Hi,
There is something called XML signature and w3c describes about that well. There are many additional tags like <Singnature>,<SignedInfo>, etc.
IBM has a tool to do this, called xmlsecuritysuite. You can go to alphawotks and download it.
Also w3c defines the standard . You can go to that site and see it.
There is a yahoo group discussing these issues. It is called XKMS.
Contact me if you need more information.
good luck,
mansooralip@hotmail.com
Mansoor