diff --git a/certificate_checker.h b/certificate_checker.h
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+++ b/certificate_checker.h
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+//
+// Copyright (C) 2011 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef UPDATE_ENGINE_CERTIFICATE_CHECKER_H_
+#define UPDATE_ENGINE_CERTIFICATE_CHECKER_H_
+
+#include <curl/curl.h>
+#include <openssl/ssl.h>
+
+#include <string>
+
+#include <base/macros.h>
+#include <gtest/gtest_prod.h>  // for FRIEND_TEST
+
+namespace chromeos_update_engine {
+
+class PrefsInterface;
+
+// Wrapper for openssl operations with the certificates.
+class OpenSSLWrapper {
+ public:
+  OpenSSLWrapper() = default;
+  virtual ~OpenSSLWrapper() = default;
+
+  // Takes an openssl X509_STORE_CTX, extracts the corresponding certificate
+  // from it and calculates its fingerprint (SHA256 digest). Returns true on
+  // success and false otherwise.
+  //
+  // |x509_ctx| is the pointer to the openssl object that holds the certificate.
+  // |out_depth| is the depth of the current certificate, in the certificate
+  // chain.
+  // |out_digest_length| is the length of the generated digest.
+  // |out_digest| is the byte array where the digest itself will be written.
+  // It should be big enough to hold a SHA1 digest (e.g. EVP_MAX_MD_SIZE).
+  virtual bool GetCertificateDigest(X509_STORE_CTX* x509_ctx,
+                                    int* out_depth,
+                                    unsigned int* out_digest_length,
+                                    uint8_t* out_digest) const;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(OpenSSLWrapper);
+};
+
+// The values in this enum are replicated in the metrics server. See metrics.h
+// for instructions on how to update these values in the server side.
+enum class CertificateCheckResult {
+  // The certificate is valid and the same as seen before or the first time we
+  // see a certificate.
+  kValid,
+  // The certificate is valid, but is different than a previously seen
+  // certificate for the selected server.
+  kValidChanged,
+  // The certificate validation failed.
+  kFailed,
+
+  // This value must be the last entry.
+  kNumConstants
+};
+
+// These values are used to generate the keys of files persisted via prefs.
+// This means that changing these will cause loss of information on metrics
+// reporting, during the transition. These values are also mapped to a metric
+// name in metrics.cc, so adding values here requires to assign a new metric
+// name in that file.
+enum class ServerToCheck {
+  kUpdate = 0,
+  kDownload,
+
+  // Ignore this server.
+  kNone,
+};
+
+// Responsible for checking whether update server certificates change, and
+// reporting to UMA when this happens. Since all state information is persisted,
+// and openssl forces us to use a static callback with no data pointer, this
+// class is entirely static.
+class CertificateChecker {
+ public:
+  class Observer {
+   public:
+    virtual ~Observer() = default;
+
+    // Called whenever a certificate is checked for the server |server_to_check|
+    // passing the result of said certificate check.
+    virtual void CertificateChecked(ServerToCheck server_to_check,
+                                    CertificateCheckResult result) = 0;
+  };
+
+  CertificateChecker(PrefsInterface* prefs, OpenSSLWrapper* openssl_wrapper);
+  ~CertificateChecker();
+
+  // This callback is called by libcurl just before the initialization of an
+  // SSL connection after having processed all other SSL related options. Used
+  // to check if server certificates change. |cert_checker| is expected to be a
+  // pointer to the CertificateChecker instance.
+  static CURLcode ProcessSSLContext(CURL* curl_handle,
+                                    SSL_CTX* ssl_ctx,
+                                    void* cert_checker);
+
+  // Initialize this class as the singleton instance. Only one instance can be
+  // initialized at a time and it should be initialized before other methods
+  // can be used.
+  void Init();
+
+  // Set the certificate observer to the passed instance. To remove the
+  // observer, pass a nullptr. The |observer| instance must be valid while this
+  // CertificateChecker verifies certificates.
+  void SetObserver(Observer* observer) { observer_ = observer; }
+
+ private:
+  FRIEND_TEST(CertificateCheckerTest, NewCertificate);
+  FRIEND_TEST(CertificateCheckerTest, SameCertificate);
+  FRIEND_TEST(CertificateCheckerTest, ChangedCertificate);
+  FRIEND_TEST(CertificateCheckerTest, FailedCertificate);
+
+  // These callbacks are asynchronously called by openssl after initial SSL
+  // verification. They are used to perform any additional security verification
+  // on the connection, but we use them here to get hold of the server
+  // certificate, in order to determine if it has changed since the last
+  // connection. Since openssl forces us to do this statically, we define two
+  // different callbacks for the two different official update servers, and only
+  // assign the correspondent one. The assigned callback is then called once per
+  // each certificate on the server and returns 1 for success and 0 for failure.
+  static int VerifySSLCallbackDownload(int preverify_ok,
+                                       X509_STORE_CTX* x509_ctx);
+  static int VerifySSLCallbackUpdate(int preverify_ok,
+                                     X509_STORE_CTX* x509_ctx);
+  static int VerifySSLCallback(int preverify_ok,
+                               X509_STORE_CTX* x509_ctx,
+                               ServerToCheck server_to_check);
+
+  // Checks if server certificate stored in |x509_ctx| has changed since last
+  // connection to that same server, specified by |server_to_check|.
+  // This is called by the callbacks defined above. The result of the
+  // certificate check is passed to the observer, if any. Returns true on
+  // success and false otherwise.
+  bool CheckCertificateChange(int preverify_ok,
+                              X509_STORE_CTX* x509_ctx,
+                              ServerToCheck server_to_check);
+
+  // Notifies the observer, if any, of a certificate checking.
+  void NotifyCertificateChecked(ServerToCheck server_to_check,
+                                CertificateCheckResult result);
+
+  // The CertificateChecker singleton instance.
+  static CertificateChecker* cert_checker_singleton_;
+
+  // Prefs instance used to store the certificates seen in the past.
+  PrefsInterface* prefs_;
+
+  // The wrapper for openssl operations.
+  OpenSSLWrapper* openssl_wrapper_;
+
+  // The observer called whenever a certificate is checked, if not null.
+  Observer* observer_{nullptr};
+
+  DISALLOW_COPY_AND_ASSIGN(CertificateChecker);
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // UPDATE_ENGINE_CERTIFICATE_CHECKER_H_