Here is a lightweight simple plugin to dump a game's savedata key.
I know that there are others like SGDeemer or FreeCheat but this one works in games where
SGDeemer fails/crashes the game (like Duael Invaders), and it is much faster since it is only dumping the key,
and with FreeCheat you need to go through a bunch of options to dump the key,
just enable this plugin and it'll be ready to dump the key.

Just run the game and load or save the game to dump the savedata key.

The key is dumped to ms0:/PSP/SAVEPLAIN/(SAVEDATAFOLDER)/(GAMEID).bin.

Place the plugin in ms0:/seplugins/ and enable it in ms0:/seplugins/game.txt.