You're trying to restrict the device to only play media files that come from you?You're trying to ensure that no other players can play the media files you create?
what lengths are you willing to go to to prevent people from overcoming your restrictions?
is it not possible to do it in Arduino?
Maybe there is a simpler approach for it instead of making crpytographic libraries?
And also we're planning to minimize the sound file to couple of seconds to minimize the size ( 100kB < size < 1MB )...