|The Open–source PKI Book: A guide to PKIs and Open–source Implementations|
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Single Sign–On (SSO) is a mechanism whereby a single initial action of user authentication and authorisation can permit the user to access all computer resources where she has access persmission without the need to authenticate/authorise subsequent times.
Among the benefits of SSO is the transparency of usage of a computer system where full access control takes place but the user is not encumbered by repeated authentications and authorisations. SSO in a nutshell requires applications to use a common security mechanism and make use of the user credentials for all the session access control requirements.
The The Open Group has standardised the Pluggable Authentication Mechanism (PAM) and the corresponding standard is available at the X/Open Single Sign-On Service (XSSO) page. A general description of the SSO standard can be found at the Single Sign-On page.