Digital Signature Services (DSS) is a specification developed at OASIS:

http://www.oasis-open.org/committees/dss/

It defines a protocol to request a server to digitally sign a document, or to request verification of a signature. It has bindings to HTTP and SOAP 1.2, but an ebXML Messaging binding makes sense for communities that use ebXML, and offers various benefits such as asynchronous operation and reliable messaging. I've suggested a standardized ebXML Messaging binding for DSS in the following message to the DSS TC.

http://lists.oasis-open.org/archives/dss-comment/200611/msg00001.html