This method returns a string is to be used as an identifer
(http://www.w3.org/TR/xml11/#id) for an xml document - all the
entries in the Base64 encoding table fit this definition with the
exception of 62 (+) and 63 (/), which are account for and parsed out
in the method implementation.
SimpleTokenReference references tokens as defined in the Web Services Security UsernameToken Profile 1.1 and
Web Services Security X.509 Certificate Token Profile 1.1.