It can be as simple as two clicks. All that's needed is Bitcoin wallet app devs to implement the needed functionality.

This is how I imagine it'll work:

- Bitcoin wallet apps will have a secondary address book for authentication addresses (these can be some of the same addresses they already use or completely new addresses)
- when you sign up for an account you simple scan a QR code which will give you the option of creating a new authentication address or to pick an existing one
- to login you scan a QR code and click confirm

It couldn't be simpler, and every Bitcoin user could have this functionality already on their device due to simply being a Bitcoin user, no additional installation necessary. All we need is Bitcoin wallet app devs to implement this.