doc iconKey refers to Programme key file. Some softwares use this kind of extension. It can be opened by kaspersky software.

Some softwares save the passcodes on the file with key extension during the licencing.

For example these softwares use key files:

-AntiVir Personal Edition

-Avira product key 

-Diablo II

-Kaspersky Anti-virus

Softwares like Kaspersky may control passcodes on the internet, if you want to skip this process you can use better progammes like Nod32 instead of dealing with key file search. 
If you buy the software, you should backup your key file properly. Because, the trojan files that get into your computer will try to search the files with key extension. 

What to use key

Kaspersky Anti virus software license process

which page related to key file

key Related Searches

You can check related links about key fileextension.