utils: Enable use of DummyKeychain to avoid crypto overhead

Refs: #2233
diff --git a/helper/ndn-stack-helper.cpp b/helper/ndn-stack-helper.cpp
index b548844..bd2413e 100644
--- a/helper/ndn-stack-helper.cpp
+++ b/helper/ndn-stack-helper.cpp
@@ -45,6 +45,7 @@
 
 #include "ndn-face-container.hpp"
 #include "ndn-stack-helper.hpp"
+#include "utils/dummy-keychain.hpp"
 
 #include <limits>
 #include <map>
@@ -76,6 +77,13 @@
 {
 }
 
+KeyChain&
+StackHelper::getKeyChain()
+{
+  static ::ndn::DummyKeyChain keyChain;
+  return keyChain;
+}
+
 void
 StackHelper::SetStackAttributes(const std::string& attr1, const std::string& value1,
                                 const std::string& attr2, const std::string& value2,