So a bit of background, I'm working on an existing web application which has a set of users, who are able to log in via a traditional login screen with a user name and password, etc.

Recently we've managed to score a client (who have their own Intranet site), who are wanting to be able to have their users log into their Intranet site, and then have their users click a link on their Intranet which redirects to our application and logs them into it automatically.

I've had two suggestions on how to implement this so far:

1. Create a URL which takes 2 parameters (which are "username" and "password") and have the Intranet site pass those parameters to us (our connection is via TLS so it's all encrypted). This would work fine, but it seems a little "hacky", and also means that the logins and passwords have to be the same on both systems (and having to write some kind of web service which can update the passwords for users - which also seems a bit insecure)
2. Provide a token to the Intranet, so when the client clicks on a link on the Intranet, it sends the token to us, along with the user name (and no password) which means they're authenticated. Again, this sounds a bit hacky as isn't that essentially the same as providing everyone with the same password to log in?

So to summarise, I'm after the following things:

1. A way for the users who are already authenticated on the Intranet to log into our system without too much messing around, and without using an external system to authenticate, i.e. LDAP / Kerberos
2. Something which isn't too specific to this client, and can easily be implemented by other Intranets to log in