diff --git a/certificate_checker_unittest.cc b/certificate_checker_unittest.cc
new file mode 100644
index 0000000..20efce9
--- /dev/null
+++ b/certificate_checker_unittest.cc
@@ -0,0 +1,140 @@
+//
+// Copyright (C) 2012 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.
+//
+
+#include "update_engine/certificate_checker.h"
+
+#include <string>
+
+#include <base/strings/string_util.h>
+#include <base/strings/stringprintf.h>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include "update_engine/common/constants.h"
+#include "update_engine/common/mock_prefs.h"
+#include "update_engine/mock_certificate_checker.h"
+
+using ::testing::DoAll;
+using ::testing::Return;
+using ::testing::SetArgumentPointee;
+using ::testing::SetArrayArgument;
+using ::testing::_;
+using std::string;
+
+namespace chromeos_update_engine {
+
+class MockCertificateCheckObserver : public CertificateChecker::Observer {
+ public:
+  MOCK_METHOD2(CertificateChecked,
+               void(ServerToCheck server_to_check,
+                    CertificateCheckResult result));
+};
+
+class CertificateCheckerTest : public testing::Test {
+ protected:
+  void SetUp() override {
+    cert_key_ = base::StringPrintf("%s-%d-%d",
+                                   cert_key_prefix_.c_str(),
+                                   static_cast<int>(server_to_check_),
+                                   depth_);
+    cert_checker.Init();
+    cert_checker.SetObserver(&observer_);
+  }
+
+  void TearDown() override {
+    cert_checker.SetObserver(nullptr);
+  }
+
+  MockPrefs prefs_;
+  MockOpenSSLWrapper openssl_wrapper_;
+  // Parameters of our mock certificate digest.
+  int depth_{0};
+  unsigned int length_{4};
+  uint8_t digest_[4]{0x17, 0x7D, 0x07, 0x5F};
+  string digest_hex_{"177D075F"};
+  string diff_digest_hex_{"1234ABCD"};
+  string cert_key_prefix_{kPrefsUpdateServerCertificate};
+  ServerToCheck server_to_check_{ServerToCheck::kUpdate};
+  string cert_key_;
+
+  testing::StrictMock<MockCertificateCheckObserver> observer_;
+  CertificateChecker cert_checker{&prefs_, &openssl_wrapper_};
+};
+
+// check certificate change, new
+TEST_F(CertificateCheckerTest, NewCertificate) {
+  EXPECT_CALL(openssl_wrapper_, GetCertificateDigest(nullptr, _, _, _))
+      .WillOnce(DoAll(
+          SetArgumentPointee<1>(depth_),
+          SetArgumentPointee<2>(length_),
+          SetArrayArgument<3>(digest_, digest_ + 4),
+          Return(true)));
+  EXPECT_CALL(prefs_, GetString(cert_key_, _)).WillOnce(Return(false));
+  EXPECT_CALL(prefs_, SetString(cert_key_, digest_hex_)).WillOnce(Return(true));
+  EXPECT_CALL(observer_,
+              CertificateChecked(server_to_check_,
+                                 CertificateCheckResult::kValid));
+  ASSERT_TRUE(
+      cert_checker.CheckCertificateChange(1, nullptr, server_to_check_));
+}
+
+// check certificate change, unchanged
+TEST_F(CertificateCheckerTest, SameCertificate) {
+  EXPECT_CALL(openssl_wrapper_, GetCertificateDigest(nullptr, _, _, _))
+      .WillOnce(DoAll(
+          SetArgumentPointee<1>(depth_),
+          SetArgumentPointee<2>(length_),
+          SetArrayArgument<3>(digest_, digest_ + 4),
+          Return(true)));
+  EXPECT_CALL(prefs_, GetString(cert_key_, _))
+      .WillOnce(DoAll(SetArgumentPointee<1>(digest_hex_), Return(true)));
+  EXPECT_CALL(prefs_, SetString(_, _)).Times(0);
+  EXPECT_CALL(observer_,
+              CertificateChecked(server_to_check_,
+                                 CertificateCheckResult::kValid));
+  ASSERT_TRUE(
+      cert_checker.CheckCertificateChange(1, nullptr, server_to_check_));
+}
+
+// check certificate change, changed
+TEST_F(CertificateCheckerTest, ChangedCertificate) {
+  EXPECT_CALL(openssl_wrapper_, GetCertificateDigest(nullptr, _, _, _))
+      .WillOnce(DoAll(
+          SetArgumentPointee<1>(depth_),
+          SetArgumentPointee<2>(length_),
+          SetArrayArgument<3>(digest_, digest_ + 4),
+          Return(true)));
+  EXPECT_CALL(prefs_, GetString(cert_key_, _))
+      .WillOnce(DoAll(SetArgumentPointee<1>(diff_digest_hex_), Return(true)));
+  EXPECT_CALL(observer_,
+              CertificateChecked(server_to_check_,
+                                 CertificateCheckResult::kValidChanged));
+  EXPECT_CALL(prefs_, SetString(cert_key_, digest_hex_)).WillOnce(Return(true));
+  ASSERT_TRUE(
+      cert_checker.CheckCertificateChange(1, nullptr, server_to_check_));
+}
+
+// check certificate change, failed
+TEST_F(CertificateCheckerTest, FailedCertificate) {
+  EXPECT_CALL(observer_, CertificateChecked(server_to_check_,
+                                            CertificateCheckResult::kFailed));
+  EXPECT_CALL(prefs_, GetString(_, _)).Times(0);
+  EXPECT_CALL(openssl_wrapper_, GetCertificateDigest(_, _, _, _)).Times(0);
+  ASSERT_FALSE(
+      cert_checker.CheckCertificateChange(0, nullptr, server_to_check_));
+}
+
+}  // namespace chromeos_update_engine