I wonder ...
Is a passport an "identifier"?
Is a drivers licence an identifier?
Is a credit card an identifier?
Is a professional membership card an identifier?
Is a building access card an identifier?
Is a house key an identifier?
Is a car key an identifier?
Or putting the questions another way ...
Is a car key a "key"?
Is a house key a key?
Is a building access card a key?
Is a professional membership card a key [to access an association]?
Is a bank card a key [to a payments system]?
Is a drivers licence a key [to access the privileges of road usage]?
Is a passport a key [to enter another country]?
I look in my wallet and I see a host of plastic cards that are viewed by many people as identity documents. In Australia we have regulations that formally score the "strength" of most of my cards as proof of identity. And so the urge has arisen to try and "federate identities", in order (it is claimed) to streamline access to third party services that are unrelated to the purposes for which my various identities were originally issued.
In stark contrast I look at my keyring and I see, well, keys. None of them can be presented as "identities" to third parties.
Yet there are some gadgets on my key ring that are thought to be pushing the envelope, like USB dongles and SecurIDs issued for very specific purposes to access particular services.
Most of the cards in our wallets actually represent very precise relationships between us and their respective issuers. A card is issued as evidence of the bearer having met a prescribed set of requirements, including typically a promise to comply with certain Ts&Cs when using the card. As such, they are much more like keys than "identities".
Maybe we should re-vist the intuition that identities can (and should) be federated? I am not sure it is any more logical to federate a bank card, a Medicare card and a security association membership card, than it is to "federate" a car key, a house key and an office key. What new type of mathematics could yield a meaningful answer to the equation:
Car Key ♥ House Key ♥ Office Key = X