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.

Secure Boot is a security feature of modern motherboards, which can protect boot manager, kernel and initramfs from tampering: e.g. from installing a keylogger or bootkit able to steal your LUKS master key. Minimal configuration in which using Secure boot worth time spent:

1. UEFI considered trusted, despite it still can have backdoors or being vulnerable to Evil Maid able to flash your hardware
2. User must not use 3rd party keys, especially from Microsoft (there is at least one known boot manager, signed by Microsoft, able to load anything [1])
3. UEFI loads EFISTUB with both kernel+initram signed by user Image Signing Key, so no one can tamper it
4. Kernel mounts encrypted root and home filesystems (LVM on LUKS), so no one can tamper them, read mentioned private keys, password hashes or user sensitive data

In future, Evil Maid attack can be potentially prevented by using a TPM.

Secure Boot status

Check the 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

To check if the machine was booted with Secure Boot, use this command:

$ od --address-radix=n --format=u1 /sys/firmware/efi/efivars/SecureBoot*

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

For a verbose status, another way is to execute:

$ bootctl status

Change the status

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, 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.

PreLoader

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-signed^AUR or download them manually.

Install preloader-signed^AUR 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 --verbose --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.

Fallback

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-signed^AUR 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 --verbose --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 #Disable 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 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.

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

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-signed^AUR.

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/

Finally, create a new NVRAM entry to boot BOOTX64.efi:

# efibootmgr --verbose --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.

hash

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:

.key

PEM format private key for EFI binary signing.

.crt

PEM format certificate for sbsign.

.cer

DER format certificate for MokManager.

Create a Machine Owner Key:

$ openssl req -newkey rsa:4096 -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. You can automate the kernel signing with a pacman hook, e.g.:

/etc/pacman.d/hooks/999-sign_kernel_for_secureboot.hook

[Trigger]
Operation = Install
Operation = Upgrade
Type = Package
Target = linux
Target = linux-lts
Target = linux-hardened
Target = linux-zen

[Action]
Description = Signing kernel with Machine Owner Key for Secure Boot
When = PostTransaction
Exec = /usr/bin/find /boot/ -maxdepth 1 -name 'vmlinuz-*' -exec /usr/bin/sh -c 'if ! /usr/bin/sbverify --list {} 2>/dev/null | /usr/bin/grep -q "signature certificates"; then /usr/bin/sbsign --key MOK.key --cert MOK.crt --output {} {}; fi' ;
Depends = sbsigntools
Depends = findutils
Depends = grep

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

Warning: With secureboot active GRUB can't chainload EFI binaries even if they are signed.

For signing you can for example use the grub2-signing extension: [2]

There is also a package in the aur: grub2-signing-extension^AUR

Run gpg --gen-key as root to create a keypair.

If you get a permission denied error try:

# chown root:root $(tty)
# export GPG_TTY"="$(tty)

Activate the gpg-agent:

 /root/.gnupg/gpg.conf 

 use-agent 

 /root/.gnupg/gpg-agent.conf 

pinentry-program /usr/bin/pinentry
no-grab
default-cache-ttl 1800

Export your public key:

gpg --export -o ~/pubkey

Mount your boot partition. (Re)install GRUB2:

# grub-install --target=x86_64-efi --efi-directory=esp --bootloader-id=GRUB -k /root/pubkey --modules="gcry_sha256 gcry_dsa gcry_rsa"

Copy your publickey to your boot partiton.

cp /root/pubkey /boot/pubkey

Edit your GRUB custom config and add:

/etc/grub.d/40_Custom

insmod verify
insmod gcry_sha256
insmod gcry_dsa gcry_rsa
set check_signatures=enforce
trust (crypto0)/pubkey
insmod shim_lock

Tip: (crypt0) is the name of the partition in GRUB.

Rebuild your grub config:

grub-mkconfig > /boot/grub/grub.cfg

Ensure that you created MOK.key and signed your kernel and grubx64.efi like described in shim with key.

Sign the grub files: grub-sign

Run grub-verify and check if there are errors.

Here is a simple unsign hook:

10-unsign-grub-before-update.hook

[Trigger]
Operation = Install
Operation = Upgrade
Operation = Remove
Type = Package
Target = linux
Target = linux-lts
Target = linux-zen
Target = grub
Target = intel-ucode
Target = amd-ucode

[Action]
Description = Unsigning GRUB
When = PreTransaction
Exec = /usr/bin/grub-unsign

And a bash script you can use to sign again after the update.

 .bashrc

function sign-update () {
   /usr/bin/grub-unsign
   if [ $? -ne 0 ]; then return 1; fi
   /usr/bin/find /boot/ -maxdepth 1 -name 'vmlinuz-*' -not -name "*.sig" -exec /usr/bin/sh -c \
   'if ! /usr/bin/sbverify --list {} 2>/dev/null | /usr/bin/grep -q "signature certificates"; \
   then echo "Signing with sbsign."; /usr/bin/sbsign --key MOK.key --cert MOK.crt --output {} {}; fi' \;
   /usr/bin/grub-sign
   /usr/bin/find /boot/ -maxdepth 1 -name 'vmlinuz-*' -not -name "*.sig" -exec /usr/bin/sh -c \
   'echo ""; echo {}; /usr/bin/sbverify --list {};' \;
}

Remove shim

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

Using your own keys

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 and perform the following steps. As an alternative, install and run sbkeys^AUR to generate a new set of keys automatically.

You will need private keys and certificates in multiple formats:

.key

PEM format private keys for EFI binary and EFI signature list signing.

.crt

PEM format certificates for sbsign(1), sbvarsign(1) and sign-efi-sig-list(1).

.cer

DER format certificates for firmware.

.esl

Certificates in an EFI Signature List for sbvarsign(1), efi-updatevar(1), KeyTool and firmware.

.auth

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 rm_PK.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

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

Note: If running sbsign without --output the resulting file will be filename.signed. See sbsign(1) for more information.

Signing the kernel and boot manager

Warning: Signing kernel only will not protect the initramfs from tampering.

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

Signing the kernel with a pacman hook

You can also use mkinitcpio's pacman hook to sign the kernel on install and updates.

Copy /usr/share/libalpm/hooks/90-mkinitcpio-install.hook to /etc/pacman.d/hooks/90-mkinitcpio-install.hook and /usr/share/libalpm/scripts/mkinitcpio-install to /usr/local/share/libalpm/scripts/mkinitcpio-install.

In /etc/pacman.d/hooks/90-mkinitcpio-install.hook, replace:

Exec = /usr/share/libalpm/scripts/mkinitcpio-install

with:

Exec = /usr/local/share/libalpm/scripts/mkinitcpio-install

In /usr/local/share/libalpm/scripts/mkinitcpio-install, replace:

install -Dm644 "${line}" "/boot/vmlinuz-${pkgbase}"

with:

sbsign --key /path/to/db.key --cert /path/to/db.crt --output "/boot/vmlinuz-${pkgbase}" "${line}"

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

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. How to enter the setup utility is described in #Before booting the OS.

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", 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 -

/etc/secureboot/keys
├── db
├── dbx
├── KEK
└── PK

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). Finally use sbkeysync to enroll your keys.

# sbkeysync --verbose
# sbkeysync --verbose --pk

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

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

• Microsoft Windows Production PCA 2011 for Windows
• Microsoft Corporation UEFI CA 2011 for third-party binaries like UEFI drivers, option ROMs etc.

Create EFI Signature Lists from Microsoft's DER format 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.esl MicWinProPCA2011_2011-10-19.crt
$ sbsiglist --owner 77fa9abd-0359-4d32-bd60-28f4e78f784b --type x509 --output MS_UEFI_db.esl MicCorUEFCA2011_2011-06-27.crt
$ 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. 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) 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.

Booting an installation medium

Note: The official installation image does not support Secure Boot (FS#53864). To successfully boot the installation medium you will need to disable Secure Boot.

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.

Remastering the installation image

One might want to remaster the Install ISO in a way described by previous topics of this article. For example, the signed EFI applications PreLoader.efi and HashTool.efi from #PreLoader can be adopted to here. Another option would be to borrow the bootx64.efi (shim) and grubx64.efi from installation media of another GNU+Linux distribution that supports Secure Boot and modify the GRUB configuration to one's needs. In this case, the authentication chain of Secure Boot in said distribution's installation media should end to the grubx64.efi ( for example Ubuntu) so that GRUB would boot the unsigned kernel and initramfs from archiso. Note that up to this point, the article assumed one can access the ESP of the machine. But when installing a machine that never had an OS before, there is no ESP present. You should explore other articles, for example Unified Extensible Firmware Interface#Create UEFI bootable USB from ISO, to learn how this situation should be handled.

Platform configuration registers

Platform configuration registers (PCRs) are 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 can be used to unlock encryption keys and proving that the correct OS was booted.

PCR	Use	Notes
PCR0	Core System Firmware executable code (aka Firmware)	May change if you upgrade your UEFI
PCR1	Core System Firmware data (aka UEFI settings)
PCR2	Extended or pluggable executable code
PCR3	Extended or pluggable firmware data	Set during Boot Device Select UEFI boot phase
PCR4	Boot Manager
PCR5	GPT / Partition Table
PCR6	Resume from S4 and S5 Power State Events
PCR7	Secure Boot State
PCR 8 to 10	Reserved for Future Use
PCR11	BitLocker Access Control
PCR12	Data events and highly volatile events
PCR13	Boot Module Details
PCR14	Boot Authorities
PCR 15 to 23	Reserved for Future Use

See also

• Wikipedia:Unified Extensible Firmware Interface#Secure boot
• Dealing with Secure Boot by Rod Smith
• Controlling Secure Boot by Rod Smith
• UEFI secure booting (part 2) by Matthew Garrett
• UEFI Secure Boot by James Bottomley
• efitools README
• Will your computer's "Secure Boot" turn out to be "Restricted Boot"? — Free Software Foundation
• Free Software Foundation recommendations for free operating system distributions considering Secure Boot
• Intel's UEFI Secure Boot Tutorial^{[dead link 2020-04-03 ⓘ]}
• Secure Boot, Signed Modules and Signed ELF Binaries
• National Security Agency docs: UEFI Defensive Practices Guidance and unclassified UEFI Secure Boot customization
• sbkeysync & maintaining uefi key databases by Jeremy Kerr