You already have your keyword - and you already know the way it works ;)
gmail
The authentication process is a two-phase procedure.
FIRST:
When hitting the the portal autnetification page, the client should enter 2 credentials.
The client should enter a gmail adress and an 'unkown' password.
Of course, the client doesn't know this password.
To retrieve this password, the user should enter his gmail mail account address.
[[b]Hey, this procedure is being used by every site on the Internet !!!! - You saw it before - you used it before - it's always the SAME procedure]
The portal code should send a mail to the visitors gmail mail address with the secret, randomly generated password.
The client should login to the gmail's WEB mail interface to grab the 'challenge' mail - find the secret random.
SECOND:
Back on the portal interface, the visitor has the password, he can login.
Note:
All IP's from Google that are used to access the GMAIL web mail account servers should be listed as 'Allowed IP adress' on the portal service page.
Maybe POP/IMAP accss should be handled as well, a part of the planet nevers uses web mails, they use mail-client software like Thunderbird, Outlook, etc.
Access to SSL (https) pages can be problematic when you aren't yet authenticated with the portal.
This is a pure PHP coding exercise.
I advise you to create a mail adress (your-portal@gmail.com) that can be used by your own portal code so it can safely send maisl to other gmail accounts.
BTW: this is just an idea, not a solution. I didn't Google anything up - just used my memory.
