Stefan Mileschin

New Windows PCs come with UEFI firmware and Secure Boot enabled. Secure Boot prevents operating systems from booting unless they’re signed by a key loaded into UEFI — out of the box, only Microsoft-signed software can boot.

Microsoft mandates that PC vendors allow users to disable Secure Boot, so you can disable Secure Boot or add your own custom key to get around this limitation. Secure Boot can’t be disabled on ARM devices running Windows RT.
How Secure Boot Works

PCs that come with Windows 8 and Windows 8.1 include UEFI firmware instead of the traditional BIOS. By default, the machine’s UEFI firmware will only boot boot loaders signed by a key embedded in the UEFI firmware. This feature is known as “Secure Boot” or “Trusted Boot.” On traditional PCs without this security feature, a rootkit could install itself and become the boot loader. The computer’s BIOS would then load the rootkit at boot time, which would boot and load Windows, hiding itself from the operating system and embedding itself at a deep level.

Secure Boot blocks this — the computer will only boot trusted software, so malicious boot loaders won’t be able to infect the system.

On an Intel x86 PC (not ARM PCs), you have control over Secure Boot. You can choose to disable it or even add your own signing key. Organizations could use their own keys to ensure only approved Linux operating systems could boot, for example.

http://www.howtogeek.com/175641/how-...h-secure-boot/