These are the lock icons intended for use in protected pages created by Lê Duy Quang, which are licensed under CC-BY-SA-3.0. Feel free to use these icons as long as the credits are given to the author and the shared/modified/distributed versions of the icons are licensed under the same license as the original.
If these locks are used as the protected indicator on the title, they are recommended to be at 26px (see this test).
Semi-protected (or autoconfirmed protected).
Cascade protected (the contents in the page are also protected).