Let's suppose I go to https://www.example.com using Tor. I check that the URL is correct, and the lock icon is present. But the exit node could be run by an adversary who uses a MITM attack and intercepts all the traffic. I download the SSL cert, then download the SSL cert from the same webpage, not through Tor, and do a binary comparison. Or else I compare the cert's serial number and SHA-256 fingerprint. If they match, can I be guaranteed the website viewed through Tor is legit? Or can those details be faked?

I am assuming the genuine cert for example.com has not been compromised.

I looked at plugins like convergence, but my questions relates specifically to checking the cert manually.

  • I'm working on integration for Perspectives project into Tor toolchain, you can use it in your web browser.
    – Alexey Vesnin
    Nov 23 '15 at 1:30

You can see the certificate information and export the cert:

Click the lock,
More Information,
View Certificate,

You don't need to download two certificates and compare them (but you can if you want to). Export the cert as a file and then:
openssl x509 -noout -text -in filename ; where filename is the cert you exported.
You can also check the serial number and fingerprint at online ssl cert checkers.

The exit node should not be able to fake a cert for the domain you're connecting to.

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