A key combination with a useful history
On the original IBM PC, pressing Ctrl+Alt+Delete triggered a reboot. The combination was implemented below ordinary programs, so software running on the computer could not easily intercept it. When Windows NT introduced a more security-focused sign-in process, it adopted the same sequence for a different job: signaling that the user was ready to enter credentials.
A trusted route to the sign-in screen
That distinction mattered because a program could draw a convincing fake password box over the desktop and try to steal a password. Requiring the user to press Ctrl+Alt+Delete first gave Windows a chance to take control through a special system-level route. Ordinary applications could not simply imitate that key sequence and claim to be the genuine sign-in screen.
The sequence itself does not authenticate anyone or make a weak password stronger. Its value is that it helps ensure the password is being entered into Windows’ trusted sign-in interface, rather than into a look-alike created by another program. On systems configured to require it, the extra keystroke is a small interruption with a specific purpose: to establish who is asking for the password before the user types it.
