The key that is used for encrypting and decrypting the current buffer. See encryption and 'cryptmethod'. Careful: Do not set the key value by hand, someone might see the typed key. Use the :X command. But you can make 'key' empty:

:set key=

It is not possible to get the value of this option with ":set key" or "echo &key". This is to avoid showing it to someone who shouldn't know. It also means you cannot see it yourself once you have set it, be careful not to make a typing error! You can use "&key" in an expression to detect whether encryption is enabled. When 'key' is set it returns "*****" (five stars).

(default "")

only available when compiled with the +cryptv feature