Login to EE with external information

I need to allow login to EE with the same login from an external authentication system.

For that I have 3 scripts made available from that external authenticating site which will answer back as follow after posting a form with a login/password(MD5) pair:

if login exist

<response>
    <login>true</login>
    <title>Mr</title>
    <firstName>John</firstName>
    <lastName>Dawn</lastName>
</response>

if login doesn’t exist

<reponse>
    <login>false</login>
</reponse>

The second script will be called with the same “login” as the previous one which would be saved in a cookie or in another way. It will answer :

<response>
    <connected>true</connected>
</response>
or
<response>
    <connected>false</connected>
</response>

The third script will be called without any parameters. It will answer :

<response>
    <disconnected>true</disconnected>
</response>

or unlikely

<response>
    <disconnected>false</disconnected>
</response>

(parameters can be passed as GET or POST)

This login method is used only for one type of what I would call frontend users (no administration only posting blog comments, participating in forum and wiki)

So my question to all the people who knows EE authenticating methods well is: which route is the simplest and less time consuming method to let those “external” users login to EE to post to forum without having to re-register?