code: private Map<String, String> toMap(JsonNode messageJson) {
          Map<String, String> fields = new HashMap<String, String>(messageJson.size());
          Iterator<Map.Entry<String, JsonNode>> jsonFields = messageJson.fields();
          while (jsonFields.hasNext()) {
              Map.Entry<String, JsonNode> next = jsonFields.next();
              fields.put(next.getKey(), next.getValue().asText());
          }
          return fields;
      }
docs: Transforms the {@link JsonNode} into a map to integrate with the {@link SignatureChecker} utility.
  
  @param messageJson JSON of message.
  @return Transformed map.