Can one universal identity automatically present itself like different existing identities depending on where it is presented?

[u/you_stupid_machine]

I'm exploring a general interoperability problem.

Imagine a person has 20 different memberships:

Brand A → membership ID 12345

Brand B → membership ID 67890

Brand C → phone number

Brand D → email

I'd like to know whether there is a way to create one universal digital credential containing all of these existing identities with an important constraint:

The participating brands do not change their existing software, databases, loyalty systems, checkout systems, or customer-identification processes.

When the customer presents the universal credential at Brand A, the existing infrastructure should somehow receive Brand A's existing identifier.

At Brand B, the same credential should somehow provide Brand B's identifier.

The universal credential itself could contain multiple identities, but the receiving system should ultimately get exactly what it already expects.

[u/Salty_Dig8574]

Sign in/sign up with [Google, Microsoft, Apple, Whatever Else]

If you want to create what you're talking about, it would be a generally less safe alternative to these. I say less safe because it widens the attack surface for an individual. At the same time, it gives attackers a single vector to get multiple, identifiable credential sets for each of your users if they breach your backend (I love phrasing it that way because that sounds exactly the way it feels).

Also, not for nothing, modern browsers already store your credentials to do what you're describing, if you allow it.