diff --git a/conf/default.conf.php b/conf/default.conf.php
index 078c4e128d..bbc0a31906 100644
--- a/conf/default.conf.php
+++ b/conf/default.conf.php
@@ -65,6 +65,12 @@ return array(
   // attack difficult, but it is viable unless you isolate the file domain.
   'security.alternate-file-domain'  => null,
 
+  // Default key for HMAC digests where the key is not important (i.e., the
+  // hash itself is secret). You can change this if you want (to any other
+  // string), but doing so will break existing sessions and CSRF tokens.
+  'security.hmac-key' => '[D\t~Y7eNmnQGJ;rnH6aF;m2!vJ8@v8C=Cs:aQS\.Qw',
+
+
 // -- DarkConsole ----------------------------------------------------------- //
 
   // DarkConsole is a administrative debugging/profiling tool built into
diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
index bab5bcf691..f6aa7a6666 100644
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -471,6 +471,7 @@ phutil_register_library_map(array(
     'PhabricatorGarbageCollectorDaemon' => 'infrastructure/daemon/garbagecollector',
     'PhabricatorGoodForNothingWorker' => 'infrastructure/daemon/workers/worker/goodfornothing',
     'PhabricatorHandleObjectSelectorDataView' => 'applications/phid/handle/view/selector',
+    'PhabricatorHash' => 'infrastructure/util/hash',
     'PhabricatorHelpController' => 'applications/help/controller/base',
     'PhabricatorHelpKeyboardShortcutController' => 'applications/help/controller/keyboardshortcut',
     'PhabricatorIRCBot' => 'infrastructure/daemon/irc/bot',
diff --git a/src/applications/metamta/storage/receivedmail/PhabricatorMetaMTAReceivedMail.php b/src/applications/metamta/storage/receivedmail/PhabricatorMetaMTAReceivedMail.php
index 860994c6c6..7f83330bd9 100644
--- a/src/applications/metamta/storage/receivedmail/PhabricatorMetaMTAReceivedMail.php
+++ b/src/applications/metamta/storage/receivedmail/PhabricatorMetaMTAReceivedMail.php
@@ -176,7 +176,11 @@ class PhabricatorMetaMTAReceivedMail extends PhabricatorMetaMTADAO {
     }
 
     $expect_hash = self::computeMailHash($receiver->getMailKey(), $check_phid);
-    if ($expect_hash != $hash) {
+
+    // See note at computeOldMailHash().
+    $old_hash = self::computeOldMailHash($receiver->getMailKey(), $check_phid);
+
+    if ($expect_hash != $hash && $old_hash != $hash) {
       return $this->setMessage("Invalid mail hash!")->save();
     }
 
@@ -228,6 +232,20 @@ class PhabricatorMetaMTAReceivedMail extends PhabricatorMetaMTADAO {
   public static function computeMailHash($mail_key, $phid) {
     $global_mail_key = PhabricatorEnv::getEnvConfig('phabricator.mail-key');
 
+    $hash = PhabricatorHash::digest($mail_key.$global_mail_key.$phid);
+    return substr($hash, 0, 16);
+  }
+
+  public static function computeOldMailHash($mail_key, $phid) {
+
+    // TODO: Remove this method entirely in a couple of months. We've moved from
+    // plain sha1 to sha1+hmac to make the codebase more auditable for good uses
+    // of hash functions, but still accept the old hashes on email replies to
+    // avoid breaking things. Once we've been sending only hmac hashes for a
+    // while, remove this and start rejecting old hashes. See T547.
+
+    $global_mail_key = PhabricatorEnv::getEnvConfig('phabricator.mail-key');
+
     $hash = sha1($mail_key.$global_mail_key.$phid);
     return substr($hash, 0, 16);
   }
diff --git a/src/applications/metamta/storage/receivedmail/__init__.php b/src/applications/metamta/storage/receivedmail/__init__.php
index 6254939d3f..37d4e56863 100644
--- a/src/applications/metamta/storage/receivedmail/__init__.php
+++ b/src/applications/metamta/storage/receivedmail/__init__.php
@@ -14,6 +14,7 @@ phutil_require_module('phabricator', 'applications/metamta/parser');
 phutil_require_module('phabricator', 'applications/metamta/storage/base');
 phutil_require_module('phabricator', 'applications/people/storage/user');
 phutil_require_module('phabricator', 'infrastructure/env');
+phutil_require_module('phabricator', 'infrastructure/util/hash');
 
 phutil_require_module('phutil', 'utils');
 
diff --git a/src/applications/people/storage/log/PhabricatorUserLog.php b/src/applications/people/storage/log/PhabricatorUserLog.php
index abc0355b1f..339877e4bb 100644
--- a/src/applications/people/storage/log/PhabricatorUserLog.php
+++ b/src/applications/people/storage/log/PhabricatorUserLog.php
@@ -89,7 +89,7 @@ class PhabricatorUserLog extends PhabricatorUserDAO {
     // seeing the logs doesn't compromise all the sessions which appear in
     // them. This just prevents casual leaks, like in a screenshot.
     if (strlen($session)) {
-      $this->session = sha1($session);
+      $this->session = PhabricatorHash::digest($session);
     }
     return $this;
   }
diff --git a/src/applications/people/storage/log/__init__.php b/src/applications/people/storage/log/__init__.php
index 90f82b5ed8..3392d47eae 100644
--- a/src/applications/people/storage/log/__init__.php
+++ b/src/applications/people/storage/log/__init__.php
@@ -7,6 +7,7 @@
 
 
 phutil_require_module('phabricator', 'applications/people/storage/base');
+phutil_require_module('phabricator', 'infrastructure/util/hash');
 
 phutil_require_module('phutil', 'utils');
 
diff --git a/src/applications/people/storage/user/PhabricatorUser.php b/src/applications/people/storage/user/PhabricatorUser.php
index ca5eb1b5cd..0cb6990ad1 100644
--- a/src/applications/people/storage/user/PhabricatorUser.php
+++ b/src/applications/people/storage/user/PhabricatorUser.php
@@ -191,7 +191,7 @@ class PhabricatorUser extends PhabricatorUserDAO {
   private function generateToken($epoch, $frequency, $key, $len) {
     $time_block = floor($epoch / $frequency);
     $vec = $this->getPHID().$this->getPasswordHash().$key.$time_block;
-    return substr(sha1($vec), 0, $len);
+    return substr(PhabricatorHash::digest($vec), 0, $len);
   }
 
   /**
diff --git a/src/applications/people/storage/user/__init__.php b/src/applications/people/storage/user/__init__.php
index c93a02dfda..8cdfe8db6e 100644
--- a/src/applications/people/storage/user/__init__.php
+++ b/src/applications/people/storage/user/__init__.php
@@ -14,6 +14,7 @@ phutil_require_module('phabricator', 'applications/phid/constants');
 phutil_require_module('phabricator', 'applications/phid/storage/phid');
 phutil_require_module('phabricator', 'applications/search/index/indexer/user');
 phutil_require_module('phabricator', 'infrastructure/env');
+phutil_require_module('phabricator', 'infrastructure/util/hash');
 phutil_require_module('phabricator', 'storage/qsprintf');
 phutil_require_module('phabricator', 'storage/queryfx');
 
diff --git a/src/docs/userguide/diffusion.diviner b/src/docs/userguide/diffusion.diviner
index d59960ced5..e4a33dae7d 100644
--- a/src/docs/userguide/diffusion.diviner
+++ b/src/docs/userguide/diffusion.diviner
@@ -58,7 +58,7 @@ of 7-character hashes:
 
 Because 7-character hashes are likely to collide for even moderately large
 repositories, Diffusion generally uses either a 16-character prefix (which makes
-collisions very unlikely) or the full 40-character SHA1 (which makes collisions
+collisions very unlikely) or the full 40-character hash (which makes collisions
 astronomically unlikely).
 
 = Adding Repositories =
diff --git a/src/infrastructure/util/hash/PhabricatorHash.php b/src/infrastructure/util/hash/PhabricatorHash.php
new file mode 100644
index 0000000000..9a54d29c10
--- /dev/null
+++ b/src/infrastructure/util/hash/PhabricatorHash.php
@@ -0,0 +1,31 @@
+<?php
+
+/*
+ * Copyright 2011 Facebook, Inc.
+ *
+ * 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.
+ */
+
+final class PhabricatorHash {
+
+  public static function digest($string) {
+    $key = PhabricatorEnv::getEnvConfig('security.hmac-key');
+    if (!$key) {
+      throw new Exception(
+        "Set a 'security.hmac-key' in your Phabricator configuration!");
+    }
+
+    return hash_hmac('sha1', $string, $key);
+  }
+
+}
diff --git a/src/infrastructure/util/hash/__init__.php b/src/infrastructure/util/hash/__init__.php
new file mode 100644
index 0000000000..071754f5cb
--- /dev/null
+++ b/src/infrastructure/util/hash/__init__.php
@@ -0,0 +1,12 @@
+<?php
+/**
+ * This file is automatically generated. Lint this module to rebuild it.
+ * @generated
+ */
+
+
+
+phutil_require_module('phabricator', 'infrastructure/env');
+
+
+phutil_require_source('PhabricatorHash.php');