Hi! I'm Luca.Fraga and I'm new here (I'll register when I finish to write this message), and I've coded (with Aurelio Mannara) a homebrew application for Nintendo DS Flashcard, DSecure.
DSecure can protect your Flashcard with a password, replacing the card loader with itself. Homebrew is for now compatible with EZ Flash V/V+/Vi, AceKard 2 (but not 2i ) , Original R4 and Top Toy DS (DSTT).
You can find downloads, installation guides and more else at its official site ( http://it.dsecure.wikia.com/wiki/Pagina_principale ).
(excuse me for my bad english in official site and in this message).
Enjoy!