<ds:SignatureProperty xmlns:ds="http://www.w3.org/2000/09/xmldsig#" Target="https://simplesamlphp.org/some/target" Id="abc123">
  <ssp:HSMSerialNumber xmlns:ssp="urn:x-simplesamlphp:namespace">1234567890</ssp:HSMSerialNumber>
</ds:SignatureProperty>