diff --git a/src/security/sec-tpm.cpp b/src/security/sec-tpm.cpp
index 2011a95..3191095 100644
--- a/src/security/sec-tpm.cpp
+++ b/src/security/sec-tpm.cpp
@@ -27,10 +27,10 @@
 #include "../encoding/buffer-stream.hpp"
 #include "cryptopp.hpp"
 
-using namespace std;
-
 namespace ndn {
 
+using std::string;
+
 ConstBufferPtr
 SecTpm::exportPrivateKeyPkcs5FromTpm(const Name& keyName, const string& passwordStr)
 {