diff --git a/ndn-cpp/key-chain.hpp b/ndn-cpp/key-chain.hpp
index 97991e9..8de7a62 100644
--- a/ndn-cpp/key-chain.hpp
+++ b/ndn-cpp/key-chain.hpp
@@ -13,14 +13,25 @@
 class KeyChain {
 public:
   /**
-   * 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.
+   * In data, set the signed info publisher public key digest and key locator key to the public key and set the 
+   * signature using the private key.
    * Note: the caller must make sure the timestamp is correct, for example with 
    * data.getSignedInfo().setTimestampMilliseconds(time(NULL) * 1000.0).
-   * @param data The Data object to sign and set the signature.
+   * @param data The Data object to sign and set the key and signature.
+   * @param publicKeyDer A pointer to a buffer with the DER-encoded public key.
+   * @param publicKeyDerLength The number of bytes in publicKeyDer.
+   * @param privateKeyDer A pointer to a buffer with the DER-encoded private key.
+   * @param privateKeyDerLength The number of bytes in privateKeyDer.
+   */
+  static void sign
+    (Data &data, const unsigned char *publicKeyDer, unsigned int publicKeyDerLength, 
+     const unsigned char *privateKeyDer, unsigned int privateKeyDerLength);
+
+  /**
+   * Call sign with the default public and private keys.
+   * @param data
    */
   static void defaultSign(Data &data);
-
 };
 
 }