Separate User selection from authentication about omejdn-server HOT 3 OPEN

Not sure how to do this. We cannot let any user select any account for authentication, right?
I think we definitely have to check the sub claim request and if it matches the authenticated user if we already have a session which is currently not done.
I am really not sure how the account selection could work. Maybe we would have to implement a username screen followed by a password screen and then store the subs which were successfully authenticated against in the session.

It would be "better" subjectively to have a single page with user and password login prompt and IF the user previously authetnticated with at least one other account we should display an account selection (and optional reauthentication) unless we have to enforce the "sub" request.

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As an idea, this is how Microsoft seems to handle this (determined by experiment):
They first have a page for the user to enter a uid (email, username,...), and afterwards have a second page to enter their password. But this way they are also able to automatically determine other providers and request tokens from there. For example, entering a mail adress ending in fraunhofer.de, the user agent is automatically redirected to the Fraunhofer SSO page, with the email address already pre-entered.

Unrelated, one simple solution for us would be to deactivate the username input field when we have to reauthenticate a specific user. The account selection page is a good idea, though we will need to support multiple sessions in the first place to do this.

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Yes, maybe two screens do make sense. Then, we also do not have to deactivate the input field.

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