Single sign-on (SSO) is a property of access control of multiple related, but independent software systems. With this property a user logs in once and gains access to all systems without being prompted to log in again at each of them. Single sign-off is the reverse property whereby a single action of signing out terminates access to multiple software systems.
As different applications and resources support different authentication mechanisms, single sign-on has to internally translate to and store different credentials compared to what is used for initial authentication.
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Benefits include:
SSO uses centralized authentication servers that all other applications and systems utilize for authentication purposes, and combines this with techniques to ensure that users do not have to actively enter their credentials more than once.
SSO users need not remember so many passwords to login to different systems or applications.
The term enterprise reduced sign-on is preferred by some authors who believe single sign-on to be impossible in real use cases.
As single sign-on provides access to many resources once the user is initially authenticated ("keys to the castle"), it increases the negative impact in case the credentials are available to other persons and misused. Therefore, single sign-on requires an increased focus on the protection of the user credentials, and should ideally be combined with strong authentication methods like smart cards and one-time password tokens.
Single sign-on also makes the authentication systems highly critical; a loss of their availability can result in denial of access to all systems unified under the SSO. SSO can thus be undesirable for systems to which access must be guaranteed at all times, such as security or plant-floor systems.
Windows environment - Windows login fetches TGT. Active Directory-aware applications fetch service tickets, so user is not prompted to re-authenticate.
UNIX/Linux environment - Login via Kerberos PAM modules fetches TGT. Kerberized client applications such as Evolution, Firefox, and SVN use service tickets, so user is not prompted to re-authenticate.
Initial sign on prompts the user for the smart card. Additional software applications also use the smart card, without prompting the user to re-enter credentials. Smart card-based single sign-on can either use certificates or passwords stored on the smart card.
Also referred to as one-time password token. Two-factor authentication with OTP tokens [1] follows industry best practices for authenticating users.[2] This OTP token method is more secure and effective at prohibiting unauthorized access than other authentication methods.[3]
Integrated Windows Authentication is a term associated with Microsoft products and refers to the SPNEGO, Kerberos, and NTLMSSP authentication protocols with respect to SSPI functionality introduced with Microsoft Windows 2000 and included with later Windows NT-based operating systems. The term is used more commonly for the automatically authenticated connections between Microsoft Internet Information Services and Internet Explorer. Cross-platform Active Directory integration vendors have extended the Integrated Windows Authentication paradigm to UNIX, Linux and Mac systems.
Single sign on requires that users literally sign in once to establish their credentials. Systems which require the user to log in multiple times to the same identity are inherently not single sign on. For example, an environment where users are prompted to log in to their desktop, then log in to their email using the same credentials, is not single sign on.