Can we agree that with a legitimate version of the TBB, even with a MITM, it's very hard to know the exit node of your Tor circuit ?
If an attacker can decrypt your connection, he can read the data. He doesn't need to know your exit node. That is much harder to do with a Tor circuit than with an HTTPS connection. Like the poster above said, HTTPS connections can be intercepted at the exit nodes with widely available programs like sslstrip. However, this interception is noticeable if you are paying attention, because you lose the lock icon. This is why mixed content is bad, it gets us used to losing the lock icon.
I wonder how strong is the encryption used in Tor. Would it really hold long supercomputers ?
Tor uses the TLS protocol with a 128 bit AES stream cipher and 1024 bit RSA keys for authentication. If you want the nitty gritty details, you can read the protocol spec:
https://gitweb.torproject.org/torspec.git?a=blob_plain;hb=HEAD;f=tor-spec.txt
In terms of encrypting the stream, Tor circuits are basically the same as HTTPS connections (they could be made stronger with AES-256 encryption, but AES-128 would take longer than your lifetime to decrypt with today's technology). The important difference between Tor circuits and HTTPS connections, as I said in a previous post, is that it's much harder to break Tor's authentication mechanism.
Oh and finally, can someone confirm that when you connect to a .onion website, the full path to server is encrypted ?
Since you're connecting Tor client to Tor client, yes, the entire path is encrypted.
Concerning clearnet sites through Tor: Why is there so much warnings about accessing clearnets sites over Tor ? Is the end server of a clearnet website able to know your own IP address ? Or it's just because of the flash/scripts and other stuff that can give this information to the end server.
Yeah, it's mostly the dangers of Flash and Java. I think the dangers of JavaScript are exaggerated. Yes, JavaScript can be dangerous too, but it's much better sandboxed inside the browser. Flash and Java are run by plugins that are separate processes and can more easily bypass the browser's proxy settings. That's why NoScript was added to the browser bundle. It blocks Flash and Java even when it is disabled.
In other words: Would a perfectly configured/UpToDate TBB leave any compromising information on a clearnet site ?
Well, if you post your name on the site, there's nothing Tor can do to save you.
But I get what you're saying, and the answer is that the browser bundle is specially configured to greatly reduce data leaks and fingerprinting attacks that could be used to identify you. Nothing is perfect of course, but browsing clearnet with TBB in its default configuration is considered safe enough by the Tor developers that they distribute it that way.