mod_auth_pubtkt is a simple Web single sign-on (SSO) solution for Apache. It
validates authentication tickets provided by the client in a cookie using
public-key cryptography (DSA or RSA). Thus, only the login server that
generates the tickets needs to possess the private key, while Web servers
can verify tickets given only the public key. The implementation of the
login server is left to the user, but an example and a library in PHP are
provided with the distribution.

WWW: https://neon1.net/mod_auth_pubtkt/