Microsoft was a successful participant in the XMLDSIG interoperability testing, missing only a few features relating to RetrievalMethod and XPath features that are optional in the standard.
The Microsoft XMLDSIG implementation is documented in the ".NET" Framework SDK see System.Security.Cryptography.Xml. It is available for download from MSDN. Microsoft expects to add XML Encryption support soon.
Microsoft maintains client and server sample code (ASP .NET) at