The login is broken.. I haven't taken the time to fix it, I think the zulip migration might have broken this...
If you've already signed up, skip the key field. You only need to enter your zulip key once.
Why do I have to give you my zulip key?
1) We need to find your last read location.
2) We also use it to find which threads you have not muted.
3) There isn't any other way to get this information (like oauth)
Are my private messages listed?
No. Only the threads you have not muted are listed.
Can others log in as me?
Yes! But they will only get a hint as to what theads you are subscribed to and when you last read zulip. Other private information like private messages are not shown.