Tor Is Not Just for Anonymity


Anonymity, privacy, and security. All good things, you'll agree if you're reading this post. But what we can't all quite agree on is where one stops and the next begins.

It is my opinion that security is an umbrella term: a more general word that encompasses both of the others. Everything that makes you more anonymous (or private) also makes you more secure.

There is a notion that comes up every now and then that Tor doesn't make you more secure, but it only makes you anonymous. This is obviously not true under my definitions of the terms (if it makes you anonymous, it by definition of the words must also make you more secure). But here I'll strive to convince you that even if you disagree with my definitions, there are benefits to Tor onion services that cannot be construed as anonymity.

The setup

The framing for this is two scenarios. Either you (1) visit a website on the regular web using TLS (HTTPS), or (2) visit a regular onion service. Everything in both scenarios is setup in a typical/sane way. Here are the assumptions:

  • The onion service is a regular 6 hop one. It is not setup to use the non-anonymous 3 hop design.
  • The user's and server's Tor clients are extremely close to the end point software such that for the purpose of discussion here, they are the end point software.
  • The regular website uses TLS in a typical way. A reputable Certificate Authority (CA) is used, sane ciphers are used, and TLS 1.2 or 1.3 is used.
  • DNSSEC is not used. Effectively no one uses it, and if they do, not fully correctly.
  • DNS over HTTPS is not used. If it is, then it's still vulnerable to attacks on TLS and the DNS server changing or censoring results.
  • There are no security destroying bugs in TLS libraries or in the relevant parts of Tor code.
  • Encryption algorithms are not broken. AES is secure, as is whatever the TLS client/server negotiate. Crypto is not a weak link.

Tor onion services are not limited to being HTTP servers (they could be IRC, SMTP, or any other TCP-based daemon), but in this post we ignore everything but HTTP servers.

Tor benefits that I'd consider anonymity

  • 5/6ths of the path between the user and the server does not know the user. I do not just mean the relays, but also the connections between them.

  • Similarly, 5/6ths doesn't know the destination. Well actually ...

    • The onion service's guard can tell based on traffic patterns that it is a guard for an onion service. Yet while the onion service's guard knows its IP address, the guard doesn't know which onion service it is acting as a guard for. This means no one knows the destination onion address. Nice! Well actually ...

    • If the guard already knows the onion address (say it's a popular publicly known address), it can perform a timing attack to confirm that it is the guard for the onion service. And now we're back to 5/6ths of the path doesn't known the destination.

  • 100% of the path doesn't know both.

Tor benefits that are not anonymity

  • DNS hijacking is impossible. DNS is simply not used.

  • The DNS-like process Tor uses is secure (find onion's Introduction Points (IP) in the DHT, connect to an IP and to tell onion what Rendezvous Point (RP) to use, both parties connect to RP). No one knows what onion was looked up in the DHT: not even the relay in the DHT that held the answer. DNS is laughably insecure compared to this. The DNS server knows everything (user, question, and answer). If any part of the link between user and server is insecure, that's a place everything can be determined by anybody watching. The DNS server can forge or censor responses whenever it wants. Likewise for anyone on an unprotected part of the link.

  • BGP hijacking is impossible. Every interaction a Tor client has with a relay or onion service is authenticated such that you are guaranteed to be interacting with the relay/onion that you intend to be.

  • MITM attacks are impossible. When visiting a regular website, you have to assume the none of the 100s of CAs that you trust have maliciously or mistakenly issued a certificate for the domain you're visiting when they shouldn't have. There are zero places a corporate firewall can inject itself to decrypt the traffic. There are zero places and zero parties between the Tor clients that a MITM attack can be performed. TLS attempts this and usually succeeds. Tor guarantees it.

If it looks like you've successfully connected to an onion service, you have and you have done so securely. TLS offers no such guarantee.

Moving on ...

Matt, you're just continuing to describe a reimplementation of Transport Layer Security.

No, I'm not. To restate what I've exhaustively said above:

  1. TLS isn't used in every part of the process of connecting to a regular website. Tor makes the whole process secure.

  2. TLS tries its best to ensure your link is secured, but Tor guarantees it. With TLS you have to make assumptions you shouldn't have to make in order to believe it is working. Or put another way: Tor requires fewer assumptions, so Tor is more secure.

Accessing Tor onion services is designed to be more secure than accessing something over TLS.

TLS has patches upon patches trying to make it better: Certificate Transparency, HSTS, OCSP ... These help! Absolutely these make TLS better and more secure.

But Tor is more secure. Every onion service is secure and you make fewer assumptions.

Will the next regular website you visit use HSTS? If you check after you connect in order to determine the answer, and you find that it isn't using HSTS, is that because you're being attacked or does the site legitimately not use HSTS?

Does your browser check CT logs, and if it does, what does it do in response to failing to find the certificate in CT logs? Do you assume the CT lookup process is secure? Are the parties you're communicating with misbehaving? Are your lookups in CT logs being logged and associated with you?

Tor is not just for anonymity. It provides real, measurable, enumerable security benefits over TLS.

Matt Traudt (pastly)

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