boot a OS from linux

Hi,
I have to check a sign on a crypted FS in a usb-key and then, after
autenthication, boot a OS. I need a library to read in that FS and so I
cannot boot directly from a loader (grub, lilo ). I think I can create a
small linux distribution but I don't' know how to boot a new OS after the
authentication... any suggestions ? (I cannot spend days & days  to read
the lilo or grub sources .. :-))
tnx, empo