Added the default private key and finish implementing defaultSign.
diff --git a/ndn-cpp/key-chain.hpp b/ndn-cpp/key-chain.hpp
index cef43e1..2259064 100644
--- a/ndn-cpp/key-chain.hpp
+++ b/ndn-cpp/key-chain.hpp
@@ -13,7 +13,9 @@
class KeyChain {
public:
/**
- * In data, set the signed info key to the default public key and set the signature using the default private key.
+ * In data, set the signed info publisher public key digest and key locator key to the default public key and set the
+ * signature using the default private key.
+ * Note: the caller must make sure the timestamp is correct with data.getSignedInfo().setTimestampMilliseconds.
* @param data The Data object to sign and set the signature.
*/
static void defaultSign(Data &data);