The XML Apache Project has released XML Security C++ 1.0, a C++ library that implements XML digital signatures.  This is the first stable release.  I have used Java version of the package but haven't tried the C++ version, so I don't have any opinions other than that the library depends on C++ versions of Xerces and Xalan which tend to support latest standards at the cost of larger footprint and lower performance than other XML and XSLT implementations.

Still, it is nice to have alternatives to XML Security Library (XMLSec) by Aleksey Sanin.  Latest version of XMLSec is 1.1.0 released on August 5th.  XMLSec uses LibXML2 which was written by Daniel Veillard for Gnome project.

Both libraries are cross-platform and Win32 binaries are available.

This makes me wonder why there isn't a C++ version of Batik.