Overview#A universal identity system must support both "omni-directional" identifiers for use by public entities and "unidirectional" identifiers for use by private entities, thus facilitating discovery while preventing unnecessary release of correlation handles.
Technical identity is always asserted with respect to some other identity or set of identities. To make an analogy with the physical world, we can say identity has direction, not just magnitude. One special "set of identities" is that of all other identities (the public). Other important sets exist (for example, the identities in an enterprise, an arbitrary domain, or a peer group).
Entities that are public can have identifiers that are invariant and well known. These public identifiers can be thought of as beacons—emitting identity to anyone who shows up. And beacons are "omni-directional" (they are willing to reveal their existence to the set of all other identities).
A corporate Web site with a well-known URL and public key certificate is a good example of such a public entity. There is no advantage—in fact there is a great disadvantage—in changing a public URL. It is fine for every visitor to the site to examine the public key certificate. It is equally acceptable for everyone to know the site is there: its existence is public.
A second example of such a public entity is a publicly visible device like a video projector. The device sits in a conference room in an enterprise. Visitors to the conference room can see the projector and it offers digital services by advertising itself to those who come near it. In the thinking outlined here, it has an omni-directional identity.
On the other hand, a consumer visiting a corporate Web site is able to use the identity beacon of that site to decide whether she wants to establish a relationship with it. Her system can then set up a "unidirectional" identity relation with the site by selecting an identifier for use with that site and no other. A unidirectional identity relation with a different site would involve fabricating a completely unrelated identifier. Because of this, there is no correlation handle emitted that can be shared between sites to assemble profile activities and preferences into super-dossiers.
When a computer user enters a conference room equipped with the projector described above, its omni-directional identity beacon could be utilized to decide (as per the Law of Control) whether she wants to interact with it. If she does, a short-lived unidirectional identity relation could be established between the computer and the projector—providing a secure connection while divulging the least possible identifying information in accordance with the law of minimal disclosure.
Bluetooth and other wireless technologies have not so far conformed to the Law of Directed Identity. They use public beacons for private entities. This explains the consumer backlash innovators in these areas are currently wrestling with.
Public key certificates have the same problem when used to identify individuals in contexts where privacy is an issue. It may be more than coincidental that certificates have so far been widely used when in conformance with this law (i.e., in identifying public Web sites) and generally ignored when it comes to identifying private individuals.
Another example involves the proposed usage of RFID technology in passports and student tracking applications. RFID devices currently emit an omni-directional public beacon. This is not appropriate for use by private individuals.
Passport readers are public devices and therefore should employ an omni-directional beacon. But passports should only respond to trusted readers. They should not be emitting signals to any eavesdropper that identify their bearers and peg them as nationals of a given country. Examples have been given of unmanned devices that could be detonated by these beacons. In California we are already seeing the first legislative measures being taken to correct abuse of identity directionality. It shows a failure of vision among technologists that legislators understand these issues before we do.