SAML 2.0

From Wikipedia, the free encyclopedia

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 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, Liberty ID-FF 1.2, and Shibboleth 1.3.

Contents

[edit] SAML 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 <saml:AuthnStatement> and a <saml:AttributeStatement>, 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>
     <saml:NameID
       Format="urn:oasis:names:tc:SAML:2.0:nameid-format:transient">
       3f7b3dcf-1674-4ecd-92c8-1544f346baf8
     </saml:NameID>
     <saml:SubjectConfirmation
       Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
       <saml:SubjectConfirmationData
         InResponseTo="aaf23196-1773-2113-474a-fe114412ab72"
         Recipient="https://sp.example.com/SAML2/SSO/POST"
         NotOnOrAfter="2004-12-05T09:27:05Z"/>
     </saml:SubjectConfirmation>
   </saml:Subject>
   <saml:Conditions
     NotBefore="2004-12-05T09:17:05Z"
     NotOnOrAfter="2004-12-05T09:27:05Z">
     <saml:AudienceRestriction>
       <saml:Audience>https://sp.example.com/SAML2</saml:Audience>
     </saml:AudienceRestriction>
   </saml:Conditions>
   <saml:AuthnStatement
     AuthnInstant="2004-12-05T09:22:00Z"
     SessionIndex="b07b804c-7c29-ea16-7300-4f3d6f7928ac">
     <saml:AuthnContext>
       <saml:AuthnContextClassRef>
         urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport
      </saml:AuthnContextClassRef>
     </saml:AuthnContext>
   </saml:AuthnStatement>
   <saml:AttributeStatement>
     <saml:Attribute
       xmlns:x500="urn:oasis:names:tc:SAML:2.0:profiles:attribute:X500"
       x500:Encoding="LDAP"
       NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
       Name="urn:oid:1.3.6.1.4.1.5923.1.1.1.1"
       FriendlyName="eduPersonAffiliation">
       <saml:AttributeValue
         xsi:type="xs:string">member</saml:AttributeValue>
       <saml:AttributeValue
         xsi:type="xs:string">staff</saml:AttributeValue>
     </saml:Attribute>
   </saml:AttributeStatement>
 </saml:Assertion>

Note that the <saml:Assertion> element contains the following subelements:

  • a <saml:Issuer> element, which contains the unique identifier of the identity provider
  • a <ds:Signature> element, which contains an integrity-preserving digital signature (not shown) over the <saml:Assertion> element
  • a <saml:Subject> 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 <saml:Conditions> element, which gives the conditions under which the assertion is to be considered valid
  • a <saml:AuthnStatement> element, which describes the act of authentication at the identity provider
  • a <saml:AttributeStatement> 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 <saml:Subject> 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 <saml:Subject> element is a staff member at this institution.

[edit] SAML 2.0 Protocols

The following protocols are specified in #SAMLCore:

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

[edit] Authentication Request Protocol

Recall that the SAML 1.1 Web Browser SSO Profiles are IdP-initiated, that is, an unsolicited <samlp:Response> 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 <samlp:AuthnRequest> 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">
   <saml:Issuer>https://sp.example.com/SAML2</saml:Issuer>
   <samlp:NameIDPolicy
     AllowCreate="true"
     Format="urn:oasis:names:tc:SAML:2.0:nameid-format:transient"/>
 </samlp:AuthnRequest>

The above <samlp:AuthnRequest> 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).

[edit] 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 <samlp:ArtifactResolve> 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 <samlp:ArtifactResolve> 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">
   <saml:Issuer>https://idp.example.org/SAML2</saml:Issuer>
   <ds:Signature
     xmlns:ds="http://www.w3.org/2000/09/xmldsig#">...</ds:Signature>
   <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.

[edit] SAML 2.0 Bindings

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

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.

[edit] 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:

 <form method="post" action="https://idp.example.org/SAML2/SSO/POST" ...>
   <input type="hidden" name="SAMLRequest" value="request" />
   ...
   <input type="submit" value="Submit" />
 </form>

The value of the SAMLRequest parameter is the base64 encoding of a <samlp:AuthnRequest> 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:

 <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, 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]).

[edit] 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 <samlp:AuthnRequest> 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 <samlp:ArtifactResolve> request (such as the #ArtifactResolveRequest shown earlier) directly to the service provider via a back channel. Finally, the service provider returns a <samlp:ArtifactResponse> element containing the referenced <samlp:AuthnRequest> 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">
   <ds:Signature
     xmlns:ds="http://www.w3.org/2000/09/xmldsig#">...</ds:Signature>
   <samlp:Status>
     <samlp:StatusCode
       Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
   </samlp:Status>
   <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">
     <saml:Issuer>https://sp.example.com/SAML2</saml:Issuer>
     <samlp:NameIDPolicy
       AllowCreate="false"
       Format="urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress"/>
   </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.

[edit] 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.

[edit] SAML 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:

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).

[edit] 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.

[edit] SP 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.

SAML 2.0 Web Browser SSO (POST)
SAML 2.0 Web Browser SSO (POST)

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 2–7.

2. Respond with an XHTML form

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

 <form method="post" action="https://idp.example.org/SAML2/SSO/POST" ...>
   <input type="hidden" name="SAMLRequest" value="request" />
   <input type="hidden" name="RelayState" value="token" />
   ...
   <input type="submit" value="Submit" />
 </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 <samlp:AuthnRequest> 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">
   <saml:Issuer>https://sp.example.com/SAML2</saml:Issuer>
   <samlp:NameIDPolicy
     AllowCreate="true"
     Format="urn:oasis:names:tc:SAML:2.0:nameid-format:transient"/>
 </samlp:AuthnRequest>

Before the <samlp:AuthnRequest> 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 <samlp:AuthnRequest> 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:

 <form method="post" action="https://sp.example.com/SAML2/SSO/POST" ...>
   <input type="hidden" name="SAMLResponse" value="response" />
   <input type="hidden" name="RelayState" value="token" />
   ...
   <input type="submit" value="Submit" />
 </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 <samlp:Response> 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">
   <saml:Issuer>https://idp.example.org/SAML2</saml:Issuer>
   <samlp:Status>
     <samlp:StatusCode
       Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
   </samlp:Status>
   <saml:Assertion
     xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
     ID="identifier_3"
     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>
       <saml:NameID
         Format="urn:oasis:names:tc:SAML:2.0:nameid-format:transient">
         3f7b3dcf-1674-4ecd-92c8-1544f346baf8
       </saml:NameID>
       <saml:SubjectConfirmation
         Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
         <saml:SubjectConfirmationData
           InResponseTo="identifier_1"
           Recipient="https://sp.example.com/SAML2/SSO/POST"
           NotOnOrAfter="2004-12-05T09:27:05Z"/>
       </saml:SubjectConfirmation>
     </saml:Subject>
     <saml:Conditions
       NotBefore="2004-12-05T09:17:05Z"
       NotOnOrAfter="2004-12-05T09:27:05Z">
       <saml:AudienceRestriction>
         <saml:Audience>https://sp.example.com/SAML2</saml:Audience>
       </saml:AudienceRestriction>
     </saml:Conditions>
     <saml:AuthnStatement
       AuthnInstant="2004-12-05T09:22:00Z"
       SessionIndex="identifier_3">
       <saml:AuthnContext>
         <saml:AuthnContextClassRef>
           urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport
        </saml:AuthnContextClassRef>
       </saml:AuthnContext>
     </saml:AuthnStatement>
   </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.

[edit] SP 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.

SAML 2.0 Web Browser SSO (Artifact)
SAML 2.0 Web Browser SSO (Artifact)

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 2–11.

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 <samlp:ArtifactResolve> 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">
   <saml:Issuer>https://idp.example.org/SAML2</saml:Issuer>
   <ds:Signature
     xmlns:ds="http://www.w3.org/2000/09/xmldsig#">...</ds:Signature>
   <samlp:Artifact>artifact_1</samlp:Artifact>
 </samlp:ArtifactResolve>

where the value of the <samlp:Artifact> 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 <samlp:ArtifactResponse> element (containing an <samlp:AuthnRequest> 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">
   <ds:Signature
     xmlns:ds="http://www.w3.org/2000/09/xmldsig#">...</ds:Signature>
   <samlp:Status>
     <samlp:StatusCode
       Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
   </samlp:Status>
   <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">
     <saml:Issuer>https://sp.example.com/SAML2</saml:Issuer>
     <samlp:NameIDPolicy
       AllowCreate="false"
       Format="urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress"/>
   </samlp:AuthnRequest>
 </samlp:ArtifactResponse>

The SSO service processes the <samlp:AuthnRequest> 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 <samlp:ArtifactResolve> 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">
   <saml:Issuer>https://sp.example.com/SAML2</saml:Issuer>
   <ds:Signature
     xmlns:ds="http://www.w3.org/2000/09/xmldsig#">...</ds:Signature>
   <samlp:Artifact>artifact_2</samlp:Artifact>
 </samlp:ArtifactResolve>

where the value of the <samlp:Artifact> 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 <samlp:ArtifactResponse> element (containing an <samlp:Response> 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">
   <ds:Signature
     xmlns:ds="http://www.w3.org/2000/09/xmldsig#">...</ds:Signature>
   <samlp:Status>
     <samlp:StatusCode
       Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
   </samlp:Status>
   <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">
     <saml:Issuer>https://idp.example.org/SAML2</saml:Issuer>
     <ds:Signature
       xmlns:ds="http://www.w3.org/2000/09/xmldsig#">...</ds:Signature>
     <samlp:Status>
       <samlp:StatusCode
         Value="urn:oasis:names:tc:SAML:2.0:status:Success"/>
     </samlp:Status>
     <saml:Assertion
       xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
       ID="identifier_7"
       Version="2.0"
       IssueInstant="2004-12-05T09:22:05Z">
       <saml:Issuer>https://idp.example.org/SAML2</saml:Issuer>
       <saml:Subject>
         <saml:NameID
           Format="urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress">
           user@mail.example.org
         </saml:NameID>
         <saml:SubjectConfirmation
           Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
           <saml:SubjectConfirmationData
             InResponseTo="identifier_3"
             Recipient="https://sp.example.com/SAML2/SSO/Artifact"
             NotOnOrAfter="2004-12-05T09:27:05Z"/>
         </saml:SubjectConfirmation>
       </saml:Subject>
       <saml:Conditions
         NotBefore="2004-12-05T09:17:05Z"
         NotOnOrAfter="2004-12-05T09:27:05Z">
         <saml:AudienceRestriction>
           <saml:Audience>https://sp.example.com/SAML2</saml:Audience>
         </saml:AudienceRestriction>
       </saml:Conditions>
       <saml:AuthnStatement
         AuthnInstant="2004-12-05T09:22:00Z"
         SessionIndex="identifier_7">
         <saml:AuthnContext>
           <saml:AuthnContextClassRef>
             urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport
          </saml:AuthnContextClassRef>
         </saml:AuthnContext>
       </saml:AuthnStatement>
     </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.

[edit] 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.

[edit] 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 <samlp:AssertionIDRequest> element, which is used to request an assertion given its unique identifier (ID)
  • the <samlp:SubjectQuery> element, which is an abstract extension point that allows new subject-based SAML queries to be defined
  • the <samlp:AuthnQuery> element, which is used to request existing authentication assertions about a given subject from an Authentication Authority
  • the <samlp:AttributeQuery> element, which is used to request attributes about a given subject from an Attribute Authority
  • the <samlp:AuthzDecisionQuery> 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.

[edit] SAML 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=trscavo@uiuc.edu
   </saml:Issuer>
   <saml:Subject>
     <saml:NameID
       Format="urn:oasis:names:tc:SAML:1.1:nameid-format:X509SubjectName">
       C=US, O=NCSA-TEST, OU=User, CN=trscavo@uiuc.edu
     </saml:NameID>
   </saml:Subject>
   <saml:Attribute
     NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
     Name="urn:oid:2.5.4.42"
     FriendlyName="givenName">
   </saml:Attribute>
   <saml:Attribute
     NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
     Name="urn:oid:1.3.6.1.4.1.1466.115.121.1.26"
     FriendlyName="mail">
   </saml:Attribute>
 </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 <samlp:Response> 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>
   <ds:Signature>...</ds:Signature>
   <saml:Subject>
     <saml:NameID
       Format="urn:oasis:names:tc:SAML:1.1:nameid-format:X509SubjectName">
       C=US, O=NCSA-TEST, OU=User, CN=trscavo@uiuc.edu
     </saml:NameID>
     <saml:SubjectConfirmation
       Method="urn:oasis:names:tc:SAML:2.0:cm:holder-of-key">
       <saml:SubjectConfirmationData>
         <ds:KeyInfo>
           <ds:X509Data>
             <ds:X509Certificate>
 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==
             </ds:X509Certificate>
           </ds:X509Data>
         </ds:KeyInfo>
       </saml:SubjectConfirmationData>
     </saml:SubjectConfirmation>
   </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">
     <saml:AuthnContext>
       <saml:AuthnContextClassRef>
           urn:oasis:names:tc:SAML:2.0:ac:classes:TLSClient
       </saml:AuthnContextClassRef>
     </saml:AuthnContext>
   </saml:AuthnStatement>
   <saml:AttributeStatement>
     <saml:Attribute
       xmlns:x500="urn:oasis:names:tc:SAML:2.0:profiles:attribute:X500"
       x500:Encoding="LDAP"
       NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
       Name="urn:oid:2.5.4.42"
       FriendlyName="givenName">
       <saml:AttributeValue
         xsi:type="xs:string">Tom</saml:AttributeValue>
     </saml:Attribute>
     <saml:Attribute
       xmlns:x500="urn:oasis:names:tc:SAML:2.0:profiles:attribute:X500"
       x500:Encoding="LDAP"
       NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
       Name="urn:oid:1.3.6.1.4.1.1466.115.121.1.26"
       FriendlyName="mail">
       <saml:AttributeValue
         xsi:type="xs:string">trscavo@gmail.com</saml:AttributeValue>
     </saml:Attribute>
   </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.

[edit] SAML 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 <samlp:AuthnRequest> 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.

[edit] Identity Provider Metadata

An identity provider publishes data about itself in an <md:EntityDescriptor> 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">
   <!-- insert ds:Signature element -->
   <!-- insert md:IDPSSODescriptor element (below) -->
   <!-- insert md:AttributeAuthorityDescriptor element (not shown) -->
   <md:Organization>
     <md:OrganizationName xml:lang="en">
       SAML Identity Provider 
     </md:OrganizationName>
     <md:OrganizationDisplayName xml:lang="en">
       SAML Identity Provider @ Some Location
     </md:OrganizationDisplayName>
     <md:OrganizationURL xml:lang="en">
       http://www.idp.example.org/
     </md:OrganizationURL>
   </md:Organization>
   <md:ContactPerson contactType="technical">
     <md:SurName>SAML IdP Support</md:SurName>
     <md:EmailAddress>mailto:saml-support@idp.example.org</md:EmailAddress>
   </md:ContactPerson>
 </md:EntityDescriptor>

The entityID attribute is the unique identifier of the identity provider. Note that the details of the digital signature (in the <ds:Signature> 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 <md:AttributeAuthorityDescriptor> element is not shown.

[edit] SSO Service Metadata

The SSO service at the identity provider is described in an <md:IDPSSODescriptor> element:

 <md:IDPSSODescriptor
   protocolSupportEnumeration="urn:oasis:names:tc:SAML:2.0:protocol">
   <md:KeyDescriptor use="signing">
     <ds:KeyInfo>
       <ds:KeyName>IdP SSO Key</ds:KeyName>
     </ds:KeyInfo>
   </md:KeyDescriptor>
   <md:ArtifactResolutionService isDefault="true" index="0"
     Binding="urn:oasis:names:tc:SAML:2.0:bindings:SOAP"
     Location="https://idp.example.org/SAML2/ArtifactResolution"/>
   <md:NameIDFormat>
     urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress
   </md:NameIDFormat>
   <md:NameIDFormat>
     urn:oasis:names:tc:SAML:2.0:nameid-format:transient
   </md:NameIDFormat>
   <md:SingleSignOnService
     Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST"
     Location="https://idp.example.org/SAML2/SSO/POST"/>
   <md:SingleSignOnService
     Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Artifact"
     Location="https://idp.example.org/SAML2/Artifact"/>
   <saml:Attribute
     NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
     Name="urn:oid:1.3.6.1.4.1.5923.1.1.1.1"
     FriendlyName="eduPersonAffiliation">
     <saml:AttributeValue>member</saml:AttributeValue>
     <saml:AttributeValue>student</saml:AttributeValue>
     <saml:AttributeValue>faculty</saml:AttributeValue>
     <saml:AttributeValue>employee</saml:AttributeValue>
     <saml:AttributeValue>staff</saml:AttributeValue>
   </saml:Attribute>
 </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 <md:ArtifactResolutionService> element indicates that the SAML SOAP binding (#SAMLBind) should be used for artifact resolution.
  • The Location attribute of the <md:ArtifactResolutionService> element is used in step 8 of the "double artifact" profile.
  • The value of the index attribute of the <md:ArtifactResolutionService> element is used as the EndpointIndex in the construction of a SAML type 0x0004 artifact.
  • The <md:NameIDFormat> elements indicate what SAML name identifier formats (#SAMLCore) the SSO service supports.
  • The Binding attributes of the <md:SingleSignOnService> elements are standard URIs specified in the SAML 2.0 Binding specification (#SAMLBind).
  • The Location attribute of the <md:SingleSignOnService> element that supports the HTTP POST binding is used in step 2 of the "double POST" profile.
  • The Location attribute of the <md:SingleSignOnService> element that supports the HTTP Artifact binding is used in step 2 of the "double artifact" profile.
  • The <saml:Attribute> element describes an attribute that the identity provider is willing to assert (subject to policy). The <saml:AttributeValue> elements enumerate the possible values the attribute may take on.

[edit] Service Provider Metadata

A service provider also publishes data about itself in an <md:EntityDescriptor> 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">
   <!-- insert ds:Signature element -->
   <!-- insert md:SPSSODescriptor element (see below) -->
   <md:Organization>
     <md:OrganizationName xml:lang="en">
       SAML Service Provider 
     </md:OrganizationName>
     <md:OrganizationDisplayName xml:lang="en">
       SAML Service Provider @ Some Location
     </md:OrganizationDisplayName>
     <md:OrganizationURL xml:lang="en">
       http://www.sp.example.com/
     </md:OrganizationURL>
   </md:Organization>
   <md:ContactPerson contactType="technical">
     <md:SurName>SAML SP Support</md:SurName>
     <md:EmailAddress>mailto:saml-support@sp.example.com</md:EmailAddress>
   </md:ContactPerson>
 </md:EntityDescriptor>

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

[edit] Assertion Consumer Service Metadata

The assertion consumer service is represented by an <md:SPSSODescriptor> element:

 <md:SPSSODescriptor
   protocolSupportEnumeration="urn:oasis:names:tc:SAML:2.0:protocol">
   <md:KeyDescriptor use="signing">
     <ds:KeyInfo>
       <ds:KeyName>SP SSO Key</ds:KeyName>
     </ds:KeyInfo>
   </md:KeyDescriptor>
   <md:ArtifactResolutionService isDefault="true" index="0"
     Binding="urn:oasis:names:tc:SAML:2.0:bindings:SOAP"
     Location="https://sp.example.com/SAML2/ArtifactResolution"/>
   <md:NameIDFormat>
     urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress
   </md:NameIDFormat>
   <md:NameIDFormat>
     urn:oasis:names:tc:SAML:2.0:nameid-format:transient
   </md:NameIDFormat>
   <md:AssertionConsumerService isDefault="true" index="0"
     Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST"
     Location="https://sp.example.com/SAML2/SSO/POST"/>
   <md:AssertionConsumerService index="1"
     Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-Artifact"
     Location="https://sp.example.com/SAML2/Artifact"/>
   <md:AttributeConsumingService isDefault="true" index="0">
     <md:ServiceName xml:lang="en">
       Service Provider Portal
     </md:ServiceName>
     <md:RequestedAttribute
       NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
       Name="urn:oid:1.3.6.1.4.1.5923.1.1.1.1"
       FriendlyName="eduPersonAffiliation">
     </md:RequestedAttribute>
   </md:AttributeConsumingService>
 </md:SPSSODescriptor>

Note the following details about the <md:SPSSODescriptor> metadata element:

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

[edit] References

[edit] See also