There is a great variety of user authentication technologies, and some of them are very different from each other. Consider, for example, one-time passwords, OAuth, Idemix, and ICAM’s Backend Attribute Exchange: any two of them have little in common.
Different authentication technologies have been developed by different communities, which have created their own vocabularies to describe them. Furthermore, some of the technologies are extremely complex: U-Prove and Idemix are based on mathematical theories that may be impenetrable to non-specialists; and OpenID Connect, which is an extension of OAuth, adds seven specifications to a large number of OAuth specifications. As a result, it is difficult to compare authentication technologies to each other.
This is unfortunate because decision makers in corporations and governments need to decide what technologies or combinations of technologies should replace passwords, which have been rendered even more inadequate by the shift from traditional personal computers to smart phones and tablets. Decision makers need to evaluate and compare the security, usability, deployability, interoperability and, last but not least, privacy, provided by the very large number of very different authentication technologies that are competing in the marketplace of technology innovations.
But all these technologies are trying to do the same thing: authenticate the user. So it should be possible to develop a common conceptual framework that makes it possible to describe them in functional terms without getting lost in the details, to compare their features, and to evaluate their adequacy to different use cases.
The paper that we presented at the recent ID360 conference can be viewed as a step in that direction. It focuses on privacy, an aspect of authentication technology which I think is in need of particular attention. It surveys eighteen technologies, including: four flavors of passwords and one-time passwords; the old Microsoft Passport (of historical interest); the browser SSO profile of SAML; Shibboleth; OpenID; the ICAM profile of OpenID; OAuth; OpenID Connect; uncertified key pairs; public key certificates; structured certificates; Idemix pseudonyms; Idemix anonymous credentials; U-Prove tokens; and ICAM’s Backend Attribute Exchange.
The paper classifies the technologies along four different dimensions or facets, and builds a matrix indicating which of the technologies provide seven privacy features: unobservability by an identity or attribute provider; free choice of identity or attribute provider; anonymity; selective disclosure; issue-show unlinkability; multishow unlinkability by different parties; and multishow unlinkability by the same party. I will not try to recap the details here; instead I will elaborate on observations made in the paper regarding privacy enhancements that have been used to improve the privacy postures of some closed-loop authentication technologies.
Privacy Enhancements for Closed-Loop Authentication
One of the classification facets that the paper considers for authentication technologies is the distinction between closed-loop and open-loop authentication, which I discussed in an earlier post. Closed-loop authentication means that the credential authority that issues or registers a credential is later responsible for verifying possession of the credential at authentication time. Closed-loop authentication may involve two parties, or may use a third-party as a credential authority, which is usually referred to as an identity provider. Examples of third-party closed-loop authentication technologies include the browser SSO profile of SAML, Shibboleth, OpenID, OAuth, and OpenID Connect.
I’ve pointed out before that third-party closed-loop authentication lacks unobservability by the identity provider. Most third-party closed-loop authentication technologies also lack anonymity and multishow unlinkability. However, some of them implement privacy enhancements that provide anonymity and a form of multishow unlinkability. There are two such enhancements, suitable for two different use cases.
The first enhancement consists of omitting the user identifier that the identity provider usually conveys to the relying party. The credential authority is then an attribute provider rather than an identity provider: it conveys attributes that do not necessarily identify the user. This enhancement provides anonymity, and multishow unlinkability assuming no collusion between the attribute provider and the relying parties. It is useful when the purpose of authentication is to verify that the user is entitled to access a service without necessarily having an account with the service provider. This functionality is provided by Shibboleth, which can be used, e.g., to allow a student enrolled in one educational institution to access the library services of another institution without having an account at that other institution.
The core OpenID 2.0 specification specifies how an identity provider conveys an identifier to a relying party. Extensions of the protocol such as the Simple Registration Extension specify methods by which the identity provider can convey user attributes in addition to the user identifier; and the core specification hints that the identifier could be omitted when extensions are used. It would be interesting to know whether any OpenID server or client implementations allow the identifier to be omitted. Any comments?
The second enhancement consists of requiring the identity provider to convey different identifiers for the same user to different relying parties. The identity provider can meet the requirement without allocating large amounts of storage by computing a user identifier specific to a relying party as a cryptographic hash of a generic user identifier and an identifier of the relying party such as a URL. This privacy enhancement is required by the ICAM profile of OpenID. It achieves user anonymity and multishow unlinkability by different parties assuming no collusion between the identity provider and the relying parties; but not multishow unlinkability by the same party. It is useful for returning user authentication.