Security Assertion Markup Language (SAML, pronounced SAM-el, ) is an open standard for exchanging authentication and authorization data between parties, in particular, between an identity provider and a service provider. SAML is an XML-based markup language for security assertions (statements that service providers use to make access-control decisions). SAML is also: A set of XML-based protocol messages A set of protocol message bindings A set of profiles (utilizing all of the above)
An important use case that SAML addresses is web-browser single sign-on (SSO). Single sign-on is relatively easy to accomplish within a security domain (using cookies, for example) but extending SSO across security domains is more difficult and resulted in the proliferation of non-interoperable proprietary technologies. The SAML Web Browser SSO profile was specified and standardized to promote interoperability.J. Hughes et al. Profiles for the OASIS Security Assertion Markup Language (SAML) 2.0. OASIS Standard, March 2005. Document identifier: saml-profiles-2.0-os https://docs.oasis-open.org/security/saml/v2.0/saml-profiles-2.0-os.pdf (for the latest working draft of this specification with errata, see: https://www.oasis-open.org/committees/download.php/56782/sstc-saml-profiles-errata-2.0-wd-07.pdf) In practice, SAML SSO is most commonly used for authentication into cloud-based business software.
Overview
The SAML specification defines three roles: the principal (typically a human user), the identity provider (IdP) and the service provider (SP). In the primary use case addressed by SAML, the principal requests a service from the service provider. The service provider requests and obtains an authentication assertion from the identity provider. On the basis of this assertion, the service provider can make an access control decision, that is, it can decide whether to perform the service for the connected principal.
At the heart of the SAML assertion is a subject (a principal within the context of a particular security domain) about which something is being asserted. The subject is usually (but not necessarily) a human. As in the SAML 2.0 Technical Overview,N. Ragouzis et al. Security Assertion Markup Language (SAML) 2.0 Technical Overview. OASIS Committee Draft 02, March 2008. Document identifier: sstc-saml-tech-overview-2.0-cd-02 https://wiki.oasis-open.org/security/Saml2TechOverview the terms subject and principal are used interchangeably.
Before delivering the subject-based assertion from Identity Provider to the Service Provider, the Identity Provider may request some information from the principal (such as a user name and password) in order to authenticate the principal. SAML specifies the content of the assertion that is passed from the Identity Provider to the Service Provider. In SAML, one Identity Provider may provide SAML assertions to many Service Providers. Similarly, one Service Provider (SP) may rely on and trust assertions from many independent Identity Providers (IdP).
SAML does not specify the method of authentication at the identity provider. The IdP may use a username and password, or some other form of authentication, including multi-factor authentication or Kerberos tickets. A directory service such as RADIUS or LDAP that allows users to log in with a user name and password is a typical source of authentication tokens at an identity provider. The popular Internet social networking services also provide identity services that in theory could be used to support SAML exchanges.
History
The Organization for the Advancement of Structured Information Standards (OASIS) Security Services Technical Committee (SSTC), which met for the first time in January 2001, was chartered "to define an XML framework for exchanging authentication and authorization information." To this end, the following intellectual property was contributed to the SSTC during the first two months of that year: Security Services Markup Language (S2ML) from Netegrity AuthXML from Securant XML Trust Assertion Service Specification (X-TASS) from VeriSign Information Technology Markup Language (ITML) from Jamcracker
Building on these initial contributions, in November 2002 OASIS announced the Security Assertion Markup Language (SAML) 1.0 specification as an OASIS Standard.
Meanwhile, the Liberty Alliance, a large consortium of companies, non-profit and government organizations, proposed an extension to the SAML standard called the Liberty Identity Federation Framework (ID-FF). Like its SAML predecessor, Liberty ID-FF proposed a standardized, cross-domain, web-based, single sign-on framework. In addition, Liberty described a circle of trust where each participating domain is trusted to accurately document the processes used to identify a user, the type of authentication system used, and any policies associated with the resulting authentication credentials. Other members of the circle of trust could then examine these policies to determine whether to trust such information.
While Liberty was developing ID-FF, the SSTC began work on a minor upgrade to the SAML standard. The resulting SAML 1.1 specification was ratified by the SSTC in September 2003. Then, in November of that same year, Liberty contributed ID-FF 1.2 to OASIS, thereby sowing the seeds for the next major version of SAML. In March 2005, SAML 2.0 was announced as an OASIS Standard. SAML 2.0 represents the convergence of Liberty ID-FF and proprietary extensions contributed by the Shibboleth project, as well as early versions of SAML itself. Most SAML implementations support v2.0 while many still support v1.1 for backward compatibility. By January 2008, deployments of SAML 2.0 became common in government, higher education, and commercial enterprises worldwide.
Versions
SAML has undergone one minor and one major revision since 1.0. SAML 1.0 was adopted as an OASIS Standard in November 2002 SAML 1.1 was ratified as an OASIS Standard in September 2003 SAML 2.0 became an OASIS Standard in March 2005
The Liberty Alliance contributed its Identity Federation Framework (ID-FF) to the OASIS SSTC in September 2003: ID-FF 1.1 was released in April 2003 ID-FF 1.2 was finalized in November 2003
Versions 1.0 and 1.1 of SAML are similar even though small differences exist., however, the differences between SAML 2.0 and SAML 1.1 are substantial. Although the two standards address the same use case, SAML 2.0 is incompatible with its predecessor.
Although ID-FF 1.2 was contributed to OASIS as the basis of SAML 2.0, there are some important differences between SAML 2.0 and ID-FF 1.2. In particular, the two specifications, despite their common roots, are incompatible.
Design
SAML is built upon a number of existing standards:
Extensible Markup Language (XML): Most SAML exchanges are expressed in a standardized dialect of XML, which is the root for the name SAML (Security Assertion Markup Language). XML Schema (XSD): SAML assertions and protocols are specified (in part) using XML Schema. XML Signature: Both SAML 1.1 and SAML 2.0 use digital signatures (based on the XML Signature standard) for authentication and message integrity. XML Encryption: Using XML Encryption, SAML 2.0 provides elements for encrypted name identifiers, encrypted attributes, and encrypted assertions (SAML 1.1 does not have encryption capabilities). XML Encryption is reported to have severe security concerns. Hypertext Transfer Protocol (HTTP): SAML relies heavily on HTTP as its communications protocol. Simple Object Access Protocol (SOAP): SAML specifies the use of SOAP, specifically SOAP 1.1 .SOAP 1.1
SAML defines XML-based assertions and protocols, bindings, and profiles. The term SAML Core refers to the general syntax and semantics of SAML assertions as well as the protocol used to request and transmit those assertions from one system entity to another. SAML protocol refers to what is transmitted, not how (the latter is determined by the choice of binding). So SAML Core defines "bare" SAML assertions along with SAML request and response elements.
A SAML binding determines how SAML requests and responses map onto standard messaging or communications protocols. An important (synchronous) binding is the SAML SOAP binding.
A SAML profile is a concrete manifestation of a defined use case using a particular combination of assertions, protocols and bindings.
Assertions
A SAML assertion contains a packet of security information:
<saml:Assertion ...> .. </saml:Assertion>
Loosely speaking, a relying party interprets an assertion as follows:
Assertion A was issued at time t by issuer R regarding subject S provided conditions C are valid.
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 principal 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 principal 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.
Protocols
A SAML protocol describes how certain SAML elements (including assertions) are packaged within SAML request and response elements, and gives the processing rules that SAML entities must follow when producing or consuming these elements. For the most part, a SAML protocol is a simple request-response protocol.
The most important type of SAML protocol request is called a query. A service provider makes a query directly to an identity provider over a secure back channel. Thus query messages are typically bound to SOAP.
Corresponding to the three types of statements, there are three types of SAML queries:
Authentication query Attribute query Authorization decision query
The result of an attribute query is a SAML response containing an assertion, which itself contains an attribute statement. See the SAML 2.0 topic for an example of attribute query/response.
Beyond queries, SAML 1.1 specifies no other protocols.
SAML 2.0 expands the notion of protocol considerably. The following protocols are described in detail in SAML 2.0 Core: Assertion Query and Request Protocol Authentication Request Protocol Artifact Resolution Protocol Name Identifier Management Protocol Single Logout Protocol Name Identifier Mapping Protocol
Most of these protocols are new in SAML 2.0.
Bindings
A SAML binding is a mapping of a SAML protocol message onto standard messaging formats and/or communications protocols. For example, the SAML SOAP binding specifies how a SAML message is encapsulated in a SOAP envelope, which itself is bound to an HTTP message.
SAML 1.1 specifies just one binding, the SAML SOAP Binding. In addition to SOAP, implicit in SAML 1.1 Web Browser SSO are the precursors of the HTTP POST Binding, the HTTP Redirect Binding, and the HTTP Artifact Binding. These are not defined explicitly, however, and are only used in conjunction with SAML 1.1 Web Browser SSO. The notion of binding is not fully developed until SAML 2.0.
SAML 2.0 completely separates the binding concept from the underlying profile. In fact, there is a brand new binding specification in SAML 2.0 that defines the following standalone bindings: