diff --git a/src/applications/auth/factor/PhabricatorTOTPAuthFactor.php b/src/applications/auth/factor/PhabricatorTOTPAuthFactor.php
index 471fe89ed7..e2f0dc74b3 100644
--- a/src/applications/auth/factor/PhabricatorTOTPAuthFactor.php
+++ b/src/applications/auth/factor/PhabricatorTOTPAuthFactor.php
@@ -1,438 +1,476 @@
 <?php
 
 final class PhabricatorTOTPAuthFactor extends PhabricatorAuthFactor {
 
   const DIGEST_TEMPORARY_KEY = 'mfa.totp.sync';
 
   public function getFactorKey() {
     return 'totp';
   }
 
   public function getFactorName() {
     return pht('Mobile Phone App (TOTP)');
   }
 
   public function getFactorDescription() {
     return pht(
       'Attach a mobile authenticator application (like Authy '.
       'or Google Authenticator) to your account. When you need to '.
       'authenticate, you will enter a code shown on your phone.');
   }
 
   public function processAddFactorForm(
     AphrontFormView $form,
     AphrontRequest $request,
     PhabricatorUser $user) {
 
     $totp_token_type = PhabricatorAuthTOTPKeyTemporaryTokenType::TOKENTYPE;
 
     $key = $request->getStr('totpkey');
     if (strlen($key)) {
       // If the user is providing a key, make sure it's a key we generated.
       // This raises the barrier to theoretical attacks where an attacker might
       // provide a known key (such attacks are already prevented by CSRF, but
       // this is a second barrier to overcome).
 
       // (We store and verify the hash of the key, not the key itself, to limit
       // how useful the data in the table is to an attacker.)
 
       $token_code = PhabricatorHash::digestWithNamedKey(
         $key,
         self::DIGEST_TEMPORARY_KEY);
 
       $temporary_token = id(new PhabricatorAuthTemporaryTokenQuery())
         ->setViewer($user)
         ->withTokenResources(array($user->getPHID()))
         ->withTokenTypes(array($totp_token_type))
         ->withExpired(false)
         ->withTokenCodes(array($token_code))
         ->executeOne();
       if (!$temporary_token) {
         // If we don't have a matching token, regenerate the key below.
         $key = null;
       }
     }
 
     if (!strlen($key)) {
       $key = self::generateNewTOTPKey();
 
       // Mark this key as one we generated, so the user is allowed to submit
       // a response for it.
 
       $token_code = PhabricatorHash::digestWithNamedKey(
         $key,
         self::DIGEST_TEMPORARY_KEY);
 
       $unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();
         id(new PhabricatorAuthTemporaryToken())
           ->setTokenResource($user->getPHID())
           ->setTokenType($totp_token_type)
           ->setTokenExpires(time() + phutil_units('1 hour in seconds'))
           ->setTokenCode($token_code)
           ->save();
       unset($unguarded);
     }
 
     $code = $request->getStr('totpcode');
 
     $e_code = true;
     if ($request->getExists('totp')) {
-      $okay = $this->verifyTOTPCode(
-        $user,
+      $okay = (bool)$this->getTimestepAtWhichResponseIsValid(
+        $this->getAllowedTimesteps($this->getCurrentTimestep()),
         new PhutilOpaqueEnvelope($key),
-        $code);
+        (string)$code);
 
       if ($okay) {
         $config = $this->newConfigForUser($user)
           ->setFactorName(pht('Mobile App (TOTP)'))
           ->setFactorSecret($key);
 
         return $config;
       } else {
         if (!strlen($code)) {
           $e_code = pht('Required');
         } else {
           $e_code = pht('Invalid');
         }
       }
     }
 
     $form->addHiddenInput('totp', true);
     $form->addHiddenInput('totpkey', $key);
 
     $form->appendRemarkupInstructions(
       pht(
         'First, download an authenticator application on your phone. Two '.
         'applications which work well are **Authy** and **Google '.
         'Authenticator**, but any other TOTP application should also work.'));
 
     $form->appendInstructions(
       pht(
         'Launch the application on your phone, and add a new entry for '.
         'this Phabricator install. When prompted, scan the QR code or '.
         'manually enter the key shown below into the application.'));
 
     $prod_uri = new PhutilURI(PhabricatorEnv::getProductionURI('/'));
     $issuer = $prod_uri->getDomain();
 
     $uri = urisprintf(
       'otpauth://totp/%s:%s?secret=%s&issuer=%s',
       $issuer,
       $user->getUsername(),
       $key,
       $issuer);
 
     $qrcode = $this->renderQRCode($uri);
     $form->appendChild($qrcode);
 
     $form->appendChild(
       id(new AphrontFormStaticControl())
         ->setLabel(pht('Key'))
         ->setValue(phutil_tag('strong', array(), $key)));
 
     $form->appendInstructions(
       pht(
         '(If given an option, select that this key is "Time Based", not '.
         '"Counter Based".)'));
 
     $form->appendInstructions(
       pht(
         'After entering the key, the application should display a numeric '.
         'code. Enter that code below to confirm that you have configured '.
         'the authenticator correctly:'));
 
     $form->appendChild(
       id(new PHUIFormNumberControl())
         ->setLabel(pht('TOTP Code'))
         ->setName('totpcode')
         ->setValue($code)
         ->setError($e_code));
 
   }
 
   protected function newIssuedChallenges(
     PhabricatorAuthFactorConfig $config,
     PhabricatorUser $viewer,
     array $challenges) {
 
     $current_step = $this->getCurrentTimestep();
 
     // If we already issued a valid challenge, don't issue a new one.
     if ($challenges) {
       return array();
     }
 
     // Otherwise, generate a new challenge for the current timestep and compute
     // the TTL.
 
     // When computing the TTL, note that we accept codes within a certain
     // window of the challenge timestep to account for clock skew and users
     // needing time to enter codes.
 
     // We don't want this challenge to expire until after all valid responses
     // to it are no longer valid responses to any other challenge we might
     // issue in the future. If the challenge expires too quickly, we may issue
     // a new challenge which can accept the same TOTP code response.
 
     // This means that we need to keep this challenge alive for double the
     // window size: if we're currently at timestep 3, the user might respond
     // with the code for timestep 5. This is valid, since timestep 5 is within
     // the window for timestep 3.
 
     // But the code for timestep 5 can be used to respond at timesteps 3, 4, 5,
     // 6, and 7. To prevent any valid response to this challenge from being
     // used again, we need to keep this challenge active until timestep 8.
 
     $window_size = $this->getTimestepWindowSize();
     $step_duration = $this->getTimestepDuration();
 
     $ttl_steps = ($window_size * 2) + 1;
     $ttl_seconds = ($ttl_steps * $step_duration);
 
     return array(
       $this->newChallenge($config, $viewer)
         ->setChallengeKey($current_step)
         ->setChallengeTTL(PhabricatorTime::getNow() + $ttl_seconds),
     );
   }
 
   public function renderValidateFactorForm(
     PhabricatorAuthFactorConfig $config,
     AphrontFormView $form,
     PhabricatorUser $viewer,
     PhabricatorAuthFactorResult $result) {
 
     $value = $result->getValue();
     $error = $result->getErrorMessage();
     $is_wait = $result->getIsWait();
 
     if ($is_wait) {
       $control = id(new AphrontFormMarkupControl())
         ->setValue($error)
         ->setError(pht('Wait'));
     } else {
       $control = id(new PHUIFormNumberControl())
         ->setName($this->getParameterName($config, 'totpcode'))
         ->setDisableAutocomplete(true)
         ->setValue($value)
         ->setError($error);
     }
 
     $control
       ->setLabel(pht('App Code'))
       ->setCaption(pht('Factor Name: %s', $config->getFactorName()));
 
     $form->appendChild($control);
   }
 
   protected function newResultFromIssuedChallenges(
     PhabricatorAuthFactorConfig $config,
     PhabricatorUser $viewer,
     array $challenges) {
 
     // If we've already issued a challenge at the current timestep or any
     // nearby timestep, require that it was issued to the current session.
     // This is defusing attacks where you (broadly) look at someone's phone
     // and type the code in more quickly than they do.
     $session_phid = $viewer->getSession()->getPHID();
     $now = PhabricatorTime::getNow();
 
     $engine = $config->getSessionEngine();
     $workflow_key = $engine->getWorkflowKey();
 
+    $current_timestep = $this->getCurrentTimestep();
+
     foreach ($challenges as $challenge) {
       $challenge_timestep = (int)$challenge->getChallengeKey();
       $wait_duration = ($challenge->getChallengeTTL() - $now) + 1;
 
       if ($challenge->getSessionPHID() !== $session_phid) {
         return $this->newResult()
           ->setIsWait(true)
           ->setErrorMessage(
             pht(
               'This factor recently issued a challenge to a different login '.
               'session. Wait %s second(s) for the code to cycle, then try '.
               'again.',
               new PhutilNumber($wait_duration)));
       }
 
       if ($challenge->getWorkflowKey() !== $workflow_key) {
         return $this->newResult()
           ->setIsWait(true)
           ->setErrorMessage(
             pht(
               'This factor recently issued a challenge for a different '.
               'workflow. Wait %s second(s) for the code to cycle, then try '.
               'again.',
               new PhutilNumber($wait_duration)));
       }
+
+      // If the current realtime timestep isn't a valid response to the current
+      // challenge but the challenge hasn't expired yet, we're locking out
+      // the factor to prevent challenge windows from overlapping. Let the user
+      // know that they should wait for a new challenge.
+      $challenge_timesteps = $this->getAllowedTimesteps($challenge_timestep);
+      if (!isset($challenge_timesteps[$current_timestep])) {
+        return $this->newResult()
+          ->setIsWait(true)
+          ->setErrorMessage(
+            pht(
+              'This factor recently issued a challenge which has expired. '.
+              'A new challenge can not be issued yet. Wait %s second(s) for '.
+              'the code to cycle, then try again.',
+              new PhutilNumber($wait_duration)));
+      }
     }
 
     return null;
   }
 
   protected function newResultFromChallengeResponse(
     PhabricatorAuthFactorConfig $config,
     PhabricatorUser $viewer,
     AphrontRequest $request,
     array $challenges) {
 
     $code = $request->getStr($this->getParameterName($config, 'totpcode'));
-    $key = new PhutilOpaqueEnvelope($config->getFactorSecret());
 
     $result = $this->newResult()
       ->setValue($code);
 
-    if ($this->verifyTOTPCode($viewer, $key, (string)$code)) {
+    // We expect to reach TOTP validation with exactly one valid challenge.
+    if (count($challenges) !== 1) {
+      throw new Exception(
+        pht(
+          'Reached TOTP challenge validation with an unexpected number of '.
+          'unexpired challenges (%d), expected exactly one.',
+          phutil_count($challenges)));
+    }
+
+    $challenge = head($challenges);
+
+    $challenge_timestep = (int)$challenge->getChallengeKey();
+    $current_timestep = $this->getCurrentTimestep();
+
+    $challenge_timesteps = $this->getAllowedTimesteps($challenge_timestep);
+    $current_timesteps = $this->getAllowedTimesteps($current_timestep);
+
+    // We require responses be both valid for the challenge and valid for the
+    // current timestep. A longer challenge TTL doesn't let you use older
+    // codes for a longer period of time.
+    $valid_timestep = $this->getTimestepAtWhichResponseIsValid(
+      array_intersect_key($challenge_timesteps, $current_timesteps),
+      new PhutilOpaqueEnvelope($config->getFactorSecret()),
+      (string)$code);
+
+    if ($valid_timestep) {
       $result->setIsValid(true);
     } else {
       if (strlen($code)) {
         $error_message = pht('Invalid');
       } else {
         $error_message = pht('Required');
       }
       $result->setErrorMessage($error_message);
     }
 
     return $result;
   }
 
   public static function generateNewTOTPKey() {
     return strtoupper(Filesystem::readRandomCharacters(32));
   }
 
-  private function verifyTOTPCode(
-    PhabricatorUser $user,
-    PhutilOpaqueEnvelope $key,
-    $code) {
-
-    $now = (int)(time() / 30);
-
-    // Allow the user to enter a code a few minutes away on either side, in
-    // case the server or client has some clock skew.
-    for ($offset = -2; $offset <= 2; $offset++) {
-      $real = self::getTOTPCode($key, $now + $offset);
-      if (phutil_hashes_are_identical($real, $code)) {
-        return true;
-      }
-    }
-
-    // TODO: After validating a code, this should mark it as used and prevent
-    // it from being reused.
-
-    return false;
-  }
-
-
   public static function base32Decode($buf) {
     $buf = strtoupper($buf);
 
     $map = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ234567';
     $map = str_split($map);
     $map = array_flip($map);
 
     $out = '';
     $len = strlen($buf);
     $acc = 0;
     $bits = 0;
     for ($ii = 0; $ii < $len; $ii++) {
       $chr = $buf[$ii];
       $val = $map[$chr];
 
       $acc = $acc << 5;
       $acc = $acc + $val;
 
       $bits += 5;
       if ($bits >= 8) {
         $bits = $bits - 8;
         $out .= chr(($acc & (0xFF << $bits)) >> $bits);
       }
     }
 
     return $out;
   }
 
   public static function getTOTPCode(PhutilOpaqueEnvelope $key, $timestamp) {
     $binary_timestamp = pack('N*', 0).pack('N*', $timestamp);
     $binary_key = self::base32Decode($key->openEnvelope());
 
     $hash = hash_hmac('sha1', $binary_timestamp, $binary_key, true);
 
     // See RFC 4226.
 
     $offset = ord($hash[19]) & 0x0F;
 
     $code = ((ord($hash[$offset + 0]) & 0x7F) << 24) |
             ((ord($hash[$offset + 1]) & 0xFF) << 16) |
             ((ord($hash[$offset + 2]) & 0xFF) <<  8) |
             ((ord($hash[$offset + 3])       )      );
 
     $code = ($code % 1000000);
     $code = str_pad($code, 6, '0', STR_PAD_LEFT);
 
     return $code;
   }
 
 
   /**
    * @phutil-external-symbol class QRcode
    */
   private function renderQRCode($uri) {
     $root = dirname(phutil_get_library_root('phabricator'));
     require_once $root.'/externals/phpqrcode/phpqrcode.php';
 
     $lines = QRcode::text($uri);
 
     $total_width = 240;
     $cell_size = floor($total_width / count($lines));
 
     $rows = array();
     foreach ($lines as $line) {
       $cells = array();
       for ($ii = 0; $ii < strlen($line); $ii++) {
         if ($line[$ii] == '1') {
           $color = '#000';
         } else {
           $color = '#fff';
         }
 
         $cells[] = phutil_tag(
           'td',
           array(
             'width' => $cell_size,
             'height' => $cell_size,
             'style' => 'background: '.$color,
           ),
           '');
       }
       $rows[] = phutil_tag('tr', array(), $cells);
     }
 
     return phutil_tag(
       'table',
       array(
         'style' => 'margin: 24px auto;',
       ),
       $rows);
   }
 
   private function getTimestepDuration() {
     return 30;
   }
 
   private function getCurrentTimestep() {
     $duration = $this->getTimestepDuration();
     return (int)(PhabricatorTime::getNow() / $duration);
   }
 
-  private function getAllowedTimesteps() {
-    $current_step = $this->getCurrentTimestep();
+  private function getAllowedTimesteps($at_timestep) {
     $window = $this->getTimestepWindowSize();
-    return range($current_step - $window, $current_step + $window);
+    $range = range($at_timestep - $window, $at_timestep + $window);
+    return array_fuse($range);
   }
 
   private function getTimestepWindowSize() {
+    // The user is allowed to provide a code from the recent past or the
+    // near future to account for minor clock skew between the client
+    // and server, and the time it takes to actually enter a code.
     return 2;
   }
 
+  private function getTimestepAtWhichResponseIsValid(
+    array $timesteps,
+    PhutilOpaqueEnvelope $key,
+    $code) {
+
+    foreach ($timesteps as $timestep) {
+      $expect_code = self::getTOTPCode($key, $timestep);
+      if (phutil_hashes_are_identical($code, $expect_code)) {
+        return $timestep;
+      }
+    }
+
+    return null;
+  }
+
+
 
 }