What is .KEY file?
Generic file extension used by various programs when registering legal copies of the software; may be saved in a plain text format but generally contains some form of encrypted key string that authenticates the purchase and registers the software.
Since KEY files are used by various software products, no single program will be able to open them. Instead, you will have to know which program a KEY file corresponds to in order to use it to register the software.
Recommended Software Programs to open .KEY file