diff --git a/src/protocol-detail/challenge.hpp b/src/protocol-detail/challenge.hpp
new file mode 100644
index 0000000..1ff74fd
--- /dev/null
+++ b/src/protocol-detail/challenge.hpp
@@ -0,0 +1,39 @@
+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+/**
+ * Copyright (c) 2017-2020, Regents of the University of California.
+ *
+ * This file is part of ndncert, a certificate management system based on NDN.
+ *
+ * ndncert is free software: you can redistribute it and/or modify it under the terms
+ * of the GNU General Public License as published by the Free Software Foundation, either
+ * version 3 of the License, or (at your option) any later version.
+ *
+ * ndncert is distributed in the hope that it will be useful, but WITHOUT ANY
+ * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+ * PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received copies of the GNU General Public License along with
+ * ndncert, e.g., in COPYING.md file.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * See AUTHORS.md for complete list of ndncert authors and contributors.
+ */
+
+#ifndef NDNCERT_PROTOCOL_DETAIL_CHALLENGE_HPP
+#define NDNCERT_PROTOCOL_DETAIL_CHALLENGE_HPP
+
+#include "../certificate-request.hpp"
+#include <ndn-cxx/encoding/block.hpp>
+
+namespace ndn {
+namespace ndncert {
+
+class CHALLENGE {
+public:
+  static Block
+  encodeDataPayload(const CertificateRequest& request);
+};
+
+}  // namespace ndncert
+}  // namespace ndn
+
+#endif // NDNCERT_PROTOCOL_DETAIL_HPP
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