SAML 2.0

Security Assertion Markup Language (SAML) is an XML standard for exchanging authentication and authorization data between security domains. SAML is a standard set by the OASIS [http://www.oasis-open.org/committees/tc_home.php?wg_abbrev=security Security Services Technical Committee] .

"SAML 2.0" was ratified as an OASIS Standard in March 2005. The critical aspects of SAML 2.0 are covered in detail in the official documents #SAMLConform, #SAMLCore, #SAMLBind, and #SAMLProf. If you are new to SAML, you should probably read the introductory SAML topic first, and then the #SAMLOverview document from OASIS.

Some 30 individuals from more than two dozen companies and organizations were involved with the creation of SAML 2.0. In particular, and of special note, Liberty Alliance donated its Identity Federation Framework (ID-FF) specification to OASIS, which became the basis of the SAML 2.0 specification. Thus SAML 2.0 represents the convergence of SAML 1.1, [http://www.projectliberty.org/resource_center/specifications/liberty_alliance_id_ff_1_2_specifications Liberty ID-FF 1.2] , and [http://shibboleth.internet2.edu/docs/internet2-mace-shibboleth-arch-protocols-200509.pdf Shibboleth 1.3] .

AML 2.0 Assertions

An important type of SAML assertion is the so-called "bearer" assertion used to facilitate Web Browser SSO. Here is an example of a short-lived bearer assertion issued by an identity provider (https://idp.example.org/SAML2) to a service provider (https://sp.example.com/SAML2). The assertion includes both a and a , which presumably the service provider uses to make an access control decision.

<saml:Assertion xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" ID="b07b804c-7c29-ea16-7300-4f3d6f7928ac" Version="2.0" IssueInstant="2004-12-05T09:22:05Z"> <saml:Issuer>https://idp.example.org/SAML2</saml:Issuer> <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">...</ds:Signature> <saml:Subject> 3f7b3dcf-1674-4ecd-92c8-1544f346baf8 </saml:Subject> <saml:Conditions NotBefore="2004-12-05T09:17:05Z" NotOnOrAfter="2004-12-05T09:27:05Z"> https://sp.example.com/SAML2 </saml:Conditions> <saml:AuthnStatement AuthnInstant="2004-12-05T09:22:00Z" SessionIndex="b07b804c-7c29-ea16-7300-4f3d6f7928ac"> urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport </saml:AuthnStatement> <saml:AttributeStatement> member staff </saml:AttributeStatement> </saml:Assertion>

Note that the element contains the following subelements:

* a element, which contains the unique identifier of the identity provider
* a element, which contains an integrity-preserving digital signature (not shown) over the element
* a element, which identifies the authenticated principal (but in this case the identity of the principal is hidden behind an opaque transient identifier, for reasons of privacy)
* a element, which gives the conditions under which the assertion is to be considered "valid"
* a element, which describes the act of authentication at the identity provider
* a element, which asserts a multi-valued attribute associated with the authenticated principal

In words, the assertion encodes the following information:

The assertion ("b07b804c-7c29-ea16-7300-4f3d6f7928ac") was issued at time "2004-12-05T09:22:05Z" by identity provider (https://idp.example.org/SAML2) regarding subject (3f7b3dcf-1674-4ecd-92c8-1544f346baf8) exclusively for service provider (https://sp.example.com/SAML2).

The authentication statement, in particular, asserts the following:

The principal identified in the element was authenticated at time "2004-12-05T09:22:00Z" by means of a password sent over a protected channel.

Likewise the attribute statement asserts that

The principal identified in the element is a staff member at this institution.

AML 2.0 Protocols

The following protocols are specified in #SAMLCore:

* Assertion Query and Request Protocol
* #Authentication Request Protocol
* #Artifact Resolution Protocol
* Name Identifier Management Protocol
* Single Logout Protocol
* Name Identifier Mapping Protocol

The most important of these protocols&mdash;the Authentication Request Protocol&mdash;is discussed in detail below.

Authentication Request Protocol

Recall that the SAML 1.1 Web Browser SSO Profiles are IdP-initiated, that is, an unsolicited element is transmitted from the identity provider to the service provider (via the browser). In SAML 2.0, however, the flow begins at the service provider who issues an explicit authentication request to the identity provider. The resulting "Authentication Request Protocol" is a significant new feature of SAML 2.0.

When a principal (or an entity acting on the principal's behalf) wishes to obtain assertions containing authentication statements, a element is transmitted to the identity provider:

<samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="aaf23196-1773-2113-474a-fe114412ab72" Version="2.0" IssueInstant="2004-12-05T09:21:59Z" AssertionConsumerServiceIndex="0" AttributeConsumingServiceIndex="0"> https://sp.example.com/SAML2 </samlp:AuthnRequest>

The above element, which implicitly requests an assertion containing an authentication statement, was evidently issued by a service provider (https://sp.example.com/SAML2) and subsequently presented to the identity provider (via the browser). The identity provider authenticates the principal (if necessary) and issues an authentication response, which is transmitted back to the service provider (again via the browser).

Artifact Resolution Protocol

A SAML message is transmitted from one entity to another either "by value" or "by reference". A reference to a SAML message is called an "artifact". The receiver of an artifact resolves the reference by sending a request directly to the issuer of the artifact, who then responds with the actual message referenced by the artifact.

Suppose, for example, that an identity provider sends the following request directly to a service provider (via a back channel):

<samlp:ArtifactResolve xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="_cce4ee769ed970b501d680f697989d14" Version="2.0" IssueInstant="2004-12-05T09:21:58Z" Destination="https://sp.example.com/SAML2/ArtifactResolution"> https://idp.example.org/SAML2 ... <samlp:Artifact>AAQAAMh48/1oXIM+sDo7Dh2qMp1HM4IF5DaRNmDj6RdUmllwn9jJHyEgIi8=</samlp:Artifact> </samlp:ArtifactResolve>

In response, the service provider returns the SAML element referenced by the enclosed artifact. This protocol forms the basis of the #HTTP Artifact Binding.

AML 2.0 Bindings

The "bindings" supported by SAML 2.0 are outlined in the Bindings specification (#SAMLBind):

* SAML SOAP Binding (based on SOAP 1.1)
* Reverse SOAP (PAOS) Binding
* HTTP Redirect (GET) Binding
* #HTTP POST Binding
* #HTTP Artifact Binding
* SAML URI Binding

For Web Browser SSO, the HTTP POST Binding is commonly used. Either the service provider or the identity provider (or both) use HTTP POST to transmit a protocol message. An entity's choice of binding is independent of its partner's choice of binding. For example, the service provider may use HTTP POST while the identity provider uses HTTP Artifact.

HTTP POST Binding

In the following example, both the service provider and the identity provider use an HTTP POST Binding. Initially, the service provider responds to a request from the user agent with a document containing an XHTML form:

...

The value of the SAMLRequest parameter is the base64 encoding of a element, which is transmitted to the identity provider via the browser. The SSO service at the identity provider validates the request and responds with a document containing another XHTML form:

...

The value of the SAMLResponse parameter is the base64 encoding of a element, which likewise is transmitted to the service provider via the browser.

To automate the submission of the form, the following line of JavaScript may appear anywhere on the XHTML page:

window.onload = function () { document.forms [0] .submit(); }

This assumes of course that the page contains a single form element (forms [0] ).

HTTP Artifact Binding

The HTTP Artifact Binding uses the #Artifact Resolution Protocol and the SAML SOAP Binding (over HTTP) to resolve a SAML message by reference. Consider the following specific example. Suppose a service provider wants to send a message to an identity provider. Initially, the service provider transmits an artifact to the identity provider via an HTTP redirect:

https://idp.example.org/SAML2/SSO/Artifact?SAMLart=artifact

Next the identity provider sends a request (such as the #ArtifactResolveRequest shown earlier) directly to the service provider via a back channel. Finally, the service provider returns a element containing the referenced message:

<samlp:ArtifactResponse xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" ID="_d84a49e5958803dedcff4c984c2b0d95" InResponseTo="_cce4ee769ed970b501d680f697989d14" Version="2.0" IssueInstant="2004-12-05T09:21:59Z"> ... <samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="_306f8ec5b618f361c70b6ffb1480eade" Version="2.0" IssueInstant="2004-12-05T09:21:59Z" Destination="https://idp.example.org/SAML2/SSO/Artifact" ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Artifact" AssertionConsumerServiceURL="https://sp.example.com/SAML2/SSO/Artifact"> https://sp.example.com/SAML2 </samlp:AuthnRequest> </samlp:ArtifactResponse>

Of course the flow can go in the other direction as well, that is, the identity provider may issue an artifact. See, for example, the "double artifact" profile example later in this topic.

Artifact Format

In general, a SAML 2.0 "artifact" is defined as follows (#SAMLBind):

SAML_artifact := B64 (TypeCode EndpointIndex RemainingArtifact) TypeCode := Byte1Byte2 EndpointIndex := Byte1Byte2

Thus a SAML 2.0 artifact consists of three components: a two-byte TypeCode, a two-byte EndpointIndex, and an arbitrary sequence of bytes called the RemainingArtifact. These three pieces of information are concatenated and base64-encoded to yield the complete artifact.

The TypeCode uniquely identifies the artifact format. SAML 2.0 predefines just one such artifact, of type 0x0004. The EndpointIndex is a reference to a particular artifact resolution endpoint managed by the artifact issuer (which may be either the IdP or the SP, as mentioned earlier). The RemainingArtifact, which is determined by the type definition, is the "meat" of the artifact.

The format of a "type 0x0004 artifact" is further defined as follows:

TypeCode := 0x0004 RemainingArtifact := SourceId MessageHandle SourceId := 20-byte_sequence MessageHandle := 20-byte_sequence

Thus a type 0x0004 artifact is of size 44 bytes (unencoded). The SourceId is an arbitrary sequence of bytes, although in practice, the SourceId is the SHA-1 hash of the issuer's entityID. The MessageHandle is a random sequence of bytes that references a SAML message that the artifact issuer is willing to produce on-demand.

For example, consider this hex-encoded type 0x0004 artifact:

00040000c878f3fd685c833eb03a3b0e1daa329d47338205e436913660e3e917549a59709fd8c91f2120222f

If you look closely, you can see the TypeCode (0x0004) and the EndpointIndex (0x0000) at the front of the artifact. The next 20 bytes are the SHA-1 hash of the issuer's entityID (https://idp.example.org/SAML2) followed by 20 random bytes. The base64-encoding of these 44 bytes is what you see in the #ArtifactResolveRequest example above.

AML 2.0 Profiles

In SAML 2.0, as in SAML 1.1, the primary use case is still Web Browser SSO, but the scope of SAML 2.0 is broader than previous versions of SAML, as suggested in the following exhaustive list of profiles:

* SSO Profiles
** #Web Browser SSO Profile
** Enhanced Client or Proxy (ECP) Profile
** #Identity Provider Discovery Profile
** Single Logout Profile
** Name Identifier Management Profile
* Artifact Resolution Profile
* #Assertion Query/Request Profile
* Name Identifier Mapping Profile
* SAML Attribute Profiles
** Basic Attribute Profile
** X.500/LDAP Attribute Profile
** UUID Attribute Profile
** DCE PAC Attribute Profile
** XACML Attribute Profile

Although the number of supported profiles is quite large, the Profiles specification (#SAMLProf) is simplified since the binding aspects of each profile have been factored out into a separate Bindings specification (#SAMLBind).

Web Browser SSO Profile

SAML 2.0 specifies a "Web Browser SSO Profile" involving an identity provider (IdP), a service provider (SP), and a principal wielding an HTTP user agent. The SP has four bindings from which to choose while the IdP has three, which leads to twelve (12) possible deployment scenarios. We outline two such deployment scenarios below.

P POST Request; IdP POST Response

This is a relatively simple deployment of the SAML 2.0 Web Browser SSO Profile where both the service provider (SP) and the identity provider (IdP) use the HTTP POST binding.

The message flow begins with a request for a secured resource at the SP.

1. Request the target resource at the SP

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&ndash;7.

2. Respond with an XHTML form

The service provider responds with a document containing an XHTML form:

...

The RelayState token is an opaque reference to state information maintained at the service provider. The value of the SAMLRequest parameter is the base64 encoding of the following element:

<samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="identifier_1" Version="2.0" IssueInstant="2004-12-05T09:21:59Z" AssertionConsumerServiceIndex="0"> https://sp.example.com/SAML2 </samlp:AuthnRequest>

Before the element is URL-encoded and inserted into the XHTML form, it is first deflated and base64-encoded (in that order).

3. Request the SSO Service at the IdP

The user agent issues a POST request to the SSO service at the identity provider:

POST /SAML2/SSO/POST HTTP/1.1 Host: idp.example.org Content-Type: application/x-www-form-urlencoded Content-Length: nnn
SAMLRequest=request&RelayState=token

where the values of the SAMLRequest and RelayState parameters are taken from the XHTML form at step 2. The SSO service processes the element (by URL-decoding, base64-decoding and inflating the request, in that order) 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:

...

The value of the RelayState parameter has been preserved from step 3. The value of the SAMLResponse parameter is the base64 encoding of the following element:

<samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="identifier_2" InResponseTo="identifier_1" Version="2.0" IssueInstant="2004-12-05T09:22:05Z" Destination="https://sp.example.com/SAML2/SSO/POST"> https://idp.example.org/SAML2 <saml:Assertion xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="identifier_3" Version="2.0" IssueInstant="2004-12-05T09:22:05Z"> https://idp.example.org/SAML2 ... 3f7b3dcf-1674-4ecd-92c8-1544f346baf8 https://sp.example.com/SAML2 urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport </saml:Assertion> </samlp:Response>

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:

POST /SAML2/SSO/POST HTTP/1.1 Host: sp.example.com Content-Type: application/x-www-form-urlencoded Content-Length: nnn
SAMLResponse=response&RelayState=token

where the values of the SAMLResponse and RelayState parameters are 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.

P Redirect Artifact; IdP Redirect Artifact

This is a complex deployment of the SAML 2.0 Web Browser SSO Profile where both the service provider (SP) and the identity provider (IdP) use the HTTP Artifact binding. Both artifacts are delivered to their respective endpoints via HTTP GET.

The message flow begins with a request for a secured resource at the SP:

1. Request the target resource at the SP

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&ndash;11.

2. Redirect to the Single Sign-on (SSO) Service at the IdP

The service provider redirects the user agent to the single sign-on (SSO) service at the identity provider. A RelayState parameter and a SAMLart parameter are appended to the redirect URL.

3. Request the SSO Service at the IdP

The user agent requests the SSO service at the identity provider:

https://idp.example.org/SAML2/SSO/Artifact?SAMLart=artifact_1&RelayState=token

where token is an opaque reference to state information maintained at the service provider and artifact_1 is a SAML artifact, both issued at step 2.

4. Request the Artifact Resolution Service at the SP

The SSO service dereferences the artifact by sending a element bound to a SAML SOAP message to the artifact resolution service at the service provider:

<samlp:ArtifactResolve xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="identifier_1" Version="2.0" IssueInstant="2004-12-05T09:21:58Z" Destination="https://sp.example.com/SAML2/ArtifactResolution"> https://idp.example.org/SAML2 ... <samlp:Artifact>artifact_1</samlp:Artifact> </samlp:ArtifactResolve>

where the value of the element is the SAML artifact transmitted at step 3.

5. Respond with a SAML AuthnRequest

The artifact resolution service at the service provider returns a element (containing an element) bound to a SAML SOAP message to the SSO service at the identity provider:

<samlp:ArtifactResponse xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" ID="identifier_2" InResponseTo="identifier_1" Version="2.0" IssueInstant="2004-12-05T09:21:59Z"> ... <samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="identifier_3" Version="2.0" IssueInstant="2004-12-05T09:21:59Z" Destination="https://idp.example.org/SAML2/SSO/Artifact" ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Artifact" AssertionConsumerServiceURL="https://sp.example.com/SAML2/SSO/Artifact"> https://sp.example.com/SAML2 </samlp:AuthnRequest> </samlp:ArtifactResponse>

The SSO service processes the element and performs a security check. If the user does not have a valid security context, the identity provider identifies the user (details omitted).

6. Redirect to the Assertion Consumer Service

The SSO service at the identity provider redirects the user agent to the assertion consumer service at the service provider. The previous RelayState parameter and a new SAMLart parameter are appended to the redirect URL.

7. Request the Assertion Consumer Service at the SP

The user agent requests the assertion consumer service at the service provider:

https://sp.example.com/SAML2/SSO/Artifact?SAMLart=artifact_2&RelayState=token

where token is the token value from step 3 and artifact_2 is the SAML artifact issued at step 6.

8. Request the Artifact Resolution Service at the IdP

The assertion consumer service dereferences the artifact by sending a element bound to a SAML SOAP message to the artifact resolution service at the identity provider:

<samlp:ArtifactResolve xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="identifier_4" Version="2.0" IssueInstant="2004-12-05T09:22:04Z" Destination="https://idp.example.org/SAML2/ArtifactResolution"> https://sp.example.com/SAML2 ... <samlp:Artifact>artifact_2</samlp:Artifact> </samlp:ArtifactResolve>

where the value of the element is the SAML artifact transmitted at step 7.

9. Respond with a SAML Assertion

The artifact resolution service at the identity provider returns a element (containing an element) bound to a SAML SOAP message to the assertion consumer service at the service provider:

<samlp:ArtifactResponse xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" ID="identifier_5" InResponseTo="identifier_4" Version="2.0" IssueInstant="2004-12-05T09:22:05Z"> ... <samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="identifier_6" InResponseTo="identifier_3" Version="2.0" IssueInstant="2004-12-05T09:22:05Z" Destination="https://sp.example.com/SAML2/SSO/Artifact"> https://idp.example.org/SAML2 ... <saml:Assertion xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="identifier_7" Version="2.0" IssueInstant="2004-12-05T09:22:05Z"> https://idp.example.org/SAML2 [e-mail] https://sp.example.com/SAML2 urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport </saml:Assertion> </samlp:Response> </samlp:ArtifactResponse>

10. 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.

11. 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

12. Respond with the requested resource

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

Identity Provider Discovery Profile

The SAML 2.0 "Identity Provider Discovery Profile" introduces the following concepts:

* Common Domain
* Common Domain Cookie
* Common Domain Cookie Writing Service
* Common Domain Cookie Reading Service

As a hypothetical example of a "Common Domain", let's suppose NWA (nwa.com) and KLM (klm.com) belong to the virtual organization SkyTeam Global Alliance (skyteam.com). In this example, the domain skyteam.com is the common domain. Both NWA and KLM have a presence in this domain (nwa.skyteam.com and klm.skyteam.com, resp.).

The "Common Domain Cookie" is a secure browser cookie scoped to the common domain. For each browser user, this cookie stores a history list of recently visited IdPs. The name and value of the cookie are specified in the IdP Discovery Profile (#SAMLProf).

After a successful act of authentication, the IdP requests the "Common Domain Cookie Writing Service". This service appends the IdP's unique identifier to the common domain cookie. An SP, when it receives an unauthenticated request for a protected resource, requests the "Common Domain Cookie Reading Service" to discover the browser user's most recently used IdP.

Assertion Query/Request Profile

The "Assertion Query/Request Profile" is a general profile that accommodates numerous types of so-called "queries" using the following SAML 2.0 elements:

* the element, which is used to request an assertion given its unique identifier (ID)
* the element, which is an abstract extension point that allows new subject-based SAML queries to be defined
* the element, which is used to request existing authentication assertions about a given subject from an Authentication Authority
* the element, which is used to request attributes about a given subject from an Attribute Authority
* the element, which is used to request an authorization decision from a trusted third party

The SAML SOAP binding is often used in conjunction with queries.

AML Attribute Query

The "Attribute Query" is perhaps the most important type of SAML query. Often a requester, acting on behalf of the principal, queries an identity provider for attributes. Below we give an example of a query issued by a principal directly:

<samlp:AttributeQuery xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" ID="aaf23196-1773-2113-474a-fe114412ab72" Version="2.0" IssueInstant="2006-07-17T20:31:40Z"> <saml:Issuer Format="urn:oasis:names:tc:SAML:1.1:nameid-format:X509SubjectName"> C=US, O=NCSA-TEST, OU=User, CN=[e-mail] </saml:Issuer> <saml:Subject> C=US, O=NCSA-TEST, OU=User, CN=[e-mail] </saml:Subject> </samlp:AttributeQuery>

Note that the Issuer is the Subject in this case. This is sometimes called an "attribute self-query". An identity provider might return the following assertion, wrapped in a element (not shown):

<saml:Assertion xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" ID="_33776a319493ad607b7ab3e689482e45" Version="2.0" IssueInstant="2006-07-17T20:31:41Z"> <saml:Issuer>https://idp.example.org/SAML2</saml:Issuer> ... <saml:Subject> C=US, O=NCSA-TEST, OU=User, CN=[e-mail] MIICiDCCAXACCQDE+9eiWrm62jANBgkqhkiG9w0BAQQFADBFMQswCQYDVQQGEwJV UzESMBAGA1UEChMJTkNTQS1URVNUMQ0wCwYDVQQLEwRVc2VyMRMwEQYDVQQDEwpT UC1TZXJ2aWNlMB4XDTA2MDcxNzIwMjE0MVoXDTA2MDcxODIwMjE0MVowSzELMAkG A1UEBhMCVVMxEjAQBgNVBAoTCU5DU0EtVEVTVDENMAsGA1UECxMEVXNlcjEZMBcG A1UEAwwQdHJzY2F2b0B1aXVjLmVkdTCBnzANBgkqhkiG9w0BAQEFAAOBjQAwgYkC gYEAv9QMe4lRl3XbWPcflbCjGK9gty6zBJmp+tsaJINM0VaBaZ3t+tSXknelYife nCc2O3yaX76aq53QMXy+5wKQYe8Rzdw28Nv3a73wfjXJXoUhGkvERcscs9EfIWcC g2bHOg8uSh+Fbv3lHih4lBJ5MCS2buJfsR7dlr/xsadU2RcCAwEAATANBgkqhkiG 9w0BAQQFAAOCAQEAdyIcMTob7TVkelfJ7+I1j0LO24UlKvbLzd2OPvcFTCv6fVHx Ejk0QxaZXJhreZ6+rIdiMXrEzlRdJEsNMxtDW8++sVp6avoB5EX1y3ez+CEAIL4g cjvKZUR4dMryWshWIBHKFFul+r7urUgvWI12KbMeE9KP+kiiiiTskLcKgFzngw1J selmHhTcTCrcDocn5yO2+d3dog52vSOtVFDBsBuvDixO2hv679JR6Hlqjtk4GExp E9iVI0wdPE038uQIJJTXlhsMMLvUGVh/c0ReJBn92Vj4dI/yy6PtY/8ncYLYNkjg oVN0J/ymOktn9lTlFyTiuY4OuJsZRO1+zWLy9g=

</saml:Subject> <saml:Conditions NotBefore="2006-07-17T20:31:41Z" NotOnOrAfter="2006-07-18T20:21:41Z"> </saml:Conditions> <saml:AuthnStatement AuthnInstant="2006-07-17T20:31:41Z"> urn:oasis:names:tc:SAML:2.0:ac:classes:TLSClient </saml:AuthnStatement> <saml:AttributeStatement> Tom [e-mail] </saml:AttributeStatement> </saml:Assertion>

In contrast to the #BearerAssertion shown earlier, this assertion has a longer lifetime corresponding to the lifetime of the X.509 certificate that the principal used to authenticate to the identity provider. Moreover, since the assertion is signed, the user can push this assertion to a relying party, and as long as the user can prove possession of the corresponding private key (hence the name "holder-of-key"), the relying party can be assured that the assertion is authentic.

AML 2.0 Metadata

Quite literally, metadata is what makes SAML work (or work well). Let's look at some examples of metadata at work:

* An identity provider receives an element from a service provider via the browser. How does the identity provider know the service provider is authentic and not some evil service provider trying to pharm private information regarding the user? Answer: Metadata! The identity provider consults its list of trusted service providers (in metadata) before issuing an authentication response.
* In the previous scenario, how does the identity provider know where to redirect the user with the authentication response? Answer: Metadata! The identity provider looks up a pre-arranged endpoint location of the service provider (in metadata).
* How does the service provider know that the authentication response came from a trusted identity provider? Answer: Metadata! The service provider validates the signature on the assertion using the public key of the identity provider (from metadata).
* How does the service provider know where to resolve an artifact from a trusted identity provider? Answer: Metadata! The service provider looks up the pre-arranged endpoint location of the identity provider's artifact resolution service from metadata.

The list goes on and on. Metadata bootstraps a secure transaction between an identity provider and a service provider. Before metadata, trust information was encoded into the implementation in a proprietary manner. Now the sharing of trust information is facilitated by standard metadata. SAML 2.0 provides a well-defined, interoperable metadata format that entities can leverage to bootstrap the trust process.

Identity Provider Metadata

An identity provider publishes data about itself in an element:

<md:EntityDescriptor xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" entityID="https://idp.example.org/SAML2"> &lt;!-- insert ds:Signature element --&gt; &lt;!-- insert md:IDPSSODescriptor element (below) --&gt; &lt;!-- insert md:AttributeAuthorityDescriptor element (not shown) --&gt; SAML Identity Provider SAML Identity Provider @ Some Location http://www.idp.example.org/ SAML IdP Support mailto:[e-mail] </md:EntityDescriptor>

The entityID attribute is the unique identifier of the identity provider. Note that the details of the digital signature (in the element) have been omitted from this example.

The identity provider manages an SSO service and an attribute authority, each having its own descriptor. We describe SSO service metadata below while the element is not shown.

O Service Metadata

The SSO service at the identity provider is described in an element:

<md:IDPSSODescriptor protocolSupportEnumeration="urn:oasis:names:tc:SAML:2.0:protocol"> IdP SSO Key urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress urn:oasis:names:tc:SAML:2.0:nameid-format:transient member student faculty employee staff </md:IDPSSODescriptor>

The previous metadata element describes the SSO service at the identity provider. Note the following details about this element:

* Key information has been omitted for brevity.
* The Binding attribute of the element indicates that the SAML SOAP binding (#SAMLBind) should be used for artifact resolution.
* The Location attribute of the element is used in step 8 of the "double artifact" profile.
* The value of the index attribute of the element is used as the EndpointIndex in the construction of a SAML type 0x0004 artifact.
* The elements indicate what SAML name identifier formats (#SAMLCore) the SSO service supports.
* The Binding attributes of the elements are standard URIs specified in the SAML 2.0 Binding specification (#SAMLBind).
* The Location attribute of the element that supports the HTTP POST binding is used in step 2 of the "double POST" profile.
* The Location attribute of the element that supports the HTTP Artifact binding is used in step 2 of the "double artifact" profile.
* The element describes an attribute that the identity provider is willing to assert (subject to policy). The elements enumerate the possible values the attribute may take on.

ervice Provider Metadata

A service provider also publishes data about itself in an element:

<md:EntityDescriptor xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" entityID="https://sp.example.com/SAML2"> &lt;!-- insert ds:Signature element --&gt; &lt;!-- insert md:SPSSODescriptor element (see below) --&gt; SAML Service Provider SAML Service Provider @ Some Location http://www.sp.example.com/ SAML SP Support mailto:[e-mail] </md:EntityDescriptor>

The primary component managed by the service provider is the assertion consumer service, which is discussed below.

Assertion Consumer Service Metadata

The assertion consumer service is represented by an element:

<md:SPSSODescriptor protocolSupportEnumeration="urn:oasis:names:tc:SAML:2.0:protocol"> SP SSO Key urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress urn:oasis:names:tc:SAML:2.0:nameid-format:transient Service Provider Portal </md:SPSSODescriptor>

Note the following details about the metadata element:

* The index attribute of an element is used as the value of the AssertionConsumerServiceIndex attribute in a element.
* The Binding attributes of the elements are standard URIs specified in the SAML 2.0 Binding specification (#SAMLBind).
* The Location attribute of the element that supports the HTTP POST binding (index="0") is used in step 4 of the "double POST" profile.
* The Location attribute of the element that supports the HTTP Artifact binding (index="1") is used in step 6 of the "double artifact" profile.
* The element is used by the identity provider to formulate an element that is pushed to the service provider in conjunction with Web Browser SSO.
* The index attribute of the element is used as the value of the AssertionConsumingServiceIndex attribute in a element.

As noted earlier, the values of the Location attributes are used by an identity provider to route SAML messages, which minimizes the possibility of a rogue service provider orchestrating a man-in-the-middle attack.

References

* [SAMLOverview] N. Ragouzis et al., "Security Assertion Markup Language (SAML) V2.0 Technical Overview." OASIS Draft, February 2007. Document ID sstc-saml-tech-overview-2.0-draft-13 http://www.oasis-open.org/committees/download.php/22553/sstc-saml-tech-overview-2%200-draft-13.pdf

* [SAMLConform] P. Mishra et al. "Conformance Requirements for the OASIS Security Assertion Markup Language (SAML) V2.0." OASIS Standard, March 2005. Document ID saml-conformance-2.0-os http://docs.oasis-open.org/security/saml/v2.0/saml-conformance-2.0-os.pdf

* [SAMLCore] S. Cantor et al. "Assertions and Protocols for the OASIS Security Assertion Markup Language (SAML) V2.0." OASIS Standard, March 2005. Document ID saml-core-2.0-os http://docs.oasis-open.org/security/saml/v2.0/saml-core-2.0-os.pdf

* [SAMLBind] S. Cantor et al. "Bindings for the OASIS Security Assertion Markup Language (SAML) V2.0." OASIS Standard, March 2005. Document ID saml-bindings-2.0-os http://docs.oasis-open.org/security/saml/v2.0/saml-bindings-2.0-os.pdf

* [SAMLProf] S. Cantor et al. "Profiles for the OASIS Security Assertion Markup Language (SAML) V2.0." OASIS Standard, March 2005. Document ID saml-profiles-2.0-os http://docs.oasis-open.org/security/saml/v2.0/saml-profiles-2.0-os.pdf

* [SAMLMeta] S. Cantor et al. "Metadata for the OASIS Security Assertion Markup Language (SAML) V2.0." OASIS Standard, March 2005. Document ID saml-metadata-2.0-os http://docs.oasis-open.org/security/saml/v2.0/saml-metadata-2.0-os.pdf

* [SAMLAuthnCtx] J. Kemp et al. "Authentication Context for the OASIS Security Assertion Markup Language (SAML) V2.0." OASIS Standard, March 2005. Document ID saml-authn-context-2.0-os http://docs.oasis-open.org/security/saml/v2.0/saml-authn-context-2.0-os.pdf

* [SAMLSecurity] F. Hirsch et al. "Security and Privacy Considerations for the OASIS Security Assertion Markup Language (SAML) V2.0." OASIS Standard, March 2005. Document ID saml-sec-consider-2.0-os http://docs.oasis-open.org/security/saml/v2.0/saml-sec-consider-2.0-os.pdf

* [SAMLGlossary] J. Hodges et al. "Glossary for the OASIS Security Assertion Markup Language (SAML) V2.0." OASIS Standard, March 2005. Document ID saml-glossary-2.0-os http://docs.oasis-open.org/security/saml/v2.0/saml-glossary-2.0-os.pdf

ee also

* SAML
* SAML 1.1