[criley2]

So, then, the only option is multi-factor biometrics. Everything else is just "not identity", right?

And even then, biometrics can't usually differentiate between twins.

For twins, there is arguably no way for software to demonstrate identity, ever.

Social security? Proves you're holding a card.

Biometrics? Proves you're one of multiple with these exact genetics.

Etc. I literally cannot think of a way to definitively and authoritatively tell twins apart in software.

[simpss]

I have a public/private key-pair from my local government. Comes with my passport and is guaranteed by the gov to represent a single person only.

Although, I wouldn't want to give the public key to google/amazon/facebook/twitter :)

[criley2]

In the twin example, what is to prevent your twin from taking your documentation and receiving a public/private key in your name?

Or simply access that key of yours and use it?

The public/private key only prove you hold the keys, not that you are you.

Not identity, just proves you have access to the keys.