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Technical implementation of identity assurance requires system changes from InCommon Operations, IdPs, and SPs. This page captures (and its child pages) capture lessons learned, recommended practices, and outstanding issues regarding the technical aspects of identity assurance.

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Info
titleThe Use of SAML V2.0

Participation in the InCommon Identity Assurance Program requires the use of SAML V2.0 Web Browser SSO. IdP and SP operators should plan to upgrade to SAML V2.0 as soon as possible.

Metadata management

InCommon Operations will add identity assurance qualifiers (IAQs) to published metadata following notification of certification by InCommon management. IAQs will be added to the appropriate IdP entity descriptor of the certified IdP operator (IdPO).

Proposed IAQ URIs are:

Silverhttp://id.incommon.org/assurance/silver
Bronzehttp://id.incommon.org/assurance/bronze

There will likely be a need for non-production IAQs for use in interoperability testing, probably with test instances of metadata:

Silverhttp://id.incommon.org/assurance/silver-test
Bronzehttp://id.incommon.org/assurance/bronze-test

Note that all of the above URIs will resolve to actual web pages at some point.

IAQs in metadata

The following extension is the immediate child element of the IdP's <md:EntityEescriptor> element in metadata:

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<md:Extensions xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata">
  <mdattr:EntityAttributes xmlns:mdattr="urn:oasis:names:tc:SAML:metadata:attribute">
    <saml:Attribute
        xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
        NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri"
        Name="urn:oasis:names:tc:SAML:attribute:assurance-certification">
      <saml:AttributeValue>http://id.incommon.org/assurance/silver-test</saml:AttributeValue>
      <saml:AttributeValue>http://id.incommon.org/assurance/bronze-test</saml:AttributeValue>
    </saml:Attribute>
  </mdattr:EntityAttributes>
</md:Extensions>

The <mdattr:EntityAttributes> element and the name of the SAML Attribute (urn:oasis:names:tc:SAML:attribute:assurance-certification) are defined by the OASIS SAML V2.0 Metadata Extension for Entity Attributes and the OASIS SAML V2.0 Identity Assurance Profiles, respectively.

A complete, working metadata sample is attached to this wiki topic. To schema validate this sample metadata, you can use xmlsectool:

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xmlsectool.sh --validateSchema \
    --schemaDirectory schema-files --inFile incommon-idp-metadata.xml

For convenience, we provide a set of (suitably modified) schema files that permit offline schema validation.

Issues

  • The entity attribute and/or the IAQ has to be dated or versioned to indicate exactly what IAP is referred to.

SP behavior

Ideally SPs that require a particular assurance level will initiate the assurance flow by including the desired IAQ in the SAML AuthnRequest element.

SPs will receive IAQs (either in response to a specific request, or sent unsolicited) in assertions from IdPs. SPs should use metadata to check that the IdP is authorized to assert the IAQs being asserting.

SPs will rely on local policy to decide how to handle incoming IAQs. For example if the SP requires InCommon Bronze but receives InCommon Silver, that is probably acceptable.

SP Policy Use Cases

Issues

  • Some SPs may not be able to use the AuthnRequest mechanism due to software or other limitations. Are they simply out of luck?
    • One option may be to use additional software to generate requests on behalf of the broken SP, although this isn't guaranteed to work with all SPs. Otherwise, such SPs will be forced to rely on OOB configuration of IdPs.
  • How is the AuthnRequest configured using the Shib SP? The simpleSAMLphp SP?
    • Shibboleth SPs can rely on the authnContextClassRef setting to control the value requested when particular resources are accessed. To include multiple values in a request, the AuthnRequest "template" mechanism described in the SessionInitiator documentation can be used.
  • Boarding process: Since an IAQ in metadata makes a statement about certification (not live service), how does an SP determine that an IdP supports assurance operationally (ala attribute support)? One approach is to include <saml:Attribute> elements in IdP metadata. Other approaches?
    • There is no metadata support for this requirement. SPs should be able to handle errors returned by IdPs that indicate the requested assurance level was not supported. The federation should help establish guidelines for describing such errors, perhaps with a FAQ page that could be linked in.
  • Does the Shib SP software support the metadata check? Does the simpleSAMLphp SP?
    • The Shibboleth SP can extract and make available the entity attribute value in a variable along with user attributes, and use the variable in authorization policy. This is described under "metadata attribute extraction".
  • What matching rules are recommended, or acceptable?
  • How is an SP supposed to "know" that Silver is acceptable in lieu of Bronze? Is there a role for InCommon to provide "advice"?

IdP behavior

Ideally IdPs will receive a desired IAQ from an SP in an AuthnRequest to initiate the process. The IdP compares the requested IAQ to its matching rule and interacts with the local IdM system to determine if the current user meets the requirements. If so, the appropriate IAQ is returned in the AuthnContext element in the assertion.

Issues

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  • The Shibboleth IdP directly supports only "exact" matching of requested IAQs, and SAML requires that such matchiing be exact (it does not allow for "equivalence"). Supporting more flexible comparison operators requires implementing a custom login handler.

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  • Returning Silver is not permissible if the request asks for Bronze and uses "exact" matching. It would be permissible if "minimum" were specified, but a custom login handler would be needed.

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  • The AuthnContext element that various groups decided to use for expressing them does not directly support more than one IAQ at a time. In theory an "umbrella" IAQ could be defined to capture combinations, but this would be complex to express and not supported widely.

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  • It is virtually certain that suporting assurance in any but the most trivial of ways will require a custom login handler. It will take time to develop patterns and common requirements that could be factored into reusable code.

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  • It seems likely that IdPs should express the IAQ appropriate for a user's session if they can.

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  • A SAML IdP always sends some form of AuthnContext information. By convention, that is being used to carry an IAQ. In Shibboleth, the term in the IdP for this data element is the "authentication method".

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SAML V2.0 Support for Assurance

SAML's support for identity assurance is embodied in a concept called "Authentication Context". The context of an authentication event is designed to capture both technical and procedural elements that factor into the "confidence" expressed by the identity provider in the event. In terms of assurance, this maps to the concepts of technical strength and identity proofing strength that make up an assurance profile.

Every authentication statement issued by an IdP contains an <saml:AuthnContext> element that expresses the context of the authentication event. There are a variety of syntaxes supported, but the most common one is to define a "class" of authentication contexts that all share essential characteristics that are of interest to a relying party. These classes are mapped to URI constants that are expressed in an element called <saml:AuthnContextClassRef>, of which a single value can be expressed by the IdP in response to an authentication request.

In addition, SAML V2.0 SPs have the capability to include simple or complex matching requirements in their authentication requests that influence the Authentication Context supplied by the IdP. The intent is to allow IdPs that support varying levels of assurance to honor requests based on the requirements of the SP and not a one-size-fits-all policy. In practice, this approach can be tricky to implement and may depend on customization of one's software deployment.

Thus, we expect assurance deployment to be gradual, and we will continue to evolve documentation to reflect what we learn. We also encourage deployers to talk to their software suppliers about the support (or lack thereof) of these features.

InCommon Practices

See: InCommon Practices - Certification and Metadata

SP Behavior

See: Assurance - Service Provider Behavior.

IdP Behavior

See: Assurance - Identity Provider Behavior.

References

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