diff --git a/CMakeLists.txt b/CMakeLists.txt
index 58b1cf41..01aead30 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -13,6 +13,7 @@ endif()
 
 find_package(ament_cmake_python REQUIRED)
 find_package(ament_cmake_ros REQUIRED)
+find_package(tinydir_vendor REQUIRED)
 
 ament_python_install_package(${PROJECT_NAME})
 
@@ -50,7 +51,9 @@ set(rcutils_sources
   src/time.c
   ${time_impl_c}
   src/uint8_array.c
+  src/security_directory.c
 )
+
 set_source_files_properties(
   ${rcutils_sources}
   PROPERTIES language "C")
@@ -108,6 +111,10 @@ install(TARGETS ${PROJECT_NAME}
   LIBRARY DESTINATION lib
   RUNTIME DESTINATION bin)
 
+ament_target_dependencies(${PROJECT_NAME}
+  "tinydir_vendor"
+)
+
 if(BUILD_TESTING)
   if(NOT WIN32)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++14")
diff --git a/include/rcutils/security_directory.h b/include/rcutils/security_directory.h
new file mode 100644
index 00000000..9d2c6806
--- /dev/null
+++ b/include/rcutils/security_directory.h
@@ -0,0 +1,67 @@
+// Copyright 2018 Open Source Robotics Foundation, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef RCUTILS__SECURITY_DIRECTORY_H_
+#define RCUTILS__SECURITY_DIRECTORY_H_
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+#include "rcutils/allocator.h"
+#include "rcutils/visibility_control.h"
+
+#ifndef ROS_SECURITY_NODE_DIRECTORY_VAR_NAME
+  #define ROS_SECURITY_NODE_DIRECTORY_VAR_NAME "ROS_SECURITY_NODE_DIRECTORY"
+#endif
+
+#ifndef ROS_SECURITY_ROOT_DIRECTORY_VAR_NAME
+  #define ROS_SECURITY_ROOT_DIRECTORY_VAR_NAME "ROS_SECURITY_ROOT_DIRECTORY"
+#endif
+
+#ifndef ROS_SECURITY_LOOKUP_TYPE_VAR_NAME
+  #define ROS_SECURITY_LOOKUP_TYPE_VAR_NAME "ROS_SECURITY_LOOKUP_TYPE"
+#endif
+
+/// Return the secure root directory associated with a node given its validated name and namespace.
+/**
+ * E.g. for a node named "c" in namespace "/a/b", the secure root path will be
+ * "a/b/c", where the delimiter "/" is native for target file system (e.g. "\\" for _WIN32).
+ * If no exact match is found for the node name, a best match would be used instead
+ * (by performing longest-prefix matching).
+ *
+ * However, this expansion can be overridden by setting the secure node directory environment
+ * variable, allowing users to explicitly specify the exact secure root directory to be utilized.
+ * Such an override is useful for where the FQN of a node is non-deterministic before runtime,
+ * or when testing and using additional tools that may not otherwise be easily provisioned.
+ *
+ * \param[in] node_name validated node name (a single token)
+ * \param[in] node_namespace validated, absolute namespace (starting with "/")
+ * \param[in] allocator the allocator to use for allocation
+ * \returns machine specific (absolute) node secure root path or NULL on failure
+ *          returned pointer must be deallocated by the caller of this function
+ */
+RCUTILS_PUBLIC
+char * rcutils_get_secure_root(
+  const char * node_name,
+  const char * node_namespace,
+  const rcutils_allocator_t * allocator
+);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // RCUTILS__SECURITY_DIRECTORY_H_
diff --git a/package.xml b/package.xml
index 9e3a398d..543d6e48 100644
--- a/package.xml
+++ b/package.xml
@@ -10,6 +10,9 @@
   <buildtool_depend>ament_cmake_ros</buildtool_depend>
   <buildtool_depend>python3-empy</buildtool_depend>
 
+  <build_depend>tinydir_vendor</build_depend>
+  <build_export_depend>tinydir_vendor</build_export_depend>
+
   <test_depend>ament_cmake_gmock</test_depend>
   <test_depend>ament_cmake_gtest</test_depend>
   <test_depend>ament_cmake_pytest</test_depend>
diff --git a/src/security_directory.c b/src/security_directory.c
new file mode 100644
index 00000000..c4208931
--- /dev/null
+++ b/src/security_directory.c
@@ -0,0 +1,264 @@
+// Copyright 2018 Open Source Robotics Foundation, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "rcutils/security_directory.h"
+
+#include "rcutils/error_handling.h"
+#include "rcutils/filesystem.h"
+#include "rcutils/get_env.h"
+#include "rcutils/format_string.h"
+
+#ifdef __clang__
+# pragma clang diagnostic push
+# pragma clang diagnostic ignored "-Wembedded-directive"
+#endif
+#include "tinydir/tinydir.h"
+#ifdef __clang__
+# pragma clang diagnostic pop
+#endif
+
+/**
+ * A security lookup function takes in the node's name, namespace, a security root directory and an allocator;
+ *  It returns the relevant information required to load the security credentials,
+ *   which is currently a path to a directory on the filesystem containing DDS Security permission files.
+ */
+typedef char * (* security_lookup_fn_t) (
+  const char * node_name,
+  const char * node_namespace,
+  const char * ros_secure_root_env,
+  const rcutils_allocator_t * allocator
+);
+
+char * exact_match_lookup(
+  const char * node_name,
+  const char * node_namespace,
+  const char * ros_secure_root_env,
+  const rcutils_allocator_t * allocator
+);
+
+char * prefix_match_lookup(
+  const char * node_name,
+  const char * node_namespace,
+  const char * ros_secure_root_env,
+  const rcutils_allocator_t * allocator
+);
+
+security_lookup_fn_t g_security_lookup_fns[] = {
+  NULL,
+  exact_match_lookup,
+  prefix_match_lookup,
+};
+
+typedef enum ros_security_lookup_type_e
+{
+  ROS_SECURITY_LOOKUP_NODE_OVERRIDE = 0,
+  ROS_SECURITY_LOOKUP_MATCH_EXACT = 1,
+  ROS_SECURITY_LOOKUP_MATCH_PREFIX = 2,
+} ros_security_lookup_type_t;
+
+char * g_security_lookup_type_strings[] = {
+  "NODE_OVERRIDE",
+  "MATCH_EXACT",
+  "MATCH_PREFIX"
+};
+
+/// Return the directory whose name most closely matches node_name (longest-prefix match),
+/// scanning under base_dir.
+/**
+ * By using a prefix match, a node named e.g. "my_node_123" will be able to load and use the
+ * directory "my_node" if no better match exists.
+ * \param[in] base_dir
+ * \param[in] node_name
+ * \param[out] matched_name must be a valid memory address allocated with at least
+ * _TINYDIR_FILENAME_MAX characters.
+ * \return true if a match was found
+ */
+static bool get_best_matching_directory(
+  const char * base_dir,
+  const char * node_name,
+  char * matched_name)
+{
+  size_t max_match_length = 0;
+  tinydir_dir dir;
+  if (NULL == base_dir || NULL == node_name || NULL == matched_name) {
+    return false;
+  }
+  if (-1 == tinydir_open(&dir, base_dir)) {
+    return false;
+  }
+  while (dir.has_next) {
+    tinydir_file file;
+    if (-1 == tinydir_readfile(&dir, &file)) {
+      goto cleanup;
+    }
+    if (file.is_dir) {
+      size_t matched_name_length = strnlen(file.name, sizeof(file.name) - 1);
+      if (0 ==
+        strncmp(file.name, node_name,
+        matched_name_length) && matched_name_length > max_match_length)
+      {
+        max_match_length = matched_name_length;
+        memcpy(matched_name, file.name, max_match_length);
+      }
+    }
+    if (-1 == tinydir_next(&dir)) {
+      goto cleanup;
+    }
+  }
+cleanup:
+  tinydir_close(&dir);
+  return max_match_length > 0;
+}
+
+char * exact_match_lookup(
+  const char * node_name,
+  const char * node_namespace,
+  const char * ros_secure_root_env,
+  const rcutils_allocator_t * allocator)
+{
+  // Perform an exact match for the node's name in directory <root dir>/<namespace>.
+  char * node_secure_root = NULL;
+  // "/" case when root namespace is explicitly passed in
+  if (1 == strlen(node_namespace)) {
+    node_secure_root = rcutils_join_path(ros_secure_root_env, node_name, *allocator);
+  } else {
+    char * node_fqn = NULL;
+    char * node_root_path = NULL;
+    // Combine node namespace with node name
+    // TODO(ros2team): remove the hard-coded value of the root namespace
+    node_fqn = rcutils_format_string(*allocator, "%s%s%s", node_namespace, "/", node_name);
+    // Get native path, ignore the leading forward slash
+    // TODO(ros2team): remove the hard-coded length, use the length of the root namespace instead
+    node_root_path = rcutils_to_native_path(node_fqn + 1, *allocator);
+    node_secure_root = rcutils_join_path(ros_secure_root_env, node_root_path, *allocator);
+    allocator->deallocate(node_fqn, allocator->state);
+    allocator->deallocate(node_root_path, allocator->state);
+  }
+  return node_secure_root;
+}
+
+char * prefix_match_lookup(
+  const char * node_name,
+  const char * node_namespace,
+  const char * ros_secure_root_env,
+  const rcutils_allocator_t * allocator)
+{
+  // Perform longest prefix match for the node's name in directory <root dir>/<namespace>.
+  char * node_secure_root = NULL;
+  char matched_dir[_TINYDIR_FILENAME_MAX] = {0};
+  char * base_lookup_dir = NULL;
+  if (strlen(node_namespace) == 1) {
+    base_lookup_dir = (char *) ros_secure_root_env;
+  } else {
+    // TODO(ros2team): remove the hard-coded length, use the length of the root namespace instead.
+    base_lookup_dir = rcutils_join_path(ros_secure_root_env, node_namespace + 1, *allocator);
+  }
+  if (get_best_matching_directory(base_lookup_dir, node_name, matched_dir)) {
+    node_secure_root = rcutils_join_path(base_lookup_dir, matched_dir, *allocator);
+  }
+  if (base_lookup_dir != ros_secure_root_env && NULL != base_lookup_dir) {
+    allocator->deallocate(base_lookup_dir, allocator->state);
+  }
+  return node_secure_root;
+}
+
+char * rcutils_get_secure_root(
+  const char * node_name,
+  const char * node_namespace,
+  const rcutils_allocator_t * allocator)
+{
+  bool ros_secure_node_override = true;
+
+  // find out if either of the configuration environment variables are set
+  const char * env_buf = NULL;
+  if (NULL == node_name) {
+    return NULL;
+  }
+  if (rcutils_get_env(ROS_SECURITY_NODE_DIRECTORY_VAR_NAME, &env_buf)) {
+    return NULL;
+  }
+  if (!env_buf) {
+    return NULL;
+  }
+  size_t ros_secure_root_size = strlen(env_buf);
+  if (!ros_secure_root_size) {
+    // check root directory if node directory environment variable is empty
+    if (rcutils_get_env(ROS_SECURITY_ROOT_DIRECTORY_VAR_NAME, &env_buf)) {
+      return NULL;
+    }
+    if (!env_buf) {
+      return NULL;
+    }
+    ros_secure_root_size = strlen(env_buf);
+    if (!ros_secure_root_size) {
+      return NULL;  // environment variable was empty
+    } else {
+      ros_secure_node_override = false;
+    }
+  }
+
+  // found a usable environment variable, copy into our memory before overwriting with next lookup
+  char * ros_secure_root_env =
+    (char *)allocator->allocate(ros_secure_root_size + 1, allocator->state);
+  memcpy(ros_secure_root_env, env_buf, ros_secure_root_size + 1);
+  // TODO(ros2team): This make an assumption on the value and length of the root namespace.
+  // This should likely come from another (rcl/rmw?) function for reuse.
+  // If the namespace is the root namespace ("/"), the secure root is just the node name.
+
+  char * lookup_strategy = NULL;
+  char * node_secure_root = NULL;
+  if (ros_secure_node_override) {
+    node_secure_root = (char *)allocator->allocate(ros_secure_root_size + 1, allocator->state);
+    memcpy(node_secure_root, ros_secure_root_env, ros_secure_root_size + 1);
+    lookup_strategy = g_security_lookup_type_strings[ROS_SECURITY_LOOKUP_NODE_OVERRIDE];
+
+  } else {
+    // Check which lookup method to use and invoke the relevant function.
+    const char * ros_security_lookup_type = NULL;
+    if (rcutils_get_env(ROS_SECURITY_LOOKUP_TYPE_VAR_NAME, &ros_security_lookup_type)) {
+      allocator->deallocate(ros_secure_root_env, allocator->state);
+      return NULL;
+    }
+    if (0 == strcmp(ros_security_lookup_type,
+      g_security_lookup_type_strings[ROS_SECURITY_LOOKUP_MATCH_PREFIX]))
+    {
+      node_secure_root = g_security_lookup_fns[ROS_SECURITY_LOOKUP_MATCH_PREFIX]
+          (node_name, node_namespace, ros_secure_root_env, allocator);
+      lookup_strategy = g_security_lookup_type_strings[ROS_SECURITY_LOOKUP_MATCH_PREFIX];
+    } else { /* Default is MATCH_EXACT */
+      node_secure_root = g_security_lookup_fns[ROS_SECURITY_LOOKUP_MATCH_EXACT]
+          (node_name, node_namespace, ros_secure_root_env, allocator);
+      lookup_strategy = g_security_lookup_type_strings[ROS_SECURITY_LOOKUP_MATCH_EXACT];
+    }
+  }
+
+  if (NULL == node_secure_root || !rcutils_is_directory(node_secure_root)) {
+    // Check node_secure_root is not NULL before checking directory
+    if (NULL == node_secure_root) {
+      RCUTILS_SET_ERROR_MSG_WITH_FORMAT_STRING(
+        "SECURITY ERROR: unable to find a folder matching the node name in %s%s."
+        "Lookup strategy: %s",
+        ros_secure_root_env, node_namespace, lookup_strategy);
+    } else {
+      RCUTILS_SET_ERROR_MSG_WITH_FORMAT_STRING(
+        "SECURITY ERROR: directory %s does not exist. Lookup strategy: %s",
+        node_secure_root, lookup_strategy);
+    }
+    allocator->deallocate(ros_secure_root_env, allocator->state);
+    allocator->deallocate(node_secure_root, allocator->state);
+    return NULL;
+  }
+  allocator->deallocate(ros_secure_root_env, allocator->state);
+  return node_secure_root;
+}