Introduction to SAML

General Concept

samlSecurity Assertion Markup Language (SAML, pronounced “sam-el”) 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. SAML is a product of the OASIS Security Services Technical Committee. SAML dates from 2001; the most recent update of SAML is from 2005.

The single most important problem that SAML addresses is the web browser single sign-on (SSO) problem

The SAML specification defines three roles:

  • the principal (typically a user),
  • the identity provider (aka IdP),
  • and the service provider (aka 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. Conversely, one SP may rely on and trust assertions from many independent IdPs.

SAML does not specify the implementation of the identity provider service; it may use a username/password, it may use multifactor authentication, it may have an opaque implementation. A company’s directory service, which allows users to login with a user name and password, is a typical example of an identity provider. Any of the popular common internet social services also provide identity services that in theory could be used to support SAML exchanges.

The Use-case

The primary SAML use case is called Web Browser Single Sign-On (SSO). A user wielding a user agent (usually a web browser) requests a web resource protected by a SAML service provider. The service provider, wishing to know the identity of the requesting user, issues an authentication request to a SAML identity provider through the user agent. The resulting protocol flow is depicted in the following diagram.

1. Request the target resource at the SP (SAML 2.0 only)

The principal (via an HTTP user agent) requests a target resource at the service provider:

https://sp.example.com/myresource

The service provider performs a security check on behalf of the target resource. If a valid security context at the service provider already exists, skip steps 2–7.

2. Redirect to the SSO Service at the IdP (SAML 2.0 only)

The service provider determines the user’s preferred identity provider (by unspecified means) and redirects the user agent to the SSO Service at the identity provider:

 https://idp.example.org/SAML2/SSO/Redirect?SAMLRequest=request

The value of the SAMLRequest parameter is the Base64 encoding of a deflated <samlp:AuthnRequest> element.

3. Request the SSO Service at the IdP (SAML 2.0 only)

The user agent issues a GET request to the SSO service at the identity provider where the value of the SAMLRequest parameter is taken from the URL query string at step 2. The SSO service processes theAuthnRequest and performs a security check. If the user does not have a valid security context, the identity provider identifies the user (details omitted).

4. Respond with an XHTML form

The SSO service validates the request and responds with a document containing an XHTML form:

<form method="post" action="https://sp.example.com/SAML2/SSO/POST" ...>
    <input type="hidden" name="SAMLResponse" value="response" />
    ...
    <input type="submit" value="Submit" />
</form>

The value of the SAMLResponse parameter is the base64 encoding of a <samlp:Response> element.

5. Request the Assertion Consumer Service at the SP

The user agent issues a POST request to the assertion consumer service at the service provider. The value of the SAMLResponse parameter is taken from the XHTML form at step 4.

6. Redirect to the target resource

The assertion consumer service processes the response, creates a security context at the service provider and redirects the user agent to the target resource.

7. Request the target resource at the SP again

The user agent requests the target resource at the service provider (again):

 https://sp.example.com/myresource

8. Respond with requested resource

Since a security context exists, the service provider returns the resource to the user agent.

Note: In SAML 1.1, the flow begins with a request to the identity provider’s inter-site transfer service at step 3.

Some Related Information

1. The use of SOAP

In the example flow above, all depicted exchanges are front-channel exchanges, that is, an HTTP user agent (browser) communicates with a SAML entity at each step. In particular, there are no back-channel exchanges or direct communications between the service provider and the identity provider. Front-channel exchanges lead to simple protocol flows where all messages are passed by value using a simple HTTP binding (GET or POST). Indeed, the flow outlined in the previous section is sometimes called the Lightweight Web Browser SSO Profile.

Alternatively, for increased security or privacy, messages may be passed by reference. For example, an identity provider may supply a reference to a SAML assertion (called an artifact) instead of transmitting the assertion directly through the user agent. Subsequently, the service provider requests the actual assertion via a back channel. Such a back-channel exchange is specified as a SOAP message exchange (SAML over SOAP over HTTP). In general, any SAML exchange over a secure back channel is conducted as a SOAP message exchange.

On the back channel, SAML specifies the use of SOAP 1.1. The use of SOAP as a binding mechanism is optional, however. Any given SAML deployment will choose whatever bindings are appropriate.

2. SAML security

The SAML specifications recommend, and in some cases mandate, a variety of security mechanisms:

Requirements are often phrased in terms of (mutual) authentication, integrity, and confidentiality, leaving the choice of security mechanism to implementers and deployers.

[Source: WikiPedia]

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