While I doubt the downvotes are necessary, you're misunderstanding the scope of a MITM attack. In order to mitigate a MITM you must in some way communicate wit

While I doubt the downvotes are necessary, you're misunderstanding the scope of a MITM attack. In order to mitigate a MITM you must in some way communicate with someone else. Your checksum idea either gets routed through the malicious node, which means they could change it, or it doesn't, which means you're revealing to some third party other than the exit node what it is you're browsing, and you're also telling them who you are. In a sense, this is what https is for, by trusting certificates you know to be valid, a malicious node won't be able to see, let alone manipulate the data being sent to you (Though obviously they will see who you're talking to.)