code: protected AuthnRequest retrieveSamlAuthenticationRequestFromHttpRequest(final HttpServletRequest request) throws Exception {
          LOGGER.debug("Retrieving authentication request from scope");
          val requestValue = request.getParameter(SamlProtocolConstants.PARAMETER_SAML_REQUEST);
          if (StringUtils.isBlank(requestValue)) {
              throw new IllegalArgumentException("SAML request could not be determined from the authentication request");
          }
          val encodedRequest = EncodingUtils.decodeBase64(requestValue.getBytes(StandardCharsets.UTF_8));
          return (AuthnRequest) XMLObjectSupport.unmarshallFromInputStream(samlProfileHandlerConfigurationContext.getOpenSamlConfigBean().getParserPool(),
              new ByteArrayInputStream(encodedRequest));
      }
docs: Retrieve authn request authn request.
  
  @param request the request
  @return the authn request
  @throws Exception the exception