Feedback on the Paper on Privacy Postures of Authentication Technologies

Many thanks to every one who provided feedback on the paper on privacy postures of authentication technologies which was announced in the previous blog post. The paper was discussed on the Identity Commons mailing list and we also received feedback at the ID360 conference, where we presented the paper, and at IIW 16, where we showed a poster summarizing the paper. In this post I will recap the feedback that we have received and the revisions that we have made to the paper based on that feedback.

Steven Carmody pointed out that SWITCH, the Swiss InCommon federation, has developed an extension of Shibboleth called uApprove that allows the identity or attribute provider to ask the user for consent before disclosing attributes to the relying party. Ken Klingestein told us that the Scalable Privacy NSTIC pilot is developing a privacy manager that will let the user choose what attributes will be disclosed to the the relying party by the Shibboleth identity provider. We have added references to these Shibboleth extensions to Section 4.2 of the paper.

The original paper explained that, although a U-Prove token does not provide multishow unlinkability, the user may obtain multiple tokens from the issuer, and present different tokens to different relying parties. Christian Paquin said that a U-Prove credential is defined as a batch of such tokens, created simultaneously by an efficient parallel procedure. We have added this definition of a U-Prove credential to Section 4.3.

Christian Paquin also pointed out that a U-Prove token is a mathematical concept that can be embodied in a variety of technologies. He sent me a link to the WS-Trust embodiment, which was used in CardSpace. We have explained this and included the link in Section 4.3.

Tom Jones said that what we call anonymity is called pseudonymity by others. In fact, column 9, labeled “Anonymity”, covers both pseudonymity, as provided, e.g., by an Idemix pseudonym or an uncertified key pair or a combination of a user ID and a password when the user ID is freely chosen by the user, and full anonymity, as provided when a relying party learns only attributes that do not uniquely identify the user. I think it is not unreasonable to view anonymity (the service provider does not learn the user’s “name”) as encompassing pseudonymity (the service provider learns a pseudonym instead of the “real name”).

Nat Sakimura provided a lot of feedback, for which we are grateful. He said that Google and Yahoo implemented OpenID Pairwise Pseudonymous Identifiers (PPID), i.e. different identifiers for the same user provided to different relying parties, before ICAM specified its OpenID profile. We have noted this in Section 4.2 of the revised paper and changed the label of row 8 to “OpenID (without PPID)”.

He also said that OpenID Connect supports an ephemeral identifier, which provides anonymity. I was able to find a discussion of an ephemeral identifier in the archives of the OpenID Connect mailing list, but no mention of it in any of the OpenID Connect specifications; so ephemeral identifiers may be added in the future, but they are not there yet.

Nat also argued that OpenID Connect provides multishow unlinkability by different parties and by the same party. I disagree, however. The Subject Identifier in the ID Token makes OpenID Connect authentication events linkable. Furthermore, OpenID is built on top of OAuth, whose purpose is to provide the relying party with access to resources owned by the user by means of an access token. In a typical use case the relying party gets access to the user’s account at a social network such as Facebook, Twitter or Google+. It is unlikely that two relying parties who share information cannot determine that they are both accessing the same account, or that a relying party cannot determine that it has accessed the same account in two different occasions.

Nat said that OpenID Connect can be used for two-party authentication using a “Self-Issued OpenID Provider”. We have added a checkmark to row 11, column 1 of the table to indicate this, and an explanation to Section 4.2.

He also said that OpenID Connect provides group 4 functionality by allowing the relying party to obtain attributes from “distributed attribute providers”. We have mentioned this in Section 4.4 of the revised version of the paper.

Finally, Nat said:

Just by reading the paper, I was not very clear what is the requirement for Issue-show unlinkability. By issuance, I imagine it means the credential issuance. I suppose then it means that the credential verifier (in ISO 29115 | ITU-T X.1254 sense) cannot tell which credential was used though it can attest that the user has a valid credential. Is that correct? If so, much of the technology in group 2 should have n/a in the column because they are independent of the actual authentication itself. They could very well use anonymous authentication or partially anonymous authentication (ISO 29191).

The technologies in group 2 are recursive authentication technologies. The relying party directs the browser to the identity or attribute provider, which recursively authenticates the user and provides a bearer credential to the relying party based on the result of the inner authentication. In all generality there may be multiple inner authentications, as the identity or attribute provider may require multiple credentials. So the authentication process may consist of a tree of nested authentications, with internal nodes of the tree involving group 2 technologies, and leaf nodes other technologies. However, rows 5-11 (group 2) are only concerned with the usual case where the user authenticates to the identity or attribute provider as a returning user with a user ID and a password or some other form of two-party authentication; we have now made that clear in Section 4.2 of the revised paper. In that case there is no issue-show unlinkability.

We have also made a couple of other improvements to the paper, motivated in part by the feedback:

  • We have replaced the word possession with the word ownership in the definition of closed-loop authentication (Section 2), so that it now reads: authentication is closed-loop when the credential authority that issues or registers a credential is later responsible for verifying ownership of the credential at authentication time. The motivation for this change is that, in group 2, the credential is the information that the identity or attribute provider has about the user, and is thus kept by the identity or attribute provider rather than by the user.
  • We have added a distinction between two forms of multishow unlinkability, a strong form that holds even if the credential authority colludes and shares information with the relying parties, and a weak form that holds only if there is no such collusion. The technologies in group 2 that provide multishow unlinkability provide the weak form, whereas Idemix anonymous credentials provide the strong form.
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2 Responses to Feedback on the Paper on Privacy Postures of Authentication Technologies

  1. I think the paper is wrong about the “Selective Disclosure” property of OpenID (rows 8 and 9), since in OpenID Attribute Exchange and Simple Registration, it was possible to designate certain attribute as “optional”, which IdPs often relayed to the user as checkboxes that could be ticked or not.

    Are you planning to maybe add more technologies to the matrix in the future, e.g. CardSpace, WebID, BrowserID? Or did you maybe omit those because you view them simply as concrete implementations of the cryptographic approaches you already have in your Group 3?

    Thanks for this great paper, it’s a fantastic reference for all of us working in this space.

    • Francisco Corella says:

      Thank you for pointing out the support for optional attributes in OpenID extensions. We’ve updated the paper and included a reference to your comment.

      Regarding WebID and BrowserID, yes, they can be viewed as variations on public key certificates. CardSpace, before it was discontinued, was a framework supporting several different authentication technologies.

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