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The Central Authentication Service (CAS) is a single sign-on protocol for the web. Its purpose is to permit a user to access multiple applications while providing their credentials (such as userid and password) only once. It also allows web applications to authenticate users without gaining access to a user's security credentials, such as a password. The name CAS also refers to a software package that implements this protocol.
How CAS Works
The CAS protocol involves at least three parties: a client web browser, the web application requesting authentication, and the CAS server. It may also involve a back-end service, such as a database server, that does not have its own HTTP interface but communicates with a web application.
When the client visits an application desiring to authenticate to it, the application redirects it to CAS. CAS validates the client's authenticity, usually by checking a username and password against a database (such as Kerberos, LDAP or Active Directory).
If the authentication succeeds, CAS returns the client to the application, passing along a service ticket. The application then validates the ticket by contacting CAS over a secure connection and providing its own service identifier and the ticket. CAS then gives the application trusted information about whether a particular user has successfully authenticated.
CAS allows multi-tier authentication via proxy address. A cooperating back-end service, like a database or mail server, can participate in CAS, validating the authenticity of users via information it receives from web applications. Thus, a webmail client and a webmail server can all implement CAS.
Laravel with CAS requires three things.
First, you need at least php5.5. Most environments will be using php 5.6 or 7.0 by now (2016). If you are using an older version, you will need to update.
Secondly, CAS relies on a third party library called phpCAS that is automagically installed with composer. You don't need to worry about this one.
Finally, phpCAS library requires php-curl. The version of curl is determined by the version of php you use. Check your Linux distribution for the installation procedures for your version of curl, if you don't already have this dependency. Windows' users can check the php website for installation procedures.
== Check out this great wikipedia article for even more CAS information!