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Tor Overzicht

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Tor is a free to use, decentralized network designed for using the internet with as much privacy as possible. Bij correct gebruik maakt het netwerk privé en anoniem browsen en communicatie mogelijk. Omdat Tor-verkeer moeilijk te blokkeren en te traceren is, is Tor een effectief middel om censuur te omzeilen.

Tor works by routing your internet traffic through volunteer-operated servers, instead of making a direct connection to the site you're trying to visit. Dit versluiert waar het verkeer vandaan komt, en geen enkele server in het verbindingspad kan het volledige pad zien van waar het verkeer vandaan komt en naartoe gaat, wat betekent dat zelfs de servers die je gebruikt om verbinding te maken jouw anonimiteit niet kunnen doorbreken.

Safely Connecting to Tor

Before connecting to Tor, you should carefully consider what you're looking to accomplish by using Tor in the first place, and who you're trying to hide your network activity from.

If you live in a free country, are accessing mundane content via Tor, aren't worried about your ISP or local network administrators having the knowledge that you're using Tor, and want to help de-stigmatize Tor usage, you can likely connect to Tor directly via standard means like Tor Browser without worry.

If you have the ability to access a trusted VPN provider and any of the following are true, you almost certainly should connect to Tor through a VPN:

  • You already use a trusted VPN provider
  • Your threat model includes an adversary which is capable of extracting information from your ISP
  • Your threat model includes your ISP itself as an adversary
  • Your threat model includes local network administrators before your ISP as an adversary

Because we already generally recommend that the vast majority of people use a trusted VPN provider for a variety of reasons, the following recommendation about connecting to Tor via a VPN likely applies to you. There is no need to disable your VPN before connecting to Tor, as some online resources would lead you to believe.

Connecting directly to Tor will make your connection stand out to any local network administrators or your ISP. Detecting and correlating this traffic has been done in the past by network administrators to identify and deanonymize specific Tor users on their network. On the other hand, connecting to a VPN is almost always less suspicious, because commercial VPN providers are used by everyday consumers for a variety of mundane tasks like bypassing geo-restrictions, even in countries with heavy internet restrictions.

Therefore, you should make an effort to hide your IP address before connecting to the Tor network. You can do this by simply connecting to a VPN (through a client installed on your computer) and then accessing Tor as normal, through Tor Browser for example. This creates a connection chain like:

  • You → VPN → Tor → Internet

From your ISP's perspective, it looks like you're accessing a VPN normally (with the associated cover that provides you). From your VPN's perspective, they can see that you are connecting to the Tor network, but nothing about what websites you're accessing. From Tor's perspective, you're connecting normally, but in the unlikely event of some sort of Tor network compromise, only your VPN's IP would be exposed, and your VPN would additionally have to be compromised to deanonymize you.

This is not censorship circumvention advice, because if Tor is blocked entirely by your ISP, your VPN likely is as well. Rather, this recommendation aims to make your traffic blend in better with commonplace VPN user traffic, and provide you with some level of plausible deniability by obscuring the fact that you're connecting to Tor from your ISP.

We very strongly discourage combining Tor with a VPN in any other manner. Do not configure your connection in a way which resembles any of the following:

  • You → Tor → VPN → Internet
  • You → VPN → Tor → VPN → Internet
  • Any other configuration

Some VPN providers and other publications will occasionally recommend these bad configurations to evade Tor bans (exit nodes being blocked by websites) in some places. Normally, Tor frequently changes your circuit path through the network. When you choose a permanent destination VPN (connecting to a VPN server after Tor), you're eliminating this advantage and drastically harming your anonymity.

Setting up bad configurations like these is difficult to do accidentally, because it usually involves either setting up custom proxy settings inside Tor Browser, or setting up custom proxy settings inside your VPN client which routes your VPN traffic through the Tor Browser. As long as you avoid these non-default configurations, you're probably fine.

VPN/SSH Fingerprinting

The Tor Project notes that theoretically using a VPN to hide Tor activities from your ISP may not be foolproof. VPNs have been found to be vulnerable to website traffic fingerprinting, where an adversary can still guess what website is being visited, because all websites have specific traffic patterns.

Therefore, it's not unreasonable to believe that encrypted Tor traffic hidden by a VPN could also be detected via similar methods. There are no research papers on this subject, and we still consider the benefits of using a VPN to far outweigh these risks, but it is something to keep in mind.

If you still believe that pluggable transports (bridges) provide additional protection against website traffic fingerprinting that a VPN does not, you always have the option to use a bridge and a VPN in conjunction.

Determining whether you should first use a VPN to connect to the Tor network will require some common sense and knowledge of your own government's and ISP's policies relating to what you're connecting to. However, again in most cases you will be better off being seen as connecting to a commercial VPN network than directly to the Tor network. If VPN providers are censored in your area, then you can also consider using Tor pluggable transports (e.g. Snowflake or meek bridges) as an alternative, but using these bridges may arouse more suspicion than standard WireGuard/OpenVPN tunnels.

What Tor is Not

The Tor network is not the perfect privacy protection tool in all cases, and has a number of drawbacks which should be carefully considered. These things should not discourage you from using Tor if it is appropriate for your needs, but they are still things to think about when deciding which solution is most appropriate for you.

Tor is not a free VPN

The release of the Orbot mobile app has lead many people to describe Tor as a "free VPN" for all of your device traffic. However, treating Tor like this poses some dangers compared to a typical VPN.

Unlike Tor exit nodes, VPN providers are usually not actively malicious. Because Tor exit nodes can be created by anybody, they are hotspots for network logging and modification. In 2020, many Tor exit nodes were documented to be downgrading HTTPS traffic to HTTP in order to hijack cryptocurrency transactions. Other exit node attacks such as replacing downloads via unencrypted channels with malware have also been observed. HTTPS does mitigate these threats to an extent.

As we've alluded to already, Tor is also easily identifiable on the network. Unlike an actual VPN provider, using Tor will make you stick out as a person likely attempting to evade authorities. In a perfect world, Tor would be seen by network administrators and authorities as a tool with many uses (like how VPNs are viewed), but in reality the perception of Tor is still far less legitimate than the perception of commercial VPNs, so using a real VPN provides you with plausible deniability, e.g. "I was just using it to watch Netflix," etc.

Tor usage is not undetectable

Even if you use bridges and pluggable transports, the Tor Project provides no tools to hide the fact that you are using Tor from your ISP. Even using obfuscated "pluggable transports" or non-public bridges do not hide the fact that you are using a private communications channel. The most popular pluggable transports like obfs4 (which obfuscates your traffic to "look like nothing") and meek (which uses domain fronting to camouflage your traffic) can be detected with fairly standard traffic analysis techniques. Snowflake has similar issues, and can be easily detected before a Tor connection is even established.

Pluggable transports other than these three do exist, but typically rely on security through obscurity to evade detection. They aren't impossible to detect, they are just used by so few people that it's not worth the effort building detectors for them. They shouldn't be relied upon if you specifically are being monitored.

It is critical to understand the difference between bypassing censorship and evading detection. It is easier to accomplish the former because of the many real-world limitations on what network censors can realistically do en masse, but these techniques do not hide the fact that you—specifically you—are using Tor from an interested party monitoring your network.

Tor Browser is not the most secure browser

Anonymity can often be at odds with security: Tor's anonymity requires every user to be identical, which creates a monoculture (the same bugs are present across all Tor Browser users). As a cybersecurity rule of thumb, monocultures are generally regarded as bad: Security through diversity (which Tor lacks) provides natural segmentation by limiting vulnerabilities to smaller groups, and is therefore usually desirable, but this diversity is also less good for anonymity.

Additionally, Tor Browser is based on Firefox's Extended Support Release builds, which only receives patches for vulnerabilities considered Critical and High (not Medium and Low). This means that attackers could (for example):

  1. Look for new Critical/High vulnerabilities in Firefox nightly or beta builds, then check if they are exploitable in Tor Browser (this vulnerability period can last weeks).
  2. Chain multiple Medium/Low vulnerabilities together until they get the level of access they're looking for (this vulnerability period can last months or longer).

Those at risk of browser vulnerabilities should consider additional protections to defend against Tor Browser exploits, such as using Whonix in Qubes to contain your Tor browsing in a secure VM and protect against leaks.

Opbouw van het pad naar Clearnet diensten

"Clearnet diensten" zijn websites die je met elke browser kunt bezoeken, zoals privacyguides.org. Met Tor kun je anoniem verbinding maken met deze websites door je verkeer door een netwerk te leiden dat bestaat uit duizenden vrijwillig gerunde servers die nodes (of relays) worden genoemd.

Telkens wanneer je verbinding maakt met Tor, zal het drie nodes kiezen om een pad naar het internet te bouwen; dit pad wordt een "circuit" genoemd

Tor-pad waarop jouw apparaat verbinding maakt met een ingangsknooppunt, middelste knooppunt en uitgangsknooppunt voordat de website van bestemming wordt bereikt](../assets/img/how-tor-works/tor-path.svg#only-light) Tor-pad waarop jouw apparaat verbinding maakt met een ingangsknooppunt, middelste knooppunt en uitgangsknooppunt voordat de website van bestemming wordt bereikt

Tor-circuitroute

Elk van deze knooppunten heeft zijn eigen functie:

De Entry Node

De entry node, vaak de guard node genoemd, is het eerste knooppunt waarmee uw Tor-client verbinding maakt. De entry node kan uw IP-adres zien, maar het kan niet zien waarmee u verbinding maakt.

In tegenstelling tot de andere nodes, zal de Tor client willekeurig een entry node kiezen en deze twee tot drie maanden aanhouden om je te beschermen tegen bepaalde aanvallen.1

De Middle Node

De Middle node is het tweede knooppunt waarmee je Tor client verbinding maakt. Het kan zien van welk knooppunt het verkeer afkomstig is - de entry node - en naar welk knooppunt het vervolgens gaat. De middle node kan jouw IP-adres of het domein waarmee je verbinding maakt niet zien.

Voor elk nieuw circuit wordt de middle node willekeurig gekozen uit alle beschikbare Tor-knooppunten.

De Exit Node

De exit node is het punt waar je webverkeer het Tor netwerk verlaat en wordt doorgestuurd naar de gewenste bestemming. De exit node kan jouw IP-adres niet zien, maar weet wel met welke site hij verbinding maakt.

De exit node wordt willekeurig gekozen uit alle beschikbare Tor-knooppunten met een exit-relaisvlag.2

Opbouw van het pad naar onion diensten

"Onion Services" (ook wel "verborgen diensten" genoemd) zijn websites die alleen toegankelijk zijn via de Tor-browser. Deze websites hebben een lange willekeurig gegenereerde domeinnaam die eindigt op .onion.

Verbinden met een Onion Service in Tor werkt ongeveer hetzelfde als verbinden met een clearnet service, maar je verkeer wordt door een totaal van zes nodes geleid voordat het de bestemmingsserver bereikt. Net als voorheen dragen echter slechts drie van deze knooppunten bij tot jouw anonimiteit, de andere drie knooppunten beschermen de Onion Service's anonimiteit, door het ware IP en de locatie van de website te verbergen op dezelfde manier als Tor Browser die van jou verbergt.

Tor-pad dat ujouw verkeer via uw drie Tor-knooppunten leidt plus drie extra Tor-knooppunten die de identiteit van de website verbergen](../assets/img/how-tor-works/tor-path-hidden-service.svg#only-light) Tor pad dat jouw verkeer toont dat door uw drie Tor nodes wordt geleid plus drie extra Tor nodes die de identiteit van de website verbergen

Tor circuit pad met Onion Services. Knooppunten in het blauwe hek behoren tot jouw browser, terwijl knooppunten in het rode hek tot de server behoren, zodat hun identiteit voor jou verborgen blijft.

Encryptie

Tor versleutelt elk netwerk pakket ( in een blok verzonden gegevens) drie keer met de sleutels van het Exit-, middle- en entry node- in die volgorde.

Zodra Tor een circuit heeft gebouwd, verloopt de gegevensoverdracht als volgt:

  1. Ten eerste: wanneer het pakket bij het entry node aankomt, wordt de eerste encryptielaag verwijderd. In dit versleutelde pakket vindt de entry een ander versleuteld pakket met het adres van de middle node. De entry node stuurt het pakket dan door naar de middle node.

  2. Ten tweede: wanneer de middle node het pakket van de entr node ontvangt, verwijdert het ook een versleutelingslaag met zijn sleutel, en vindt ditmaal een versleuteld pakket met het adres van de exit node. De middle node stuurt het pakket dan door naar de exit node.

  3. Ten slotte: wanneer de exit node zijn pakket ontvangt, verwijdert het de laatste versleutelingslaag met zijn sleutel. De exit node ziet hierna bestemmingsadres en stuurt het pakket door naar dat adres.

Hieronder staat een alternatief schema dat het proces weergeeft. Elke node verwijdert zijn eigen versleutelings laag, en wanneer de bestemmings server gegevens terugstuurt, gebeurt hetzelfde proces volledig in omgekeerde richting. Zo weet de exit node niet wie je bent, maar wel van welk knooppunt het afkomstig is, en dus voegt het zijn eigen versleutelings laag toe en stuurt het het terug.

Tor encryption](../assets/img/how-tor-works/tor-encryption.svg#only-light) Tor encryption

Gegevens verzenden en ontvangen via het Tor Netwerk

Met Tor kunnen we verbinding maken met een server zonder dat een enkele partij het hele pad kent. De entry node weet wie je bent, maar niet waar je naartoe gaat; De middle node weet niet wie je bent of waar je naartoe gaat; en de exit node weet waar je naartoe gaat, maar niet wie je bent. Omdat de exit node de uiteindelijke verbinding maakt, zal de bestemmingsserver nooit jouw IP-adres kennen.

Opmerkingen

Hoewel Tor sterke privacygaranties biedt, moet men beseffen dat Tor niet perfect is:

  • Tor never protects you from exposing yourself by mistake, such as if you share too much information about your real identity.
  • Tor exit nodes can modify unencrypted traffic which passes through them. This means traffic which is not encrypted, such as plain HTTP traffic, can be changed by a malicious exit node. Never download files from an unencrypted http:// website over Tor, and ensure your browser is set to always upgrade HTTP traffic to HTTPS.
  • Tor exit nodes kunnen ook het verkeer controleren dat via hen verloopt. Unencrypted traffic which contains personally identifiable information can deanonymize you to that exit node. Again, we recommend only using HTTPS over Tor.
  • Powerful adversaries with the capability to passively watch all network traffic around the globe ("Global Passive Adversaries") are not something that Tor protects you against (and using Tor with a VPN doesn't change this fact).
  • Well-funded adversaries with the capability to passively watch most network traffic around the globe still have a chance of deanonymizing Tor users by means of advanced traffic analysis.

Als je Tor wilt gebruiken om op het web te surfen, raden we alleen de officiële Tor Browser aan - deze is ontworpen om vingerafdrukken te voorkomen.

Protections provided by bridges

Tor bridges are commonly touted as an alternative method to hiding Tor usage from an ISP, instead of a VPN (as we suggest using if possible). Something to consider is that while bridges may provide adequate censorship circumvention, this is only a transient benefit. They do not adequately protect you from your ISP discovering you connected to Tor in the past with historical traffic log analysis.

To illustrate this point, consider the following scenario: You connect to Tor via a bridge, and your ISP doesn’t detect it because they are not doing sophisticated analysis of your traffic, so things are working as intended. Now, 4 months go by, and the IP of your bridge has been made public. This is a very common occurrence with bridges, they are discovered and blocked relatively frequently, just not immediately.

Your ISP wants to identify Tor users 4 months ago, and with their limited metadata logging they can see that you connected to an IP address which was later revealed to be a Tor bridge. You have virtually no other excuse to be making such a connection, so the ISP can say with very high confidence that you were a Tor user at that time.

Contrast this with our recommended scenario, where you connect to Tor via a VPN. Say that 4 months later your ISP again wants to identify anybody who used Tor 4 months ago. Their logs almost certainly can identify your traffic 4 months ago, but all they would likely be able to see is that you connected to a VPN’s IP address. This is because most ISPs only retain metadata over long periods of time, not the full contents of the traffic you request. Storing the entirety of your traffic data would require a massive quantity of storage which nearly all threat actors wouldn't possess.

Because your ISP almost certainly is not capturing all packet-level data and storing it forever, they have no way of determining what you connected to with that VPN after the fact with an advanced technique like deep packet inspection, and therefore you have plausible deniability.

Therefore, bridges provide the most benefit when circumventing internet censorship in the moment, but they are not an adequate substitute for all the benefits that using a VPN alongside Tor can provide. Again, this is not advice against using Tor bridges, you should just be aware of these limitations while making your decision. In some cases bridges may be the only option (if all VPN providers are blocked, for instance), so you can still use them in those circumstances with this limitation in mind.

If you think that a bridge can aid in defending against fingerprinting or other advanced network analysis more than a VPN's encrypted tunnel already can, you always have the option to use a bridge in conjunction with a VPN as well. That way you are still protected by the pluggable transport's obfuscation techniques even if an adversary gains some level of visibility into your VPN tunnel. If you decide to go this route, we recommend connecting to an obfs4 bridge behind your VPN for optimal fingerprinting protection, rather than meek or Snowflake.

It is possible that the WebTunnel pluggable transport currently being trialed may mitigate some of these concerns. We will continue to keep an eye on that technology as it develops.

Extra bronnen

  1. De entry node in jouw circuit wordt een "bewaker" of "Guard" genoemd. Het is een snel en stabiel node dat gedurende 2-3 maanden de eerste blijft in jouw circuit, ter bescherming tegen een bekende anonimiteitsdoorbrekende aanval. De rest van je circuit verandert bij elke nieuwe website die je bezoekt, en alles bij elkaar bieden deze relays de volledige privacybescherming van Tor. Voor meer informatie over de werking van guard nodes, zie deze blogpost en paper over inloopbeveiliging. (https://support.torproject.org/tbb/tbb-2/) ↩

  2. Relaysvlag: een speciale (dis-)kwalificatie van relais voor circuitposities (bijvoorbeeld "Guard", "Exit", "BadExit"), circuiteigenschappen (bijvoorbeeld "Fast", "Stable"), of rollen (bijvoorbeeld "Authority", "HSDir"), zoals toegewezen door de directory-autoriteiten en nader gedefinieerd in de specificatie van het directory-protocol. (https://metrics.torproject.org/glossary.html/) ↩

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