SAML

Introduction
Security Assertion Markup Language (SAML, pronounced sam-el[1]) is an XML-based, open-standard data format for exchanging authentication and authorization data between parties, in particular, between an identity provider and a service provider.

How SAML works

The SAML specification defines three roles: the principal (typically a user), the Identity provider (IdP), and the service provider (SP). In the use case addressed by SAML, the principal requests a service from the service provider. The service provider requests and obtains an identity assertion from the identity provider. On the basis of this assertion, the service provider can make an access control decision – in other words it can decide whether to perform some service for the connected principal.

Before delivering the identity assertion to the SP, the IdP may request some information from the principal – such as a user name and password – in order to authenticate the principal. SAML specifies the assertions between the three parties: in particular, the messages that assert identity that are passed from the IdP to the SP. In SAML, one identity provider may provide SAML assertions to many service providers. Similarly, one SP may rely on and trust assertions from many independent IdPs.

SAML does not specify the method of authentication at the identity provider; it may use a username and password, or other form of authentication, including multi-factor authentication. A directory service such as LDAP, RADIUS, or Active Directory that allows users to log in with a user name and password is a typical source of authentication tokens at an identity provider.[3] The popular Internet social networking services also provide identity services that in theory could be used to support SAML exchanges.

SAML Assertions
SAML assertions are usually transferred from identity providers to service providers. Assertions contain statements that service providers use to make access-control decisions. Three types of statements are provided by SAML:

    Authentication statements
    Attribute statements
    Authorization decision statements

Authentication statements assert to the service provider that the principal did indeed authenticate with the identity provider at a particular time using a particular method of authentication. Other information about the authenticated principal (called the authentication context) may be disclosed in an authentication statement.

An attribute statement asserts that a subject is associated with certain attributes. An attribute is simply a name-value pair. Relying parties use attributes to make access-control decisions.

An authorization decision statement asserts that a subject is permitted to perform action A on resource R given evidence E. The expressiveness of authorization decision statements in SAML is intentionally limited. More-advanced use cases are encouraged to use XACML instead.

SAML wiki link: https://en.wikipedia.org/wiki/Security_Assertion_Markup_Language