Magento Rest-Api Category List

I would like to create a Category List Extension for the Magento Rest Api.

There is a thread posted at stackoverflow:

Create new magento Rest api to get category list in magento

But it shows only a small incomplete piece of code which is not working. It seems that the developer just copied a short snippet from the following extension by Marko Andreini:

https://github.com/marcoandreini/magento-extendedrestapi

But I can't get it to work with Magento 1.9.

I alway get an 403 -access denied when calling /magento/api/rest/categories

I would be very pleased if someone has an idea what's the problem and how to fix it.

this is working for me:

                    $_helper = Mage::helper('catalog/category');
                    $_categories = $_helper->getStoreCategories();

                    $result = array();


                    if (count($_categories) > 0) {

                        $i = 0;
                        foreach ($_categories as $_category) {

                            $katId = $_category->getId();

                                $result[$i]['katId'] = $katId;
                                $result[$i]['url'] = $_helper->getCategoryUrl($_category);
                                $result[$i]['katName'] = $_category->getName();
                                $result[$i]['katSubs'] = null;

                                $_category = Mage::getModel('catalog/category')->load($_category->getId());
                                $_subcategories = $_category->getChildrenCategories();

                                $j = 0;
                                if (count($_subcategories) > 0) {

                                    foreach ($_subcategories as $_subcategory) {

                                        $subKatId = $_subcategory->getId();
                                        $subKatName = $_subcategory->getName();

                                        if ($subKatId && $subKatName && $subKatName != "") {

                                            $result[$i]['katSubs'][$j]['parentKatId'] = $katId;
                                            $result[$i]['katSubs'][$j]['subKatId'] = $subKatId;
                                            $result[$i]['katSubs'][$j]['url'] =  $_helper->getCategoryUrl($_subcategory);
                                            $result[$i]['katSubs'][$j]['subKatName'] = $subKatName;

                                            $j++;
                                        }
                                    }

                                }

                            $i++;
                        }
                    }