Tuesday, 28 May 2013

Why can a TOR exit node decrypt data, but not the entry node?

Why can a TOR exit node decrypt data, but not the entry node?

Me -> Node A -> Node B -> Node C -> destination
The documentation on TOR always states that only the exit node C can see plain text data. How is this possible without me talking to Node C directly?
If I have some plain text data, and want to send it encrypted to Node A, I'd usually do a Diffie-Hellman key exchange, and send the data over. But with that scheme, Node A could decrypt the data.
If Node C was somehow sharing its public key with me, couldn't Node B or Node A MITM the key?
How exactly does TOR manage its PKI? What keys are used to encrypt data where?

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