简要概述CAS(Central Authentication Service)
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.(摘自：http://en.wikipedia.org/wiki/Central_Authentication_Service)
CAS作为web单点登录协议，目的是允许用户访问多个应用仅需提供证书一次.也允许web应用认证用户无需访问安全证书，例如密码.
图1，使用CAS执行认证
原文地址：http://www.developertutorials.com/tutorials/java/single-sign-on-1399/
CAS server（服务端）
This distribution contains the CAS 2.0 server – that is, the web application that authenticates users, provides CAS tickets, and validates these tickets. The CAS 2.0 server is written entirely in Java and is deployable in any compliant Servlet 2.3 container (e.g., Tomcat). See below for installation instructions.
In most environments, you'll also be interested in the CAS clients we provide for many platforms. The clients are available in a separate distribution ('casclient'), available from http://www.yale.edu/tp/cas. This site also contains general information about CAS and specific white papers that cover CAS's architecture. Note that while the CAS server requires a Java Servlet container, the CAS protocol is open and accessible from most modern platforms.（摘自：https://wiki.jasig.org/display/CAS2/cas-server-2.0.12+README）
Kerberos
Kerberos is a network authentication protocol. It is designed to provide strong authentication for client/server applications by using secret-key cryptography. A free implementation of this protocol is available from the Massachusetts Institute of Technology. Kerberos is available in many commercial products as well.（摘自：http://web.mit.edu/kerberos/）
Kerberos 是网络认证协议，设计通过密钥加密提供强大认证c/s应用程序，免费使用这协议从麻省理工技术学院。被使用在多个商业产品.
SSO Framework
http://www.slideshare.net/craigsdickson/fast-and-free-sso-a-survey-of-open-source-solutions-to-single-sign-on
https://opensso.dev.java.net/
http://www.josso.org/confluence/display/JOSSO1/JOSSO+-+Java+Open+Single+Sign-On+Project+Home