Secure attention key
A secure attention key (SAK) is a special key or key combination to be pressed on a computer keyboard before a login screen must be trusted by a user. The operating system kernel, which interacts directly with the hardware, is able to detect whether the secure attention key has been pressed. When this event is detected, the kernel starts the trusted login processing.
The secure attention key is designed to make login spoofing impossible, as the kernel will suspend any program, including those masquerading as the computer's login process, before starting a trustable login operation.
Users must always be suspicious of login prompts that are displayed, without having pressed this key combination.
Some examples are:
- Ctrl+Alt+Del for Windows NT-based systems (called Secure Attention Sequence) 
- Ctrl+Alt+Pause or the SysRq+K sequence for Linux 
- Ctrl+X Ctrl+R for AIX
- Break for OpenVMS
- ⇧ Shift+Stop for PLATO IV in the 1970s.
Use by BIOS 
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