Tor with VPNs (Don’t!)

I see a lot of questions on forums by people asking how to “use Tor with a VPN” for “added security”, and a lot of poor advice given in response. Proposals fall in two categories:

The first is useless and unnecessary, the second is catastrophically harmful. Let’s dig in.

Using a VPN to connect to Tor

In the first case, users want to connect to Tor through a VPN, with one of the following goals:

  1. Add more levels of proxies between them and the ‘net for safety

  2. Hide that they’re connecting to Tor from their ISP

  3. Hide that they’re connecting to Tor from Tor

The first goal is theoretically an alright idea, especially if you know little about Tor’s design or haven’t thought much about your threat model. More proxies = safer, right? In practice, it doesn’t add much: any adversary able to break Tor’s three-level onion routing is probably not to have any trouble breaking a single-hop VPN, either through legal coercion or traffic analysis. Adding a VPN here won’t hurt, but you’re losing money and slowing down your connection for a questionable improvement in “security” or “anonymity”.

The second goal is a good idea if you live in a country which forbids use of Tor - but there are better solutions here. If Tor is legal in your country, then your ISP can’t identify anything about your Tor usage besides when you were connected to Tor, and approximately how much data you moved. If Tor is not legal in your country, the Tor Project provides ‘bridges’, which are special proxies designed to hide that you are connecting to Tor. These bridges don’t stand out as much as a VPN, and don’t have any money trail tying them to you, and so are probably safer.

The last objective, hiding your IP address from Tor, is silly. Because of the onion routing design, Tor can’t see anything but your IP address and approximately how much data you’ve moved. Tor doesn’t care who you are, and can’t see what you’re doing. But sure, a VPN could hide your IP address from the Tor entry guard.

Using Tor to connect to a VPN

This is where we enter the danger zone. To explain why this is a horrible idea, we need to expand the original diagram:

When you connect to “Tor”, you aren’t connecting to a single proxy server, but to a series of three proxy servers. All of your data is encrypted in layers, like an envelope stuffed inside another envelope. When you communicate with the Tor entry guard, it can see that you’re sending encrypted data destined for a Tor relay, but doesn’t know anything else, so it removes the outermost envelope and sends the message along. When the relay receives the envelope it doesn’t know that you’re the original sender, it only knows that it received data from an entry guard, destined for an exit node. The relay strips off the outermost envelope and forwards along. The exit node receives an envelope from a relay destined for some host on the Internet, dutifully strips the envelope and sends the final data to the Internet host. When the exit node receives a response, the entire process runs in reverse, using a clever ephemeral key system, so each computer in the circuit still only knows who its two neighbors are.

The safety and anonymity in Tor comes from the fact that no server involved knows both who you are, and who you’re talking to. Each proxy server involved can see a small piece of the puzzle, but not enough to put all the details together. Compromising Tor requires either finding a critical bug in the code, or getting the entry guard, relay, and exit node to collude to identify you.

When you add a VPN after Tor, you’re wrecking Tor’s entire anonymity guarantee: The VPN can see where you’re connecting to, because it just received the data from the Tor exit node, and it knows who you are, because you’re paying the VPN provider. So now the VPN is holding all the pieces of the puzzle, and an attacker only needs to compromise that VPN to deanonymize you and see all your decrypted network traffic.

(There is one use-case for placing a proxy after Tor: If you are specifically trying to visit a website that blocks Tor exit nodes. However, this is still a compromise, sacrificing anonymity for functionality.)

What if the VPN doesn’t know who I am?

How are you pulling that off? Paying the VPN with cryptocurrency? Cool, this adds one extra financial hop, so the VPN doesn’t have your name and credit card, but it has your wallet address. If you use that wallet for any other purchases, that’s leaking information about you. If you filled that wallet through a cryptocurrency exchange, and you paid the exchange with a credit card or paypal, then they know who you are.

Even if you use a dedicated wallet just for this VPN, and filled it through mining, so there’s no trail back to you whatsoever, using the same VPN account every time you connect is assigning a unique identifier to all of your traffic, rather than mixing it together with other users like Tor does.

What if you use a new dedicated wallet to make a new VPN account every time you connect, and all those wallets are filled independently through mining so none of them can be traced back to you or each-other? Okay, this might work, but what an incredible amount of tedious effort to fix a loss in anonymity, when you could just… not use a VPN after Tor.

Tl;dr

Just don’t! Just use Tor! Or, if you’re in a region where using Tor would make you unsafe, use Tor + bridges. VPNs are ineffectual at best and harmful at worst when combined with Tor.

Posted 6/26/2021