With many users no longer having an openid account, and Persona seeming to
be dying on the vine, and no other replacements looking very likely (except
for Oauth type stuff perhaps), it would be good to have a new easy way to
log into ikiwiki, that doesn't need pre-registration.

Importantly, I want something that is not going to go
the way of openid in the future. I think that email is here to stay; at
least anyone who wants an email address is going to be able to get one in
the forseeable future. (Google and large providers are making it harder to
run small email systems, but it's still very possible, and there are at
worst many large providers.)

I've read about email being used for login auth, and seen it once or twice.
While I can't remember any links right now, the basic idea is:

1. user enters email address into form
2. response page says "a login link has been emailed to you"
3. user opens email and clicks login link
4. user is logged in until the cookie expires or is cleared

A few points to make this more secure:

* Only 1 login link should be active at a time; old ones won't work to log in.
* A login link is only valid for a single login. Once it's used, it cannot
  be used to log in again.
* A login link is only valid for a certain period of time. 24 hours seems
  like more than enough, and 12 hours would probably be plenty too.
  This timeout means a user doesn't need to worry about their email
  archives being used to log in.

Still, this could be attacked:

* If an attacker can access a user's inbox, they can generate a new login
  link, and log in as them.
* If TLS is not used for the email transport, a MITM can snoop login links
  and use them.
* If https is not used for the login link, a MITM can intercept and proxy
  web traffic and either steal a copy of the cookie, or use the login
  link themselves without letting the user log in. This attack seems no
  worse then using password authentication w/o https, and the solution is
  of course https.
* If an attacker wants to DOS a wiki, they can try to get its domain, IP,
  whatever blacklisted as a spam source.

These attacks don't seem worth not doing it; many of the same attacks can
be performed against openid, or passwordauth. Eg, reset password and
intercept email.

Implementation notes:

* Use the email address as the username.
* Sanitize the email for display in recentchanges etc.
* The login link should be as short an url as possible, while containing
  sufficient entropy. Some email clients will let the user click on it,
  but some users will need to cut and paste.
* The `adminemail` config setting has a bit of overlap with an `adminuser`
  set to an email address. Probably worth keeping them separae though;
  the `adminemail` is an email address to display, and we may not want to
  let anyone who can read the adminemail's mailbox to log into the wiki.
* Will want to make passwordauth reject registrations that contain `@`.
  Otherwise, someone could use passwordauth to register as a username that
  looks like an email address, which would be confusing to possibly a
  security hole. Probably best to keep passwordauth and emailauth accounts
  entirely distinct.
* Currently, subscription to comments w/o registering is handled by
  passwordauth, by creating a passwordless account (making up a username,
  not using the email address as the username thankfully). That account can be
  upgraded to a passworded account if the user follows a link in comment
  mails to login. So there is considerable overhead between that and
  emailauth.
* Adapting the passwordauth reset code is probably the simplest way to
  implement emailauth. That uses a CGI::Session id as the entropy.

Thoughts anyone? --[[Joey]]