The External Login project is the continuation of the Authentication Manager project, originally developed for Joomla! 1.5, and now compatible with Joomla! 3.x. Documentation can be found in the wiki pages https://github.com/chdemko/joomla-external-login/wiki.
It allows external authentication using for example CAS (Central Authentication Service by https://www.apereo.org/projects/cas) http://download.chdemko.com/joomla/extensions/external-login/plgsystemcaslogin-220.127.116.11.zip. It has been designed to support other external authentication mecanisms using plugins.
The icon has been taken from https://commons.wikimedia.org/wiki/File:OpenAccesslogoPLoStransparent.svg