and tampered with by someone having physical access to the machine.
  5. Some further improvements may be obtained by using a TPM, although tooling and support makes this harder to implement.

A simple and fully self-reliant setup is described in #Using your own keys, while #Using a signed boot loader makes use of intermediate tools signed by a third-party.

Using the GRUB bootloader requires extra steps before enabling secure boot, see GRUB#Secure Boot support for details.

Using your own keys

Warning: Replacing the platform keys with your own can end up bricking hardware on some machines, including laptops, making it impossible to get into the firmware settings to rectify the situation. This is due to the fact that some device (e.g GPU) firmware (OpROMs), that get executed during boot, are signed using Microsoft 3rd Party UEFI CA certificate.

Secure Boot implementations use these keys:

Platform Key (PK)
Top-level key.
Key Exchange Key (KEK)
Keys used to sign Signatures Database and Forbidden Signatures Database updates.
Signature Database (db)
Contains keys and/or hashes of allowed EFI binaries.
Forbidden Signatures Database (dbx)
Contains keys and/or hashes of denylisted EFI binaries.

See The Meaning of all the UEFI Keys for a more detailed explanation.

To use Secure Boot you need at least PK, KEK and db keys. While you can add multiple KEK, db and dbx certificates, only one Platform Key is allowed.

Once Secure Boot is in "User Mode" keys can only be updated by signing the update (using sign-efi-sig-list) with a higher level key. Platform key can be signed by itself.

Backing up current variables

Before creating new keys and modifying EFI variables, it is advisable to backup the current variables, so that they may be restored in case of error.

Install the efitools package, then run the following commands to backup all four of the principal Secure Boot variables:

$ for var in PK KEK db dbx ; do efi-readvar -v $var -o old_${var}.esl ; done

If you perform this command on a new computer or motherboard, the variables you extract will most likely be the ones provided by Microsoft.

Putting firmware in "Setup Mode"

Secure Boot is in Setup Mode when the Platform Key is removed. To put firmware in Setup Mode, enter firmware setup utility and find an option to delete or clear certificates. How to enter the setup utility is described in #Before booting the OS.

Assisted process with sbctl

sbctl is a user-friendly way of setting up secure boot and signing files.

Note: sbctl does not work with all hardware. How well it will work depends on the manufacturer.

To use it, install sbctl. See also the upstream README and sbctl(8).

Creating and enrolling keys

Before starting, go to your firmware settings and set secure boot mode to Setup mode. This is different for each device: see sbctl(8) § USAGE.

Once you log back in, check the secure boot status:

$ sbctl status

You should see that sbctl is not installed and secure boot is disabled.

Then create your custom secure boot keys:

# sbctl create-keys

Enroll your keys, with Microsoft's keys, to the UEFI:

# sbctl enroll-keys -m
Warning: Some firmware is signed and verified with Microsoft's keys when secure boot is enabled. Not validating devices could brick them. To enroll your keys without enrolling Microsoft's, run: sbctl enroll-keys. Only do this if you know what you are doing.

Check the secure boot status again:

$ sbctl status

sbctl should be installed now, but secure boot will not work until the boot files have been signed with the keys you just created.


Check what files need to be signed for secure boot to work:

# sbctl verify

Now sign all the unsigned files. Usually the kernel and the boot loader need to be signed. For example:

# sbctl sign -s /boot/vmlinuz-linux
# sbctl sign -s /boot/EFI/BOOT/BOOTX64.EFI

The files that need to be signed will depend on your system's layout, kernel and boot loader.

Tip: Especially if you are dual-booting with Windows, there may be a lot of files that need to be signed. The process of signing all needed files using sbctl can be done with sed:
# sbctl verify | sed 's/✗ /sbctl sign -s /e'

This example assumes that the outputted file paths are relative to /boot.

Now you are done! Reboot your system and turn secure boot back on in the firmware settings. If the boot loader and OS load, secure boot should be working. Check with:

$ sbctl status
Automatic signing with the pacman hook

sbctl comes with a pacman hook that automatically signs all new files whenever the Linux kernel, systemd or the boot loader is updated.

Tip: If you use Systemd-boot and systemd-boot-update.service, the boot loader is only updated after a reboot, and the sbctl pacman hook will therefore not sign the new file. As a workaround, it can be useful to sign the boot loader directly in /usr/lib/, as bootctl install and update will automatically recognize and copy .efi.signed files to the ESP if present, instead of the normal .efi file. See bootctl(1).
# sbctl sign -s -o /usr/lib/systemd/boot/efi/systemd-bootx64.efi.signed /usr/lib/systemd/boot/efi/systemd-bootx64.efi

Manual process

Install efitools

Nearly all of the following sections require you to install the efitools package.

Creating keys

To generate keys, perform the following steps.

You will need private keys and certificates in multiple formats:

PEM format private keys for EFI binary and EFI signature list signing.
PEM format certificates for sbsign(1), sbvarsign(1) and sign-efi-sig-list(1).
DER format certificates for firmware.
Certificates in an EFI Signature List for sbvarsign(1), efi-updatevar(1), KeyTool and firmware.
Certificates in an EFI Signature List with an authentication header (i.e. a signed certificate update file) for efi-updatevar(1), sbkeysync, KeyTool and firmware.

Create a GUID for owner identification:

$ uuidgen --random > GUID.txt

Platform key:

$ openssl req -newkey rsa:4096 -nodes -keyout PK.key -new -x509 -sha256 -days 3650 -subj "/CN=my Platform Key/" -out PK.crt
$ openssl x509 -outform DER -in PK.crt -out PK.cer
$ cert-to-efi-sig-list -g "$(< GUID.txt)" PK.crt PK.esl
$ sign-efi-sig-list -g "$(< GUID.txt)" -k PK.key -c PK.crt PK PK.esl PK.auth

Sign an empty file to allow removing Platform Key when in "User Mode":

$ sign-efi-sig-list -g "$(< GUID.txt)" -c PK.crt -k PK.key PK /dev/null noPK.auth

Key Exchange Key:

$ openssl req -newkey rsa:4096 -nodes -keyout KEK.key -new -x509 -sha256 -days 3650 -subj "/CN=my Key Exchange Key/" -out KEK.crt
$ openssl x509 -outform DER -in KEK.crt -out KEK.cer
$ cert-to-efi-sig-list -g "$(< GUID.txt)" KEK.crt KEK.esl
$ sign-efi-sig-list -g "$(< GUID.txt)" -k PK.key -c PK.crt KEK KEK.esl KEK.auth

Signature Database key:

$ openssl req -newkey rsa:4096 -nodes -keyout db.key -new -x509 -sha256 -days 3650 -subj "/CN=my Signature Database key/" -out db.crt
$ openssl x509 -outform DER -in db.crt -out db.cer
$ cert-to-efi-sig-list -g "$(< GUID.txt)" db.crt db.esl
$ sign-efi-sig-list -g "$(< GUID.txt)" -k KEK.key -c KEK.crt db db.esl db.auth
Helper script

A helper/convenience script is offered by the author of the reference page on this topic[3] (requires python). A mildly edited version is also packaged as sbkeysAUR.

In order to use it, simply create a folder in a secure location (e.g. /etc/efi-keys/ if later use of sbupdate-gitAUR to automate unified kernel image creation and signing is planned) and run it:

# mkdir /etc/efi-keys
# cd !$
# curl -L -O https://www.rodsbooks.com/efi-bootloaders/mkkeys.sh
# chmod +x mkkeys.sh
# ./mkkeys.sh
Enter a Common Name to embed in the keys, e.g. "Secure Boot"

This will produce the required files in different formats.

Enrolling keys in firmware

Use one of the following methods to enroll db, KEK and PK certificates.

Tip: As the dbx (forbidden signatures db) is empty, it can be safely left out in the following instructions.
Warning: Enrolling Platform Key sets Secure Boot in "User Mode", leaving "Setup Mode", so it should be enrolled last in sequence.
Using sbkeysync

Install sbsigntools. Create a directory /etc/secureboot/keys with the following directory structure -

├── db
├── dbx
├── KEK
└── PK

For example using:

# mkdir -p /etc/secureboot/keys/{db,dbx,KEK,PK}

Then copy each of the .auth files that were generated earlier into their respective locations (for example, PK.auth into /etc/secureboot/keys/PK and so on).

If you want to verify the changes sbkeysync will make to the system's UEFI keystore, use:

# sbkeysync --keystore /etc/secureboot/keys --pk --dry-run --verbose

Finally, use sbkeysync to enroll your keys.

# sbkeysync --keystore /etc/secureboot/keys --verbose
# sbkeysync --keystore /etc/secureboot/keys --verbose --pk
  • If sbkeysync returns write errors, first run chattr -i /sys/firmware/efi/efivars/{PK,KEK,db}* prior to issuing commands with sbkeysync to temporarily change file attributes, enabling writing of the EFI keys within the efivars directory. See chattr(1).
  • If you get a permission denied error for PK.auth, you can enroll it with command efi-updatevar -f /etc/secureboot/keys/PK/PK.auth PK.
  • If this fails, it might be due to the firmware locking BIOS settings. On Dell & Lenovo systems, you may need to reset the BIOS password: Sysfs Firmware Authentication Documentation

On Lenovo systems:

# cat > /sys/class/firmware-attributes/thinklmi/authentication/Admin/current_password 

On Dell systems:

# cat > /sys/class/firmware-attributes/dell-wmi-sysman/authentication/Admin/current_password 
and try again.

On next boot the UEFI should be back in User Mode and enforcing Secure Boot policy.

Using firmware setup utility

Copy all *.cer, *.esl, *.auth files (except the noPK.auth file!) to a FAT formatted file system (you can use EFI system partition).

Warning: Do not copy the noPK.auth file to the EFI system partition (ESP) of your PC! If you do this, and someone e.g. steals your PC, this person can delete the personal Platform Key in the UEFI Secure Boot firmware again, turn on "Setup Mode" on your PC again and replace your Secure Boot Keys (PK, KEK, db, dbx) with their own Platform Key, thereby defeating the whole purpose of UEFI Secure Boot. Only you should be able to replace the Platform Key, so only you should have access to the noPK.auth file. Therefore keep the noPK.auth file in a secret, safe place where only you have access to. A safe place for the noPK.auth file are:

The EFI system partition of your PC must not be encrypted according to the UEFI specifications and can be mounted and read on another PC (if your PC is stolen and if the hard drive is taken out and connected to another PC). Copying the noPK.auth file to the ESP of your PC and deleting it afterwards is also not advisable, because deleted files on the FAT32 EFI system partition can be recovered with tools like PhotoRec.

Launch firmware setup utility and enroll db, KEK and PK certificates. Firmwares have various different interfaces, see Replacing Keys Using Your Firmware's Setup Utility for example how to enroll keys.

If the used tool supports it prefer using .auth and .esl over .cer.

Using KeyTool

KeyTool.efi is in efitools package, copy it to ESP. To use it after enrolling keys, sign it with sbsign.

# sbsign --key db.key --cert db.crt --output esp/KeyTool-signed.efi /usr/share/efitools/efi/KeyTool.efi

Launch KeyTool-signed.efi using firmware setup utility, boot loader or UEFI Shell and enroll keys.

See Replacing Keys Using KeyTool for explanation of KeyTool menu options.

Using systemd-boot

Since systemd version 252, systemd-boot can be used to enroll keys. To enroll keys, copy the db.auth, KEK.auth and PK.auth to the special folder on the ESP:

# cp db.auth KEK.auth PK.auth esp/loader/keys/NAME/

Where NAME can be any unique name you assign, such as MYKEYS.

After copying the keys and enabling the secure boot setup mode a new entry would appear in the boot menu that would read Enroll Secure Boot keys: MYKEYS. Activating this entry would enroll the secure boot keys.

Signing EFI binaries

When Secure Boot is active (i.e. in "User Mode"), only signed EFI binaries (e.g. applications, drivers, unified kernel images) can be launched.

With sbsigntools

Install sbsigntools to sign EFI binaries with sbsign(1).

  • To check if a binary is signed and list its signatures use sbverify --list /path/to/binary.
  • The rEFInd boot manager's refind-install script can sign rEFInd EFI binaries and copy them together with the db certificates to the ESP. See rEFInd#Using your own keys for instructions.
Note: If running sbsign without --output the resulting file will be filename.signed. See sbsign(1) for more information.

To sign your kernel and boot manager use sbsign, e.g.:

# sbsign --key db.key --cert db.crt --output /boot/vmlinuz-linux /boot/vmlinuz-linux
# sbsign --key db.key --cert db.crt --output esp/EFI/BOOT/BOOTx64.EFI esp/EFI/BOOT/BOOTx64.EFI
Warning: Signing kernel only will not protect the initramfs from tampering. See Unified kernel image to know how to produce a combined image that you can then manually sign with sbsign.
Signing the kernel with a mkinitcpio post hook

You can also use a mkinitcpio post hook to sign the kernel when the initramfs gets generated.

Create the following file an make it executable:

#!/usr/bin/env bash

[[ -n "$kernel" ]] || exit 0

# use already installed kernel if it exists

keypairs=(/path/to/db.key /path/to/db.crt)

for (( i=0; i<${#keypairs[@]}; i+=2 )); do
    key="${keypairs[$i]}" cert="${keypairs[(( i + 1 ))]}"
    if ! sbverify --cert "$cert" "$kernel" &>/dev/null; then
        sbsign --key "$key" --cert "$cert" --output "$kernel" "$kernel"

Replace /path/to/db.key and /path/to/db.crt with the paths to the key pair you want to use for signing the kernel.

If you are using systemd-boot, there is a dedicated pacman hook doing this task semi-automatically.

Signing unified kernel images with a mkinitcpio post hook

See Unified kernel image#Signing the UKIs for Secure Boot.

Fully automated unified kernel generation and signing with sbupdate
Note: sbupdate was created long before mkinitcpio and other initramfs generators acquired the ability to generate unified kernel images, providing much appreciated ease to this formerly cumbersome process. Nowadays, consider using instead the #Assisted process with sbctl.

sbupdate is a tool made specifically to automate unified kernel image generation and signing on Arch Linux. It handles installation, removal and updates of kernels through pacman hooks.

Install sbupdate-gitAUR and configure it following the instructions given on the project's homepage.[4]

Tip: If using systemd-boot, set OUT_DIR="EFI/Linux" to get your signed kernel images directly recognized without needing configuration. See systemd-boot(7) § FILES and systemd-boot#Adding loaders.

Once configured, simply run sbupdate as root for first-time image generation.

Note: sbupdate output often contains errors such as warning: data remaining[26413568 vs 26423180]: gaps between PE/COFF sections?. Those are harmless and can be safely ignored.[5]

Updating keys

Once Secure Boot is in "User Mode" any changes to KEK, db and dbx need to be signed with a higher level key.

For example, if you wanted to replace your db key with a new one:

  1. Create the new key,
  2. Convert it to EFI Signature List,
  3. Sign the EFI Signature List,
  4. Enroll the signed certificate update file.
$ cert-to-efi-sig-list -g "$(< GUID.txt)" new_db.crt new_db.esl
$ sign-efi-sig-list -g "$(< GUID.txt)" -k KEK.key -c KEK.crt db new_db.esl new_db.auth

If instead of replacing your db key, you want to add another one to the Signature Database, you need to use the option -a (see sign-efi-sig-list(1)):

$ sign-efi-sig-list -a -g "$(< GUID.txt)" -k KEK.key -c KEK.crt db new_db.esl new_db.auth

When new_db.auth is created, enroll it.

Dual booting with other operating systems

Microsoft Windows

This article or section needs expansion.

Reason: Is it possible to boot Windows by signing its bootloader with a custom key? (Discuss in Talk:Unified Extensible Firmware Interface/Secure Boot#Booting Windows with custom bootloader signature)

It is usually not possible to boot Windows by signing its bootloader (EFI/Microsoft/Boot/bootmgfw.efi) with a custom, personal key with Secure Boot Mode enabled, without enrolling the "Microsoft Windows Production PCA 2011" key in the UEFI Secure Boot variables:

  • if bootmgfw.efi contains a signature both from the "Microsoft Windows Production PCA 2011" and from your own Secure Boot db key (so two signatures), then UEFI firmware implementations like INSYDE Corp. 4096.01 (UEFI Version 2.31, Version F.70, Release Date: 07/18/2016, BIOS Revision 15.112, Firmware Revision: 29.66) will not launch bootmgfw.efi and will throw a security violation error (Selected boot image did not authenticate. Press ENTER to continue.): UEFI firmware implementation like this can probably only read the first signature - not the second one. Only the certificate for the second signature is enrolled in the UEFI Secure Boot variables, so the Secure Boot verification fails.
  • if the "Microsoft Windows Production PCA 2011" signature from the bootmgfw.efi file is stripped/removed, and only a signature from your own Secure Boot db key is added to the file, then UEFI will launch the file - but Windows will launch a recovery/repair environment: Windows complains that the Windows installation is broken (because the "Microsoft Windows Production PCA 2011" signature on bootmgfw.efi file is missing).

So to dual boot with Windows,

Create EFI Signature Lists from Microsoft's DER format db certificates using Microsoft's GUID (77fa9abd-0359-4d32-bd60-28f4e78f784b) and combine them in one file for simplicity:

$ sbsiglist --owner 77fa9abd-0359-4d32-bd60-28f4e78f784b --type x509 --output MS_Win_db_2011.esl MicWinProPCA2011_2011-10-19.crt
$ sbsiglist --owner 77fa9abd-0359-4d32-bd60-28f4e78f784b --type x509 --output MS_Win_db_2023.esl 'windows uefi ca 2023.crt'
$ sbsiglist --owner 77fa9abd-0359-4d32-bd60-28f4e78f784b --type x509 --output MS_UEFI_db_2011.esl MicCorUEFCA2011_2011-06-27.crt
$ sbsiglist --owner 77fa9abd-0359-4d32-bd60-28f4e78f784b --type x509 --output MS_UEFI_db_2023.esl 'microsoft uefi ca 2023.crt'
$ cat MS_Win_db_2011.esl MS_Win_db_2023.esl MS_UEFI_db_2011.esl MS_UEFI_db_2023.esl > MS_db.esl

Optional (for strict conformity with Microsoft UEFI Secure Boot requirements): Create an EFI Signature List from Microsoft's DER format KEK certificates using Microsoft's GUID (77fa9abd-0359-4d32-bd60-28f4e78f784b):

$ sbsiglist --owner 77fa9abd-0359-4d32-bd60-28f4e78f784b --type x509 --output MS_Win_KEK_2011.esl MicCorKEKCA2011_2011-06-24.crt
$ sbsiglist --owner 77fa9abd-0359-4d32-bd60-28f4e78f784b --type x509 --output MS_Win_KEK_2023.esl 'microsoft corporation kek 2k ca 2023.crt'
$ cat MS_Win_KEK_2011.esl MS_Win_KEK_2023.esl > MS_Win_KEK.esl

Sign a db variable update with your KEK. Use sign-efi-sig-list with option -a to add not replace a db certificate:

$ sign-efi-sig-list -a -g 77fa9abd-0359-4d32-bd60-28f4e78f784b -k KEK.key -c KEK.crt db MS_db.esl add_MS_db.auth

Optional (for strict conformity with Microsoft UEFI Secure Boot requirements): Sign a KEK variable update with your PK. Use sign-efi-sig-list with option -a to add not replace a KEK certificate:

$ sign-efi-sig-list -a -g 77fa9abd-0359-4d32-bd60-28f4e78f784b -k PK.key -c PK.crt KEK MS_Win_KEK.esl add_MS_Win_KEK.auth

Follow #Enrolling keys in firmware to enroll add_MS_db.auth and for strict conformity with Microsoft UEFI Secure Boot requirements add_MS_Win_KEK.auth into the UEFI Secure Boot Database variables.

Using a signed boot loader

Using a signed boot loader means using a boot loader signed with Microsoft's key. There are two known signed boot loaders: PreLoader and shim. Their purpose is to chainload other EFI binaries (usually boot loaders). Since Microsoft would never sign a boot loader that automatically launches any unsigned binary, PreLoader and shim use an allowlist called Machine Owner Key list, abbreviated MokList. If the SHA256 hash of the binary (Preloader and shim) or key the binary is signed with (shim) is in the MokList they execute it, if not they launch a key management utility which allows enrolling the hash or key.

Warning: What Microsoft calls "Secured-core PCs" do not ship with Microsoft 3rd Party UEFI CA certificate (Microsoft Corporation UEFI CA 2011 or Microsoft UEFI CA 2023) enrolled.[6]. The only enrolled DB certificate is the Microsoft Windows Production PCA 2011 certificate which is exclusively used to sign the Windows boot loader.

The enrollment of the Microsoft 3rd Party UEFI CA certificate needs to be enabled in firmware settings to launch EFI binaries and OpROMs signed with this certificate.


When run, PreLoader tries to launch loader.efi. If the hash of loader.efi is not in MokList, PreLoader will launch HashTool.efi. In HashTool you must enroll the hash of the EFI binaries you want to launch, that means your boot loader (loader.efi) and kernel.

Note: Each time you update any of the binaries (e.g. boot loader or kernel) you will need to enroll their new hash.
Tip: The rEFInd boot manager's refind-install script can copy the rEFInd and PreLoader EFI binaries to the ESP. See rEFInd#Using PreLoader for instructions.
Set up PreLoader
Note: PreLoader.efi and HashTool.efi in efitools package are not signed, so their usefulness is limited. You can get a signed PreLoader.efi and HashTool.efi from preloader-signedAUR or download them manually.

Install preloader-signedAUR and copy PreLoader.efi and HashTool.efi to the boot loader directory; for systemd-boot use:

# cp /usr/share/preloader-signed/{PreLoader,HashTool}.efi esp/EFI/systemd

Now copy over the boot loader binary and rename it to loader.efi; for systemd-boot use:

# cp esp/EFI/systemd/systemd-bootx64.efi esp/EFI/systemd/loader.efi

Finally, create a new NVRAM entry to boot PreLoader.efi:

# efibootmgr --unicode --disk /dev/sdX --part Y --create --label "PreLoader" --loader /EFI/systemd/PreLoader.efi

Replace X with the drive letter and replace Y with the partition number of the EFI system partition.

This entry should be added to the list as the first to boot; check with the efibootmgr command and adjust the boot-order if necessary.


If there are problems booting the custom NVRAM entry, copy HashTool.efi and loader.efi to the default loader location booted automatically by UEFI systems:

# cp /usr/share/preloader-signed/HashTool.efi esp/EFI/BOOT/
# cp esp/EFI/systemd/systemd-bootx64.efi esp/EFI/BOOT/loader.efi

Copy over PreLoader.efi and rename it:

# cp /usr/share/preloader-signed/PreLoader.efi esp/EFI/BOOT/BOOTx64.EFI

For particularly intransigent UEFI implementations, copy PreLoader.efi to the default loader location used by Windows systems:

# mkdir -p esp/EFI/Microsoft/Boot
# cp /usr/share/preloader-signed/PreLoader.efi esp/EFI/Microsoft/Boot/bootmgfw.efi
Note: If dual-booting with Windows, backup the original bootmgfw.efi first as replacing it may cause problems with Windows updates.

As before, copy HashTool.efi and loader.efi to esp/EFI/Microsoft/Boot/.

When the system starts with Secure Boot enabled, follow the steps above to enroll loader.efi and /vmlinuz-linux (or whichever kernel image is being used).

How to use while booting?

A message will show up that says Failed to Start loader... I will now execute HashTool. To use HashTool for enrolling the hash of loader.efi and vmlinuz.efi, follow these steps. These steps assume titles for a remastered archiso installation media. The exact titles you will get depends on your boot loader setup.

  • Select OK
  • In the HashTool main menu, select Enroll Hash, choose \loader.efi and confirm with Yes. Again, select Enroll Hash and archiso to enter the archiso directory, then select vmlinuz.efi and confirm with Yes. Then choose Exit to return to the boot device selection menu.
  • In the boot device selection menu choose Arch Linux archiso x86_64 UEFI CD
Remove PreLoader
Note: Since you are going to remove stuff, is a good idea to backup it.

Uninstall preloader-signedAUR and simply remove the copied files and revert configuration; for systemd-boot use:

# rm esp/EFI/systemd/{PreLoader,HashTool}.efi
# rm esp/EFI/systemd/loader.efi
# efibootmgr --unicode --bootnum N --delete-bootnum
# bootctl update

Where N is the NVRAM boot entry created for booting PreLoader.efi. Check with the efibootmgr command and adjust the boot-order if necessary.

Note: The above commands cover the easiest case; if you have created, copied, renamed or edited further files probably you have to handle with them, too. If PreLoader was your operational boot entry, you obviously also need to #Disabling Secure Boot.
Delete enrolled hash

Every entry of hashes enrolled in the MOK database eats up a little piece of space of NVRAM. You may want to delete useless hashes to free the space and to prevent outdated programs from booting.

Install efitools and copy KeyTool.efi:

# cp /usr/share/efitools/efi/KeyTool.efi esp/EFI/systemd/KeyTool.efi

Manage to boot to Preloader and you will see the KeyTool entry. You can then edit hashes in the MOK database.


This article or section needs expansion.

When run, shim tries to launch grubx64.efi. If MokList does not contain the hash of grubx64.efi or the key it is signed with, shim will launch MokManager (mmx64.efi). In MokManager you must enroll the hash of the EFI binaries you want to launch (your boot loader (grubx64.efi) and kernel) or enroll the key they are signed with.

  • If you use #shim with hash, each time you update any of the binaries (e.g. boot loader or kernel) you will need to enroll their new hash.
  • Since version 15.3, shim will not launch EFI binaries without a valid .sbat section. Run objdump -j .sbat -s /path/to/binary.efi to verify if an EFI binary has it. See the SBAT documentation for details.
  • If you are not actually interested in the security brought by Secure Boot and are only enabling it to meet the requirements posed by Windows 11, you may want to consider disabling the validation process in shim with mokutil --disable-validation. In that case you will not need to sign grub (sbat probably still needed) or the kernel images and at the same time be able to boot Windows with chainloader in grub.
Set up shim
Tip: The rEFInd boot manager's refind-install script can sign rEFInd EFI binaries and copy them along with shim and the MOK certificates to the ESP. See rEFInd#Using shim for instructions.

Install shim-signedAUR.

Rename your current boot loader to grubx64.efi

# mv esp/EFI/BOOT/BOOTx64.EFI esp/EFI/BOOT/grubx64.efi

Copy shim and MokManager to your boot loader directory on ESP; use previous filename of your boot loader as as the filename for shimx64.efi:

Note: Make sure you do not copy fbx64.efi (which is under the same directory) unless you actually have a valid bootx64.csv to use. Otherwise shim will not execute grubx64.efi but will appear to fail to work and just reset the machine.
# cp /usr/share/shim-signed/shimx64.efi esp/EFI/BOOT/BOOTx64.EFI
# cp /usr/share/shim-signed/mmx64.efi esp/EFI/BOOT/

Finally, create a new NVRAM entry to boot BOOTx64.EFI:

# efibootmgr --unicode --disk /dev/sdX --part Y --create --label "Shim" --loader /EFI/BOOT/BOOTx64.EFI

shim can authenticate binaries by Machine Owner Key or hash stored in MokList.

Machine Owner Key (MOK)
A key that a user generates and uses to sign EFI binaries.
A SHA256 hash of an EFI binary.

Using hash is simpler, but each time you update your boot loader or kernel you will need to add their hashes in MokManager. With MOK you only need to add the key once, but you will have to sign the boot loader and kernel each time it updates.

shim with hash

If shim does not find the SHA256 hash of grubx64.efi in MokList it will launch MokManager (mmx64.efi).

In MokManager select Enroll hash from disk, find grubx64.efi and add it to MokList. Repeat the steps and add your kernel vmlinuz-linux. When done select Continue boot and your boot loader will launch and it will be capable launching the kernel.

shim with key

Install sbsigntools.

You will need:

PEM format private key for EFI binary signing.
PEM format certificate for sbsign.
DER format certificate for MokManager.

Create a Machine Owner Key:

$ openssl req -newkey rsa:2048 -nodes -keyout MOK.key -new -x509 -sha256 -days 3650 -subj "/CN=my Machine Owner Key/" -out MOK.crt
$ openssl x509 -outform DER -in MOK.crt -out MOK.cer
Note: It seems that shim will not support adding a 4096 RSA key to the MokList (it might freeze when loading and verifying the grubx64.efi binary), so make sure you use a 2048 key for now. See Debian:SecureBoot#Generating a new key.

Sign your boot loader (named grubx64.efi) and kernel:

# sbsign --key MOK.key --cert MOK.crt --output /boot/vmlinuz-linux /boot/vmlinuz-linux
# sbsign --key MOK.key --cert MOK.crt --output esp/EFI/BOOT/grubx64.efi esp/EFI/BOOT/grubx64.efi

You will need to do this each time they are updated. You can automate the kernel signing with a mkinitcpio post hook. Create the following file and make it executable:

#!/usr/bin/env bash

[[ -n "$kernel" ]] || exit 0

# use already installed kernel if it exists

keypairs=(/path/to/MOK.key /path/to/MOK.crt)

for (( i=0; i<${#keypairs[@]}; i+=2 )); do
    key="${keypairs[$i]}" cert="${keypairs[(( i + 1 ))]}"
    if ! sbverify --cert "$cert" "$kernel" &>/dev/null; then
        sbsign --key "$key" --cert "$cert" --output "$kernel" "$kernel"

Copy MOK.cer to a FAT formatted file system (you can use EFI system partition).

Reboot and enable Secure Boot. If shim does not find the certificate grubx64.efi is signed with in MokList it will launch MokManager (mmx64.efi).

In MokManager select Enroll key from disk, find MOK.cer and add it to MokList. When done select Continue boot and your boot loader will launch and it will be capable launching any binary signed with your Machine Owner Key.

shim with key and GRUB

See GRUB#Shim-lock for instructions.

Delete enrolled hash/key

Every entry of hash/key enrolled in the MOK database eats up a little piece of space of NVRAM. You may want to delete useless hash/key to free the space and to prevent outdated programs from booting.

MOK database can be managed with mokutil

List enrolled keys & hashes

# mokutil --list-enrolled

Remove hash from database. The password entered here will be asked for confirmation to delete in MOK manager.

# mokutil --delete-hash <hash to remove from above command>
Input password:

Remove key from database

# mokutil --delete MOK.cer

List hashes/keys to be deleted on next reboot

# mokutil --list-delete

On next reboot, MOK manager will be initiated with option to Enroll/Delete hashes/keys. More details mokutil(1)

Remove shim

Uninstall shim-signedAUR, remove the copied shim and MokManager files and rename back your boot loader.

Protecting Secure Boot

The only way to prevent anyone with physical access from disabling Secure Boot is to protect the firmware settings with a password. Most UEFI firmwares provide such a feature, usually listed under the "Security" section in the firmware settings.

Consider enabling kernel lockdown mode. See [7].

Tips and tricks

ISO repacking

It is possible to unpack and repack the official installation image using libisoburn and mtools. This way, you can create an image that supports Secure Boot, either with custom keys or with a signed boot loader.

This article or section needs expansion.

Reason: Add instructions for repacking the ISO to include shim. (Discuss in Talk:Unified Extensible Firmware Interface/Secure Boot)

Sign the official ISO with custom keys

Support for Secure Boot using custom keys can be added to the official ISO by simply extracting the boot loader (BOOTx64.EFI and BOOTIA32.EFI), kernel, UEFI shell, signing them and then repacking the ISO with the signed files.

First extract the relevant files and El Torito boot images:

$ osirrox -indev archlinux-YYYY.MM.DD-x86_64.iso \
	-extract_boot_images ./ \
	-cpx /arch/boot/x86_64/vmlinuz-linux \
	/shellx64.efi \
	/shellia32.efi ./

xorrisofs(1) option -rational-rock as used by mkarchiso makes the files on ISO 9660 read-only which persists after extracting them. Make the files writable so that they can be modified:

$ chmod +w BOOTx64.EFI BOOTIA32.EFI shellx64.efi shellia32.efi vmlinuz-linux

Sign the files. To do so with sbsigntools, for example:

$ sbsign --key db.key --cert db.crt --output BOOTx64.EFI BOOTx64.EFI
$ sbsign --key db.key --cert db.crt --output BOOTIA32.EFI BOOTIA32.EFI
$ sbsign --key db.key --cert db.crt --output shellx64.efi shellx64.efi
$ sbsign --key db.key --cert db.crt --output shellia32.efi shellia32.efi
$ sbsign --key db.key --cert db.crt --output vmlinuz-linux vmlinuz-linux

Copy the boot loader and UEFI shell to eltorito_img2_uefi.img. It will be used as the EFI system partition and will be listed as an El Torito UEFI boot image. The size of eltorito_img2_uefi.img is fixed, but there is 1 MiB free space added by mkarchiso for the purposes of rounding/alignment and to account for reserved sectors, so the size increase from the signatures should not be an issue.

$ mcopy -D oO -i eltorito_img2_uefi.img BOOTx64.EFI BOOTIA32.EFI ::/EFI/BOOT/
$ mcopy -D oO -i eltorito_img2_uefi.img shellx64.efi shellia32.efi ::/

Repack the ISO using the modified El Torito UEFI boot image and add the signed boot loader files, UEFI shell and kernel to ISO 9660:

$ xorriso -indev archlinux-YYYY.MM.DD-x86_64.iso \
	-outdev archlinux-YYYY.MM.DD-x86_64-Secure_Boot.iso \
	-map vmlinuz-linux /arch/boot/x86_64/vmlinuz-linux \
	-map_l ./ /EFI/BOOT/ BOOTx64.EFI BOOTIA32.EFI -- \
	-map_l ./ / shellx64.efi shellia32.efi -- \
	-boot_image any replay \
	-append_partition 2 0xef eltorito_img2_uefi.img

Boot the resulting archlinux-YYYY.MM.DD-x86_64-Secure_Boot.iso.

Replacing the boot loader with PreLoader

Another way to add Secure Boot support to the official ISO is by extracting the boot loader and replacing it with #PreLoader.

First, extract the boot loader and the El Torito boot image:

$ osirrox -indev archlinux-YYYY.MM.DD-x86_64.iso \
	-extract_boot_images ./ \
	-cpx /EFI/BOOT/BOOTx64.EFI ./

Replace the BOOTx64.efi file with PreLoader:

$ mv BOOTx64.EFI loader.efi
$ cp /usr/share/preloader-signed/PreLoader.efi BOOTx64.EFI
$ cp /usr/share/preloader-signed/HashTool.efi HashTool.efi

Add the new files to the boot image:

$ mcopy -D oO -i eltorito_img2_uefi.img BOOTx64.EFI loader.efi HashTool.efi ::/EFI/BOOT/

Finally, repack the ISO using the modified boot image and the new boot loader files:

$ xorriso -indev archlinux-YYYY.MM.DD-x86_64.iso \
	-outdev archlinux-YYYY.MM.DD-x86_64-Secure_Boot.iso \
	-map_l ./ /EFI/BOOT/ BOOTx64.EFI loader.efi HashTool.efi -- \
	-boot_image any replay \
	-append_partition 2 0xef eltorito_img2_uefi.img

Enrolling Option ROM digests

Option ROMs (OpROMs), i.e. device firmware that is executed during boot, must be signed for Secure Boot otherwise the devices will not be initialized. Typically OpROMs are signed with the Microsoft 3rd Party UEFI CA certificate which may prevent implementing Secure Boot with only your own keys. To solve this, the SHA256 digests of OpROMs can be enrolled instead.

Warning: Preventing essential devices from initializing by removing the Microsoft 3rd Party UEFI CA certificate can end up bricking hardware on some machines, including laptops, making it impossible to get into the firmware settings to rectify the situation.

On systems with a TPM, it is possible to acquire Option ROM SHA256 digests from the TPM event log.

Install tpm2-tools and digest-to-efi-sig-listAUR.

Use tpm2_eventlog(1) to read /sys/kernel/security/tpm0/binary_bios_measurements and look for the digests of BOOT_SERVICES_DRIVER.[8]

For example:

# tpm2_eventlog /sys/kernel/security/tpm0/binary_bios_measurements
- EventNum: 9
  PCRIndex: 2
  DigestCount: 1
  - AlgorithmId: sha256
    Digest: "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"
- EventNum: 10
  PCRIndex: 2
  DigestCount: 1
  - AlgorithmId: sha256
    Digest: "fedcba9876543210fedcba9876543210fedcba9876543210fedcba9876543210"

Use digest-to-efi-sig-list to create an EFI signature list for each OpROM digest you find:

$ digest-to-efi-sig-list 0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef OpROM1.esl
$ digest-to-efi-sig-list fedcba9876543210fedcba9876543210fedcba9876543210fedcba9876543210 OpROM2.esl

In case you have multiple OpROMs, combine their EFI signature lists into one so that it could be signed as a single file:

$ cat OpROM1.esl OpROM2.esl > OpROM.esl

Sign the EFI signature list for appending to the Signature Database:

$ sign-efi-sig-list -a -g "$(< GUID.txt)" -k KEK.key -c KEK.crt db OpROM.esl OpROM.auth

Enroll it.

The final and most dangerous step is to remove Microsoft 3rd Party UEFI CA certificate from the Signature Database and see if the system still boots and all devices still work.

See also