diff --git a/certificate_checker.cc b/certificate_checker.cc
new file mode 100644
index 0000000..6e886e7
--- /dev/null
+++ b/certificate_checker.cc
@@ -0,0 +1,204 @@
+//
+// 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/logging.h>
+#include <base/strings/string_number_conversions.h>
+#include <base/strings/string_util.h>
+#include <base/strings/stringprintf.h>
+#include <curl/curl.h>
+#include <openssl/evp.h>
+#include <openssl/ssl.h>
+
+#include "update_engine/common/constants.h"
+#include "update_engine/common/prefs_interface.h"
+#include "update_engine/common/utils.h"
+
+using std::string;
+
+namespace chromeos_update_engine {
+
+bool OpenSSLWrapper::GetCertificateDigest(X509_STORE_CTX* x509_ctx,
+                                          int* out_depth,
+                                          unsigned int* out_digest_length,
+                                          uint8_t* out_digest) const {
+  TEST_AND_RETURN_FALSE(out_digest);
+  X509* certificate = X509_STORE_CTX_get_current_cert(x509_ctx);
+  TEST_AND_RETURN_FALSE(certificate);
+  int depth = X509_STORE_CTX_get_error_depth(x509_ctx);
+  if (out_depth)
+    *out_depth = depth;
+
+  unsigned int len;
+  const EVP_MD* digest_function = EVP_sha256();
+  bool success = X509_digest(certificate, digest_function, out_digest, &len);
+
+  if (success && out_digest_length)
+    *out_digest_length = len;
+  return success;
+}
+
+// static
+CertificateChecker* CertificateChecker::cert_checker_singleton_ = nullptr;
+
+CertificateChecker::CertificateChecker(PrefsInterface* prefs,
+                                       OpenSSLWrapper* openssl_wrapper)
+    : prefs_(prefs), openssl_wrapper_(openssl_wrapper) {
+}
+
+CertificateChecker::~CertificateChecker() {
+  if (cert_checker_singleton_ == this)
+    cert_checker_singleton_ = nullptr;
+}
+
+void CertificateChecker::Init() {
+  CHECK(cert_checker_singleton_ == nullptr);
+  cert_checker_singleton_ = this;
+}
+
+// static
+CURLcode CertificateChecker::ProcessSSLContext(CURL* curl_handle,
+                                               SSL_CTX* ssl_ctx,
+                                               void* ptr) {
+  ServerToCheck* server_to_check = reinterpret_cast<ServerToCheck*>(ptr);
+
+  if (!cert_checker_singleton_) {
+    DLOG(WARNING) << "No CertificateChecker singleton initialized.";
+    return CURLE_FAILED_INIT;
+  }
+
+  // From here we set the SSL_CTX to another callback, from the openssl library,
+  // which will be called after each server certificate is validated. However,
+  // since openssl does not allow us to pass our own data pointer to the
+  // callback, the certificate check will have to be done statically. Since we
+  // need to know which update server we are using in order to check the
+  // certificate, we hardcode Chrome OS's two known update servers here, and
+  // define a different static callback for each. Since this code should only
+  // run in official builds, this should not be a problem. However, if an update
+  // server different from the ones listed here is used, the check will not
+  // take place.
+  int (*verify_callback)(int, X509_STORE_CTX*);
+  switch (*server_to_check) {
+    case ServerToCheck::kDownload:
+      verify_callback = &CertificateChecker::VerifySSLCallbackDownload;
+      break;
+    case ServerToCheck::kUpdate:
+      verify_callback = &CertificateChecker::VerifySSLCallbackUpdate;
+      break;
+    case ServerToCheck::kNone:
+      verify_callback = nullptr;
+      break;
+  }
+
+  SSL_CTX_set_verify(ssl_ctx, SSL_VERIFY_PEER, verify_callback);
+  return CURLE_OK;
+}
+
+// static
+int CertificateChecker::VerifySSLCallbackDownload(int preverify_ok,
+                                                  X509_STORE_CTX* x509_ctx) {
+  return VerifySSLCallback(preverify_ok, x509_ctx, ServerToCheck::kDownload);
+}
+
+// static
+int CertificateChecker::VerifySSLCallbackUpdate(int preverify_ok,
+                                                X509_STORE_CTX* x509_ctx) {
+  return VerifySSLCallback(preverify_ok, x509_ctx, ServerToCheck::kUpdate);
+}
+
+// static
+int CertificateChecker::VerifySSLCallback(int preverify_ok,
+                                          X509_STORE_CTX* x509_ctx,
+                                          ServerToCheck server_to_check) {
+  CHECK(cert_checker_singleton_ != nullptr);
+  return cert_checker_singleton_->CheckCertificateChange(
+      preverify_ok, x509_ctx, server_to_check) ? 1 : 0;
+}
+
+bool CertificateChecker::CheckCertificateChange(int preverify_ok,
+                                                X509_STORE_CTX* x509_ctx,
+                                                ServerToCheck server_to_check) {
+  TEST_AND_RETURN_FALSE(prefs_ != nullptr);
+
+  // If pre-verification failed, we are not interested in the current
+  // certificate. We store a report to UMA and just propagate the fail result.
+  if (!preverify_ok) {
+    NotifyCertificateChecked(server_to_check, CertificateCheckResult::kFailed);
+    return false;
+  }
+
+  int depth;
+  unsigned int digest_length;
+  uint8_t digest[EVP_MAX_MD_SIZE];
+
+  if (!openssl_wrapper_->GetCertificateDigest(x509_ctx,
+                                              &depth,
+                                              &digest_length,
+                                              digest)) {
+    LOG(WARNING) << "Failed to generate digest of X509 certificate "
+                 << "from update server.";
+    NotifyCertificateChecked(server_to_check, CertificateCheckResult::kValid);
+    return true;
+  }
+
+  // We convert the raw bytes of the digest to an hex string, for storage in
+  // prefs.
+  string digest_string = base::HexEncode(digest, digest_length);
+
+  string storage_key =
+      base::StringPrintf("%s-%d-%d", kPrefsUpdateServerCertificate,
+                         static_cast<int>(server_to_check), depth);
+  string stored_digest;
+  // If there's no stored certificate, we just store the current one and return.
+  if (!prefs_->GetString(storage_key, &stored_digest)) {
+    if (!prefs_->SetString(storage_key, digest_string)) {
+      LOG(WARNING) << "Failed to store server certificate on storage key "
+                   << storage_key;
+    }
+    NotifyCertificateChecked(server_to_check, CertificateCheckResult::kValid);
+    return true;
+  }
+
+  // Certificate changed, we store a report to UMA and store the most recent
+  // certificate.
+  if (stored_digest != digest_string) {
+    if (!prefs_->SetString(storage_key, digest_string)) {
+      LOG(WARNING) << "Failed to store server certificate on storage key "
+                   << storage_key;
+    }
+    LOG(INFO) << "Certificate changed from " << stored_digest << " to "
+              << digest_string << ".";
+    NotifyCertificateChecked(server_to_check,
+                             CertificateCheckResult::kValidChanged);
+    return true;
+  }
+
+  NotifyCertificateChecked(server_to_check, CertificateCheckResult::kValid);
+  // Since we don't perform actual SSL verification, we return success.
+  return true;
+}
+
+void CertificateChecker::NotifyCertificateChecked(
+    ServerToCheck server_to_check,
+    CertificateCheckResult result) {
+  if (observer_)
+    observer_->CertificateChecked(server_to_check, result);
+}
+
+}  // namespace chromeos_update_engine