diff --git a/src/detail/challenge-encoder.hpp b/src/detail/challenge-encoder.hpp
new file mode 100644
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--- /dev/null
+++ b/src/detail/challenge-encoder.hpp
@@ -0,0 +1,43 @@
+/* -*- 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_DETAIL_CHALLENGE_STEP_HPP
+#define NDNCERT_DETAIL_CHALLENGE_STEP_HPP
+
+#include "../ca-state.hpp"
+#include "../requester-state.hpp"
+
+namespace ndn {
+namespace ndncert {
+
+class ChallengeEncoder
+{
+public:
+  static Block
+  encodeDataContent(const CaState& request);
+
+  static void
+  decodeDataContent(const Block& data, RequesterState& state);
+};
+
+} // namespace ndncert
+} // namespace ndn
+
+#endif // NDNCERT_DETAIL_CHALLENGE_STEP_HPP
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