The W3C XML Security Working Group has posted the first public draft of XML Signature Properties:
The SignatureProperties element defined by XML Signature [XMLDSIG2nd] offers a means to associate property values with an XML Signature. This document defines specific properties that may be used by various applications of XML Signature, without requiring those applications to define such properties on a per case basis. This document defines how these properties are to be specified and processed when used but does not require their use - specifications that reference this document may or may not require their use.
The changes proposed in this document would not be a breaking change to XML Signature, but warrant a new namespace for the properties themselves so that they can be used in various versions of XML Signature."