<HMAC name="HMAC-Fail-1">
  <!-- whitespace in algo -->
  <Algorithm> SHA-256 </Algorithm>
  <Message>{template_for_message}</Message>
  <SecretKey ref='private.secret_key'/>
  <Output encoding='base16'>output_value</Output>
  <VerificationValue ref='variable_name'/> <!-- optional -->
</HMAC>
