diff --git a/session.lib.php b/session.lib.php
index 8f30675..b5b18d2 100644
--- a/session.lib.php
+++ b/session.lib.php
@@ -25,35 +25,45 @@ if (isset($_SERVER['HTTPS'])) {
   header( 'Pragma: no-cache' ); 
 }
 
+/*
+ * Persistent logins: the cookie holds "username|key", and the database
+ * holds only a SHA-256 of the key, so a copy of the table cannot be used
+ * to log in. Each device has its own row.
+ */
+function persistent_cookie() {
+  $cookie_data = $_COOKIE['PersistentAuth'] ?? '';
+  if (!is_string($cookie_data))
+    $cookie_data = '';
+  return explode("|", $cookie_data, 2) + array('', '');
+}
+
+function persistent_key_hash($key) {
+  return hash('sha256', $key);
+}
+
 function check_persistent_login() {
   if (!array_key_exists('PersistentAuth', $_COOKIE)) {
     return false;
   }
   global $db, $is_admin;
   $table = "persistent_sessions";
+  list($username, $key) = persistent_cookie();
+  if ($username === '' || $key === '')
+    return false;
+  $key_hash = persistent_key_hash($key);
+
   open_db('open');
-
-  $cookie_data = (string)$_COOKIE['PersistentAuth'];
-  list($username, $key) = explode("|", $cookie_data, 2) + array('', '');
-
-  $query = "SELECT * FROM $table WHERE username = ? LIMIT 1";
-  $getid = $db->execute_query($query, array($username)) or die("Query failed: ".$db->error);
+  $query = "SELECT * FROM $table WHERE username = ? AND session = ? LIMIT 1";
+  $getid = $db->execute_query($query, array($username, $key_hash)) or die("Query failed: ".$db->error);
   $row = $getid->fetch_assoc();
   open_db('close');
 
-  if (isset($row)) {
-    // Set up session, and delete old cookie, and create new persistent cookie
-    $stored_session = $row['session'];
-    if ($stored_session == $key) {
-      // login
-      login_editor($username);
-      // delete cookie
-      delete_persistent_login();
-      // create new cookie
-      create_persistent_login($username);
-      return true;
-    }
-    return false;
+  if (isset($row) && hash_equals((string)$row['session'], $key_hash)) {
+    // Log in, then swap the used key for a new one
+    login_editor($username);
+    delete_persistent_login();
+    create_persistent_login($username);
+    return true;
   }
   return false;
 }
@@ -64,11 +74,11 @@ function create_persistent_login($username) {
   $table = "persistent_sessions";
   open_db('open');
 
-  $key = bin2hex(random_bytes(100));
+  $key = bin2hex(random_bytes(32));
 
-  // Add session key to database
+  // Add the hash of the session key to the database
   $query = "INSERT INTO $table (username, session) VALUES (?, ?)";
-  $db->execute_query($query, array($username, $key)) or die("Query failed: ".$db->error);
+  $db->execute_query($query, array($username, persistent_key_hash($key))) or die("Query failed: ".$db->error);
   open_db('close');
 
   // Create cookie with key
@@ -82,16 +92,18 @@ function delete_persistent_login() {
   $table = "persistent_sessions";
   open_db('open');
 
-  $cookie_data = (string)($_COOKIE['PersistentAuth'] ?? '');
-  list($username, $key) = explode("|", $cookie_data, 2) + array('', '');
+  list($username, $key) = persistent_cookie();
 
   // Remove this session key from the database
-  $query = "DELETE FROM $table WHERE username = ? AND session = ?";
-  $db->execute_query($query, array($username, $key)) or die("Query failed: ".$db->error);
+  if ($username !== '' && $key !== '') {
+    $query = "DELETE FROM $table WHERE username = ? AND session = ?";
+    $db->execute_query($query, array($username, persistent_key_hash($key))) or die("Query failed: ".$db->error);
+  }
 
-  // Reap old sessions
+  // Reap old sessions, and keys stored before they were hashed
   $query = "DELETE FROM $table " . 
-           " WHERE created < DATE_SUB(now(), INTERVAL 1 YEAR);";
+           " WHERE created < DATE_SUB(now(), INTERVAL 1 YEAR)" .
+           " OR CHAR_LENGTH(session) <> 64;";
   $db->query($query) or die("Query failed: ".$db->error);
 
   open_db('close');