diff --git a/src/security/v2/validation-state.hpp b/src/security/v2/validation-state.hpp
new file mode 100644
index 0000000..2181fae
--- /dev/null
+++ b/src/security/v2/validation-state.hpp
@@ -0,0 +1,243 @@
+/* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
+/**
+ * Copyright (c) 2013-2017 Regents of the University of California.
+ *
+ * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
+ *
+ * ndn-cxx library is free software: you can redistribute it and/or modify it under the
+ * terms of the GNU Lesser General Public License as published by the Free Software
+ * Foundation, either version 3 of the License, or (at your option) any later version.
+ *
+ * ndn-cxx library 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 Lesser General Public License for more details.
+ *
+ * You should have received copies of the GNU General Public License and GNU Lesser
+ * General Public License along with ndn-cxx, e.g., in COPYING.md file.  If not, see
+ * <http://www.gnu.org/licenses/>.
+ *
+ * See AUTHORS.md for complete list of ndn-cxx authors and contributors.
+ */
+
+#ifndef NDN_SECURITY_V2_VALIDATION_STATE_HPP
+#define NDN_SECURITY_V2_VALIDATION_STATE_HPP
+
+#include "../../tag-host.hpp"
+#include "validation-callback.hpp"
+#include "certificate.hpp"
+
+#include <unordered_set>
+#include <list>
+
+namespace ndn {
+namespace security {
+namespace v2 {
+
+class Validator;
+
+/**
+ * @brief Validation state
+ *
+ * One instance of the validation state is kept for the validation of the whole certificate
+ * chain.
+ *
+ * The state collects the certificate chain that adheres to the selected validation policy to
+ * validate data or interest packets.  Certificate, data, and interest packet signatures are
+ * verified only after the validator determines that the chain terminates with a trusted
+ * certificate (a trusted anchor or a previously validated certificate).  This model allows
+ * filtering out invalid certificate chains without incurring (costly) cryptographic signature
+ * verification overhead and mitigates some forms of denial-of-service attacks.
+ *
+ * Validation policy and/or key fetcher may add custom information associated with the
+ * validation state using tags (@sa TagHost)
+ *
+ * @sa DataValidationState, InterestValidationState
+ */
+class ValidationState : public TagHost, noncopyable
+{
+public:
+  /**
+   * @brief Create validation state
+   */
+  ValidationState();
+
+  virtual
+  ~ValidationState();
+
+  /**
+   * @brief Call the failure callback
+   */
+  virtual void
+  fail(const ValidationError& error) = 0;
+
+  /**
+   * @return Depth of certificate chain
+   */
+  size_t
+  getDepth() const;
+
+  /**
+   * @brief Check if @p certName has been previously seen and record the supplied name
+   */
+  bool
+  hasSeenCertificateName(const Name& certName);
+
+  /**
+   * @brief Add @p cert to the top of the certificate chain
+   *
+   * If m_certificateChain is empty, @p cert should be the signer of the original
+   * packet.  If m_certificateChain is not empty, @p cert should be the signer of
+   * m_certificateChain.front().
+   *
+   * @post m_certificateChain.front() == cert
+   * @note This function does not verify the signature bits.
+   */
+  void
+  addCertificate(const Certificate& cert);
+
+private: // Interface intended to be used only by Validator class
+  /**
+   * @brief Verify signature of the original packet
+   *
+   * @param trustCert The certificate that signs the original packet
+   */
+  virtual void
+  verifyOriginalPacket(const Certificate& trustedCert) = 0;
+
+  /**
+   * @brief Call success callback of the original packet without signature validation
+   */
+  virtual void
+  bypassValidation() = 0;
+
+  /**
+   * @brief Verify signatures of certificates in the certificate chain
+   *
+   * When certificate chain cannot be verified, this method will call this->fail() with
+   * INVALID_SIGNATURE error code and the appropriate diagnostic message.
+   *
+   * @retval nullptr Signatures of at least one certificate in the chain is invalid.  All unverified
+   *                 certificates have been removed from m_certificateChain.
+   * @retval Certificate to validate original data packet, either m_certificateChain.back() or
+   *         trustedCert if the certificate chain is empty.
+   *
+   * @post m_certificateChain includes a list of certificates successfully verified by
+   *       @p trustedCert.
+   */
+  const Certificate*
+  verifyCertificateChain(const Certificate& trustedCert);
+
+protected:
+  bool m_hasOutcome;
+
+private:
+  std::unordered_set<Name> m_seenCertificateNames;
+
+  /**
+   * @brief the certificate chain
+   *
+   * Each certificate in the chain signs the next certificate.  The last certificate signs the
+   * original packet.
+   */
+  std::list<v2::Certificate> m_certificateChain;
+
+  friend class Validator;
+};
+
+/**
+ * @brief Validation state for a data packet
+ */
+class DataValidationState final : public ValidationState
+{
+public:
+  /**
+   * @brief Create validation state for @p data
+   *
+   * The caller must ensure that state instance is valid until validation finishes (i.e., until
+   * after validateCertificateChain() and validateOriginalPacket() are called)
+   */
+  DataValidationState(const Data& data,
+                      const DataValidationSuccessCallback& successCb,
+                      const DataValidationFailureCallback& failureCb);
+
+  /**
+   * @brief Destructor
+   *
+   * If neither success callback nor failure callback was called, the destructor will call
+   * failure callback with IMPLEMENTATION_ERROR error code.
+   */
+  ~DataValidationState() final;
+
+  void
+  fail(const ValidationError& error) final;
+
+  /**
+   * @return Original data being validated
+   */
+  const Data&
+  getOriginalData() const;
+
+private:
+  void
+  verifyOriginalPacket(const Certificate& trustedCert) final;
+
+  void
+  bypassValidation() final;
+
+private:
+  Data m_data;
+  DataValidationSuccessCallback m_successCb;
+  DataValidationFailureCallback m_failureCb;
+};
+
+/**
+ * @brief Validation state for an interest packet
+ */
+class InterestValidationState final : public ValidationState
+{
+public:
+  /**
+   * @brief Create validation state for @p interest
+   *
+   * The caller must ensure that state instance is valid until validation finishes (i.e., until
+   * after validateCertificateChain() and validateOriginalPacket() are called)
+   */
+  InterestValidationState(const Interest& interest,
+                          const InterestValidationSuccessCallback& successCb,
+                          const InterestValidationFailureCallback& failureCb);
+
+  /**
+   * @brief Destructor
+   *
+   * If neither success callback nor failure callback was called, the destructor will call
+   * failure callback with IMPLEMENTATION_ERROR error code.
+   */
+  ~InterestValidationState() final;
+
+  void
+  fail(const ValidationError& error) final;
+
+  /**
+   * @return Original interest being validated
+   */
+  const Interest&
+  getOriginalInterest() const;
+
+private:
+  void
+  verifyOriginalPacket(const Certificate& trustedCert) final;
+
+  void
+  bypassValidation() final;
+
+private:
+  Interest m_interest;
+  InterestValidationSuccessCallback m_successCb;
+  InterestValidationFailureCallback m_failureCb;
+};
+
+} // namespace v2
+} // namespace security
+} // namespace ndn
+
+#endif // NDN_SECURITY_V2_VALIDATION_STATE_HPP