diff --git a/src/requester.hpp b/src/requester.hpp
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+++ b/src/requester.hpp
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+/* -*- 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_CLIENT_MODULE_HPP
+#define NDNCERT_CLIENT_MODULE_HPP
+
+#include "configuration.hpp"
+#include "request-state.hpp"
+#include "crypto-support/crypto-helper.hpp"
+
+namespace ndn {
+namespace ndncert {
+
+// TODO
+// For each RequesterState, create a validator instance and initialize it with CA's cert
+// The validator instance should be in CaProfile
+
+struct RequesterState {
+  explicit
+  RequesterState(security::v2::KeyChain& keyChain, const CaProfile& caItem, RequestType requestType);
+
+  CaProfile m_caItem;
+  security::v2::KeyChain& m_keyChain;
+  RequestType m_type;
+
+  Name m_identityName;
+  security::Key m_keyPair;
+  std::string m_requestId;
+  Status m_status = Status::NOT_STARTED;
+  std::string m_challengeType;
+  std::string m_challengeStatus;
+  int m_remainingTries = 0;
+  time::system_clock::TimePoint m_freshBefore;
+  Name m_issuedCertName;
+
+  ECDHState m_ecdh;
+  uint8_t m_aesKey[16] = {0};
+
+  bool m_isCertInstalled = false;
+  bool m_isNewlyCreatedIdentity = false;
+  bool m_isNewlyCreatedKey = false;
+};
+
+class Requester : noncopyable
+{
+public:
+  // INFO related helpers
+  static shared_ptr<Interest>
+  genCaProfileInterest(const Name& caName);
+
+  /**
+   * Will first verify the signature of the packet using the key provided inside the profile.
+   * The application should be cautious whether to add CaProfile into the RequesterCaCache.
+   */
+  static boost::optional<CaProfile>
+  onCaProfileResponse(const Data& reply);
+
+  // PROBE related helpers
+  static shared_ptr<Interest>
+  genProbeInterest(const CaProfile& ca, std::vector<std::tuple<std::string, std::string>>&& probeInfo);
+
+  static void
+  onProbeResponse(const Data& reply, const CaProfile& ca,
+                  std::vector<Name>& identityNames, std::vector<Name>& otherCas);
+
+  // NEW/REVOKE/RENEW related helpers
+  static shared_ptr<Interest>
+  genNewInterest(RequesterState& state, const Name& identityName,
+                      const time::system_clock::TimePoint& notBefore,
+                      const time::system_clock::TimePoint& notAfter);
+
+  static shared_ptr<Interest>
+  genRevokeInterest(RequesterState& state, const security::v2::Certificate& certificate);
+
+  static std::list<std::string>
+  onNewRenewRevokeResponse(RequesterState& state, const Data& reply);
+
+  // CHALLENGE helpers
+  static std::vector<std::tuple<std::string, std::string>>
+  selectOrContinueChallenge(RequesterState& state, const std::string& challengeSelected);
+
+  static shared_ptr<Interest>
+  genChallengeInterest(const RequesterState& state,
+                       std::vector<std::tuple<std::string, std::string>>&& parameters);
+
+  static void
+  onChallengeResponse(RequesterState& state, const Data& reply);
+
+  static shared_ptr<Interest>
+  genCertFetchInterest(const RequesterState& state);
+
+  shared_ptr<security::v2::Certificate>
+  onCertFetchResponse(const Data& reply);
+
+  void
+  endSession(RequesterState& state);
+
+private:
+  static void
+  processIfError(const Data& data);
+};
+
+} // namespace ndncert
+} // namespace ndn
+
+#endif // NDNCERT_CLIENT_MODULE_HPP