The identity assurance of a given interaction of user, IdP and SP is dependent on many factors. However, in the interest of simplicity of operation, it seems desirable will be desirable in at least some cases for an IdP to assert that a given interaction, taking everything into consideration, fits a particular identity assurance profile. NIH SPs, for example, would like simple assertions of this kind. This essentially means that an IdP would want to make sure that all aspects of a given user's identity proofing, credentials and attribute reliability fit the "bronze" or "silver" profile. Assuming that the authentication technology also fit the profile, the IdP would be warranted in asserting that the whole IdP side of the interaction matched a particular identity assurance profile.
In other cases, it is possible that the SP would like a finer level of detail, with information on multiple aspects of the authentication present in the IdP assertion. Here, too, information on which profile applied would often be of interest to the SP, either as an overall qualification as above or as applied to various elements of the authentication event. The SAML Authentication Context element would be the appropriate means of conveying this information.
In both the simpler and more complicated cases, it would be useful to have globally unique attribute/value pairs to represent a given identity assurance scheme and the particular profile within it. Properly constructed URIs provide global uniqueness without the need for a global registry. This document recommends that federations wishing to express federation-defined identity assurance profiles adopt a convention for URI-based representations of those profiles. To illustrate with an example, InCommon could opt to assign "urn:mace:incommon:iap:bronze" to the bronze identity assurance profile, and "urn:mace:incommon:iap:silver" to the silver profile. To continue with this example, in the simpler scenario outlined above, MACE-Dir might define an attribute, identityAssuranceQualifier, whose value would be one of the two URIs defined above. This attribute could be one element of a SAML attribute assertion, and the SP would then have information on the identity assurance of the assertion as a whole.