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Add integration between KDEConnect and SDDM. Only one plugin must work - remote control of computer (mouse/keyboard). User must login provide root password to pair device (maybe allow to provide password on mobile?).

I fail to understand what you actually suggests. Maybe you could revise your suggestion?

What do you mean with the second sentence? Why should the root password be required? If one is set at all? Most distributions won't allow a root login these days.
Certainly it might be possible to get to work what you suggest. But why? What for? What problem are you trying to solve? Why should it be better to use a mobile device to type in a password?

My suggestion will made better of using computer as media center or other situation, when we are far of monitor.

Pairing is necessary to disallow catching any commands from device we won't use.

I still fail to understand. No one is going to type a password on a mobile phone and _hope_ that it goes to the right place and nowhere else.

To unlock a desktop / media center / whatever the _presence_ of a previously paired device is perfectly suitable and safe. No need for a password. Passwords are 1980-ish anyway.

Imagine I wanna to place my computer in wardrobe, monitor on desk and use my mobile to control my PC/TV. I need a password, so nobody could login and read my data.