It is part of the mystery gifts
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which key there are a ton
You cannot get the secret key in pearl or diamond version. It was only available for a limited time via mystery gift on platinum version from June 2009 to July 2009. So, it is now impossible to obtain even in platinum version. Sorry, but you lucked out.
NO!!
The Secret Key that you get in the Pokemon Mansion is used to enter the Cinnabar Gym.
RSA Secret-Key Challenge was created in 1997.