The seminal Laws of Identity define a Digital Identity as a set of claims made by one digital subject about itself or another digital subject.
Crucially, this definition leaves "identity" as a metaphor. The word "identity" in day-to-day use is suggestive of a holistic property and is regarded intuitively as innate and pretty much invariant (or only slowly changing; indeed on the occasions when real life identity changes abruptly, we tend to think of that as pathological or criminal).
When we move from real world to digital, a presumption is carried over that identity can be lifted from one context and freely applied in others. So despite the careful framing of the Laws of Identity - that digital identity is different and algorithmic - many people still carry around a utopian idea of a singular digital identity. The archetypal online identity metaphor is the passport. The tacit belief in the possibility of a universal digital passport has been a long standing distraction, and terribly unhelpful, because there is actually no such thing, not in the sense the word is used by technologists!
Ever since the early days of Big PKI, there has been the beguiling idea of an all purpose credential that will let its bearer into any and all online services, enabling total strangers to "trust" one another online. Later Microsoft of course even named an early digital identity service "Passport", and the word is still commonplace in discussing authentication products. The idea is that the passport allows you to go wherever you like, yet the concept that the metaphor alludes to doesn't exist.
A real world passport simply does not let the holder into any country. To begin with, a passport is not always sufficient; you often need a visa. Then, you can't stay as long as you like in a foreign place; some countries won't let you in at all if you carry the passport of an unfriendly nation. You also need to complete a landing card and customs declarations specific to your particular journey. And finally, when you've got to the end of the arrivals queue, you are still at the mercy of an immigration officer who usually has the discretion to turn you away based on any other evidence they may have to hand. As with business transactions, there is much more to border control than identity. So if we could create the universal digital identity, we would do well to call it something other than "passport"!
Metaphors are more than wordplay; they are used to teach, and once learned, simplistic mental models like “electronic passport” can be deeply unhelpful. The dream of all-purpose digital certificates derailed PKI. When they tried to implement "digital passports", they turned out to be unwieldy, riddled with fine print and excessive identity proofing, and very rarely could such certificates be used anywhere on their own. So the passport metaphor is lousy. Yet with "open" federated identity frameworks, we're unwittingly repeating many of the missteps of early PKI, largely because people are still failing to come to grips with complexities obscured by faulty metaphors.
The well-initiated get that the Laws of Identity and well meaning schemes like NSTIC all involve a plurality of identities tuned to different contexts. Many federated identity supporters expressly deprecate the idea of having a single all-purpose cyber identity. Yet NSTIC in particular is easily confused by many with a single new ID; a crazy number of press reports represent it as a "passport" or an Internet "driver licence". It's a misunderstanding that is actually exacerbated by the strategy's own champions when they use terms like “interoperable identity” without enough care, and casually imagine that a student in future will log in to their bank using their student card.
The Laws of Identity teach that identities are context dependent. That is, you cannot expect that an ID issued in one context will operate seamlessly in another. If we recall the definition of digital identity and set aside the passport metaphor, it's actually obvious that identities don't easily interoperate.
Consider the set of claims made about me in the context of my employment; my corporate digital identity might comprise my employee number, position and department, contact details, and company role, which together amount to my employer's imprimatur to represent the organisation. On the other hand, I were enrolled at university, my student identity might consist of my student number, faculty, the stage of my course, and my eligibility to get into certain labs and access certain online collections. What do these respective sets of such claims say about me in other contexts, say banking or healthcare? Very little. I can identify as Steve Wilson in my company and Steve Wilson at university, but any interoperability of these identities across contexts only happens at the attribute level, where the identity metaphor breaks down.
"Interoperability" is actually a curious omission in the Laws of Identity. The interoperability of atomic claims like date of birth, home address, credit card number, student number or SSN is almost trivial; some services recognise these claims and have business rules that use them, while others don't care about them. But the "interoperability" of a rolled-up set of claims like "Steve Wilson is employed by Lockstep Pty Ltd." makes almost no sense. The set of claims that make up that digital identity says a lot about me to a Relying Party doing business with Lockstep, but my corporate identity means nothing to retailers, doctors, my personal bank, the police or the video store.