XML-Security-C is the C++ XML Signature and Encryption library from the Apache Software Foundation. It is used for all XML Signature and Encryption processing in OpenSAML and Shibboleth.
Project Website: http://xml.apache.org/security/c/
For now, build notes can be found in the BuildingShibboleth topic.
The as-yet-unreleased OpenSAML 2 library requires at least version 1.3.1 of this library, which is officially released, but contains some known bugs that affect some signature verification scenarios. The final release of OpenSAML 2.0 will probably require a newer xml-security version, possibly a 1.4.