Refer to LTK, and to Linux OS background here.


General interest


Regular booting


The Unified Extensible Firmware Interface (UEFI) defines a software interface between OS and firmware and replaces the Basic Input/Output System (BIOS) firmware interface. UEFI can support remote diagnostics and repair of computers, even with no operating system installed.

Intel developed the original Extensible Firmware Interface (EFI) specifications. The Linux kernel has been able to use EFI at boot time since early 2000s, using the elilo EFI boot loader or, more recently, EFI versions of GRUB.

UEFI defines a boot manager as part of the UEFI specification (it does not rely on boot sectors). When a computer is powered on, the boot manager checks the boot configuration and based on its settings, then executes the specified OS boot loader or operating system kernel. The boot configuration is defined by variables stored in NVRAM, including variables that indicate the file system paths to OS loaders or OS kernels.

UEFI provides a shell environment, which can be used to execute other UEFI applications, including UEFI boot loaders.

DebbyBuster has the InsydeH2O BIOS, the Insyde Software (Taiwan) implementation of the Intel Platform Innovation Framework for UEFI/EFI.


GRUB (GRand Unified Bootloader) can be used to boot most operating system on the intel platforms. There are two versions: Grub Legacy (Grub 1) and Grub 2. What do I have?

Secure booting

UEFI Secure Boot and Shim

The UEFI 2.3.1 Errata C specification (or higher) defines a protocol known as Secure Boot, which can secure the boot process by preventing the loading of UEFI drivers or OS boot loaders that are not signed with an acceptable digital signature. Supported by Debian since Debian 10.

Shim is a boot loader to chain-load signed boot loaders under Secure Boot. Shim becomes the root of trust for all the other distro-provided UEFI programs. It embeds a further distro-specific CA key that is itself used for signing further programs (e.g. Linux, GRUB, fwupdate). This allows for a clean delegation of trust - the distros are then responsible for signing the rest of their packages. Shim itself should ideally not need to be updated very often, reducing the workload on the central auditing and CA teams.

Windows 10 allows OEMs to decide whether or not Secure Boot can be managed by users of their x86 systems. The Machine Owner Key (MOK) allows you to add signed files.

Finding out your status: Installed shim files:

Debian and TPM

See also local files: Debian TPM tools: Do a 'dpkg -L tpm2-tools' to see the files.

Debian's tpm2-tools are based on TrouSerS, a Trusted Computing Software Stack (TSS). Installed:

Linux kernel

Linux distinguishes between: Four interfaces: And then there's the TEE...

Disks, partitions, logical volumes

Networking and network security

Within the Linux kernel

Separate firewall software

User space

Observation: there are other ways to automatically start services. E.g. autostart: Microsoft Teams is started via /home/marc/.config/autostart/teams.

Linux security

Linux kernel security

Linux kernel TEE support




Gramine (Graphene) - Intel SGX

Gramine (ex-Graphene) is a lightweight guest OS, designed to run a single Linux application with minimal host requirements. Graphene can run applications in an isolated environment with benefits comparable to running a complete OS in a virtual machine – including guest customisation, ease of porting to different host OSes, and process migration.

It supports running Linux applications using Intel SGX (Graphene-SGX) where applications are secured in hardware-encrypted memory regions (called SGX enclaves). SGX protects code and data in the enclave against privileged software attacks and against physical attacks on the hardware off the CPU package (e.g., cold-boot attacks on RAM). It is able to run unmodified applications inside SGX enclaves, without the toll of manually porting the application to the SGX environment.

Linux security/subsystem level

Linux kernel security/LSM


Linux kernel security/encryption

Linux kernel crypto API

The kernel crypto API serves the following entity types: Documentation


Device-mapper is infrastructure in the Linux kernel that provides a generic way to create virtual layers of block devices.

Device-mapper crypt (dm-crypt) provides transparent encryption of block devices using the kernel crypto API.

The user can basically specify a symmetric ciphers, an encryption mode, a key, an iv generation mode and then the user can create a new block device in /dev. Writes to this device will be encrypted and reads decrypted. One can mount the filesystem on it as usual or stack dm-crypt device with another device like RAID or LVM volume.


Policy-Based Decryption (PBD) is a collection of technologies that enable unlocking encrypted root and secondary volumes of hard drives on physical and virtual machines. PBD uses a variety of unlocking methods, such as user passwords, a Trusted Platform Module (TPM) device, a PKCS #11 device connected to a system, for example, a smart card, or a special network server.

Network Bound Disc Encryption (NBDE) is a subcategory of PBD that allows binding encrypted volumes to a special network server. Redhat's PBD is based on the Clevis framework, which offers support for tang (network based decryption) and TPM2. Tang is a DH-inspired approach, created by McCallum and Relyea (both Redhat).

Linux kernel security checking

Refer also to cybersecurity.

Integrity checking




Linux security/other

Linux Distributions


Other distributions

D-bus middleware

D-Bus is a message bus system, a simple way for applications to talk to one another. In addition to interprocess communication, D-Bus helps coordinate process lifecycle; it makes it simple and reliable to code a "single instance" application or daemon, and to launch applications and daemons on demand when their services are needed.

D-Bus supplies both a system daemon (for events such as "new hardware device added" or "printer queue changed") and a per-user-login-session daemon (for general IPC needs among user applications). Also, the message bus is built on top of a general one-to-one message passing framework, which can be used by any two apps to communicate directly (without going through the message bus daemon). Currently the communicating applications are on one computer, or through unencrypted TCP/IP suitable for use behind a firewall with shared NFS home directories.


Display servers

Application level encryption

Hardware, support, consulting

Linux printing

Legacy printers need a driver, modern printers are driverless.

CUPS - the Common Unix Printing System

Printers sold in the last 10 years or so are almost always AirPrint devices and therefore would support driverless printing when the device is connected by ethernet or wireless. Additionally, a USB connected modern printer might be capable of driverless printing if it is IPP-over-USB-capable.

Starting from GNOME 3, CUPS printing has been handled in the Settings application. The GUI can add CUPS printers and manage CUPS printers and queues. Before GNOME 3, the GNOME Print Settings (formerly called CUPS Manager) were used to fulfil these tasks.


AirPrint was devised by Apple to enable an iPhone, an iPad, ..., referred to as iOS clients to print without having to install drivers on the client device. More and more new printers come with firmware to support AirPrint. In fact, it would be unusual nowadays for a network-aware printer not to provide AirPrint support. There are two technologies central to the AirPrint facility on a printer: The broadcast mDNS packets contain information about the capabilities of the printer, its identity and its location on the network. They also utilise some extensions (not necessarily fully explained in existing literature) to the existing Bonjour specification to allow iOS clients to search specifically for AirPrint-capable printers and display them in a print dialogue.

IPP (version 2.0) is needed for print management. The client uses IPP to send the print job with information about what printer language it is in, whether it is to be duplexed, the number of copies, the resolution to be used for printing, the media output location on the printer etc.

On Debian, avahi-daemon is essential to detect the Bonjour broadcasts from a printer.

Linux mobile and embedded

Linux media servers and music applications

Linux and language

Raspberry Pi

Backing up a Windows PC