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Is there a minimum amount of nodes traffic goes through before reaching it's destination? To my understanding of Tor, every node increases the effectiveness of the encrypted communication.

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All Tor circuits contain 3 nodes by default (passing over hidden services which work a little differently and essentially use two circuits chained to a gateway node). Any more than this does not add any extra anonymity.

While it is possible to change the number of nodes by modifying the source code, this is not recommended. Any less than three nodes and your anonymity is more easily compromised, and any more and your latency (and the load on the network) is increased without providing you with any real benefit.

To understand why three nodes is the optimal choice, let's start with one and build our way up. With one node (effectively a proxy) the node knows both who is sending it traffic, and where that traffic is going (no anonymity). With two nodes one node knows who is sending traffic, and what node the traffic will be coming out of (or what the guard node is). This makes it easy to perform traffic correlation. However, with 3 nodes, there is a buffer between these two nodes. Therefore one node knows who you are, one node knows nothing, and the last node knows where the traffic is going but is fully isolated from the input. Adding anymore nodes doesn't do anything else except add more nodes that know nothing between the guard and exit nodes (slowing down everything).

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    Is it true that middle nodes know nothing? They may not know users or where traffic is going, but they do know where packets come from, and where they're going. Given that there are so many possible circuits, and that each circuit is only used for ten minutes, it would be very hard for adversaries to compromise the requisite middle nodes in real time. But what stops them is not just that middle nodes know nothing. – mirimir Oct 29 '13 at 5:09
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    By "nothing" I meant "nothing about the user or the destination". They do know the previous and next nodes in the circuit. – Sam Whited Oct 29 '13 at 12:13
  • doesn't adding nodes improve the security in the case we might have a lot of corrupted nodes? – David 天宇 Wong Feb 1 '15 at 19:46
  • @David天宇Wong No, the only two nodes that really matter in that case are the start and end nodes. Adding more essentially does nothing but slow things down. – Sam Whited Feb 1 '15 at 19:48
  • what you are saying is that if both 3 nodes are corrupted by the NSA for exemple, adding more nodes in the mix won't change anything? – David 天宇 Wong Feb 1 '15 at 23:02

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