authenticate user inside iframe
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I have an website with an iframe pointing to a different domain. I need to check if the user is authenticated in the website, and auto login it in the page loaded by iframe. I can pass the user id in the query string (the user id is allways the same in the both websites) but that would be very unsecure. Maybe i could encrypt the user id with some secrey key and decrypt it inside the iframe.
The process should be the most transparent possible to the user (he shouldn´t note any difference when loading the iframe and doing the login.

Any suggestions?