Secure Boot

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Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Need to explain the relationship with Win8 which is already document here. Not sure how to integrate the info without duplication. (Discuss in Talk:Secure Boot#)

For an overview about Secure Boot in Linux see Rodsbooks' Secure Boot article. This article focuses on how to set up Secure Boot in Arch Linux.

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 a whitelist called Machine Owner Key list. 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.

Booting archiso

Tango-view-refresh-red.pngThis article or section is out of date.Tango-view-refresh-red.png

Reason: Since archlinux-2016.06.01-dual.iso, bootx64.efi (PreLoader) and HashTool.efi in archiso are not signed. (Discuss in Talk:Secure Boot#)

Booting the archiso with Secure Boot enabled is possible since the EFI applications PreLoader.efi and HashTool.efi have been added to it. 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.

  • 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

The archiso boots, and you are presented with a shell prompt, automatically logged in as root. To check if the archiso was booted with Secure Boot, use this command:

$ od -An -t u1 /sys/firmware/efi/efivars/SecureBoot-XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX

The characters denoted by XXXX differ from machine to machine. To help with this, you can use tab completion or list the EFI variables.

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

6  0  0  0  1

For a verbose status, another way is to execute:

# bootctl status


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.

Set up PreLoader

Warning: 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 --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).

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 -b N -B
# 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 #Disable Secure Boot.


Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Testing needed. (Discuss in Talk:Secure Boot#shim)

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 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.

Note: 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.

Set up shim

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:

# cp /usr/share/shim-signed/shimx64.efi esp/EFI/BOOT/BOOTX64.efi
# cp /usr/share/shim-signed/mmx64.efi esp/EFI/BOOT/

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 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

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.

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 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.

Remove shim

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

Using your own keys

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: instructions needed, testing too, a subsection on backing up existing keys prior to replacing them should be added (Discuss in Talk:Secure Boot#)
  • It is advised to read Rod Smith's Controlling Secure Boot.
  • You can use cryptboot-efikeys script from cryptbootAUR package for simplified creating keys, enrolling keys, signing bootloader and verifying signatures.
  • Note that cryptbootAUR requires the encrypted /boot partition to be specified in /etc/crypttab before it runs, and if you are using it in combination with sbupdate-gitAUR, sbupdate expects the /boot/efikeys/db.* files created by cryptboot to be capitalized like DB.* unless otherwise configured in /etc/default/sbupdate. Users who do not use systemd to handle encryption may not have anything in their /etc/crypttab file and would need to create an entry.

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 blacklisted EFI binaries.

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

Custom keys

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.

Creating keys

To generate keys, install efitools.

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.
DER format certificates for firmware.
Certificates in EFI Signature List for KeyTool and/or firmware.
Certificates in EFI Signature List with authentication header (i.e. a signed certificate update file) for KeyTool and/or firmware.

Create a GUID for owner identification:

$ uuidgen --random > GUID.txt

Platform key:

$ openssl req -newkey rsa:2048 -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 rm_PK.auth

Key Exchange Key:

$ openssl req -newkey rsa:2048 -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:2048 -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

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.

Signing bootloader and kernel

When Secure Boot is active (i.e. in "User Mode") you will only be able to launch signed binaries, so you need to sign your kernel and boot loader.

Install sbsigntools.

Note: If running sbsign without --output the resulting file will be filename.signed. See sbsign(1) for more information.
# 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
  • To check if a binary is signed and list its signatures use sbverify --list /path/to/binary.
  • You can use sbupdate-gitAUR to automatically sign your kernels on update. This will also take care of embedding the otherwise unprotected initramfs and kernel command line into the signed UEFI image.
  • When using sbupdate-gitAUR, CMDLINE_DEFAULT must be set in /etc/default/sbupdate in order for the .efi image to be bootable.
  • You may want to consider using Direct UEFI boot with efibootmgr after generating the signed .efi file.

Signing kernel with pacman hook

You can also create your own pacman hook to sign kernel on install and updates.

Operation = Install
Operation = Upgrade
Type = Package
Target = linux

Description = Signing Kernel for SecureBoot
When = PostTransaction
Exec = /usr/bin/sbsign --key db.key --cert db.crt --output /boot/vmlinuz-linux /boot/vmlinuz-linux
Depends = sbsigntools

Put 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.

Enroll keys in firmware

Copy all *.cer, *.esl, *.auth to a FAT formatted file system (you can use EFI System Partition).

Launch firmware setup utility or KeyTool and enroll db, KEK and PK certificates.

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

Warning: Enrolling Platform Key sets Secure Boot in "User Mode", so it should be enrolled last.

Using firmware setup utility

Firmwares have various different interfaces, see Replacing Keys Using Your Firmware's Setup Utility for example how to enroll keys.

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.

Dual booting with other operating systems

Microsoft Windows

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Is it possible to boot Windows by signing its bootloader with a custom key? (Discuss in Talk:Secure Boot#)

To dual boot with Windows, you would need to add Microsoft's certificates to the Signature Database. Microsoft has two db certificates:

Microsoft's certificates are in DER format, convert them to PEM format with openssl:

$ openssl x509 -inform DER -outform PEM -in MicWinProPCA2011_2011-10-19.crt -out MicWinProPCA2011_2011-10-19.crt.pem
$ openssl x509 -inform DER -outform PEM -in MicCorUEFCA2011_2011-06-27.crt -out MicCorUEFCA2011_2011-06-27.crt.pem

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

$ cert-to-efi-sig-list -g 77fa9abd-0359-4d32-bd60-28f4e78f784b MicWinProPCA2011_2011-10-19.crt.pem MS_Win_db.esl
$ cert-to-efi-sig-list -g 77fa9abd-0359-4d32-bd60-28f4e78f784b MicCorUEFCA2011_2011-06-27.crt.pem MS_UEFI_db.esl
$ cat MS_Win_db.esl MS_UEFI_db.esl > MS_db.esl

Sign a db 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

Follow #Enroll keys in firmware to add add_MS_db.auth to Signature Database.

Disable Secure Boot

The Secure Boot feature can be disabled via the UEFI firmware interface. 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.

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) 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.

See also