Onion Services Properties¶

Onion Services have many interesting and useful properties, making it a good fit for privacy-preserving applications.

Summary¶

The table below offers a good summary, comparing regular websites, regular websites over Tor, and Onion Services (check the Terminology page for definitions such as "Regular website"):

Property	Regular website	Regular website over Tor	Onion Service
Censorship resistance	Poor: Website can easily be censored	Good: Censorship still possible via exit nodes	Very good: Accessible as long as Tor is reachable, address not censorable
Privacy safeguards	Very low: only minimal safeguards via HTTPS, susceptible to traffic surveillance	Good: Data correlation is not an eliminated risk	Very good: End-to-end encryption for user and service, anonymity for both
Metadata elimination	Poor: Data about online activity recorded by websites and entities passing traffic	Good: Data about online activity can be recorded by website if user logs in and identifies themselves	Very good: Metadata logging eliminated on both ends, but website can record data if user logs in

Some of these properties arise from the addressing mechanisms built in the technology (The Onion Space), while others comes from the way clients connects to services (The Rendezvous).

So, roughly speaking, the technology can be explained in a higher level as

Onion Services = The Onion Space + The Rendezvous

Applications can them benefit from these addressing and routing systems along with other protocols (such as HTTPS and SSH).

The Onion Space¶

What is the Onion Space?¶

As explained in the Onion Services specification, an .onion address is basically a random-looking number that is used as a public key.

The Onion Space consists in all possible .onion addresses, and this number is huge (in the order of \(2^{251}\) possible addresses, check the discussion below), which is bigger than the estimated number of atoms in the galaxy¹.

This does not mean that there are so many running Onion Services out there. What is important is that:

The actual number of Onion Services should be way smaller than the total number of possible Onion Services.
The number of possible Onion Services should be so big that everyone can have as many addresses as needed and use them to chat, to share files or even running an onionsite, even if they just use an address only once and then discard it.

Connecting to an Onion Service address means using the .onion address through the Tor network to establish a connection to the remote computer that has control over the corresponding .onion address' private key.

The Onion space is a border-less, country-agnostic space¶

Onion Services offer a country-agnostic Web experience. You're not visiting an onionsite in/from a specific country, you're simply visiting a website in a virtual, border-less, protected space.

When accessing a website over VPN, you're exiting from the VPN server which is located in a specific country, and the experience of navigating that website will differ based on which country you're visiting it from (i.e. where the VPN server is located). When accessing a website over Tor, this depends on where the exit node is located.

When accessing an Onion Service on the other hand, you're not "exiting" from anywhere, rather you're meeting the website inside the Tor network. The communication between you and the website is also protected by end-to-end encryption.

There are positives to this (not necessarily technical – rather affective):

A country-agnostic visit implies one that removes risks and pressures put forward by different jurisdictions (similar to what VPN providers and Tor exit nodes face).
A border-less Internet is akin to a quiet and peaceful room away from the noise bustling outside of the Tor network.

While this argument can easily be used to idealize Onion Services, it's not the aim here.

The Onion Space Size¶

The Onion Services version 3 have, in theory, 256 bits of space size, but the actual number is a little smaller, since not any possible 256 bit number can be a valid public key: Tor clients should check if the key has a "torsion component", as described at Client-side validation of onion addresses:

As part of the address validation, Tor clients should check that the underlying ed25519 key does not have a torsion component. If Tor accepted ed25519 keys with torsion components, attackers could create multiple equivalent onion addresses for a single ed25519 key, which would map to the same service. We want to avoid that because it could lead to phishing attacks and surprising behaviors (e.g. imagine a browser plugin that blocks onion addresses, but could be bypassed using an equivalent onion address with a torsion component).

This implies that not all of the possible 256 bit combinations can be used as public keys, but a slightly smaller number: it's estimated that this makes the space around 5 bits smaller, having a total keyspace in the order of \(2^{256-x}\) for \(x \approx 5\).

That still means that allocation space it's still bigger (almost twice) the size of the IPv6 space (which supports up to 128 bits of address space)!

With onion service public keys, with the torsion component taken into account, the key clamping, etc. would you still say the key space for onion keys are around the size of \(2^{256}\) or would you say it's smaller?

Enumerating / brute forcing the Onion Space¶

Besides being huge, it's not easy to enumerate the Onion Space².

With the arrival of version 3 Onion Services, Tor added a protection mechanism to the Onion Services subsystem in Tor that prevents relays from enumerating which Onion Services they may be involved with providing access to.

This means there is no longer an efficient way of enumerating Onion Services by participating in the Tor network as a relay. This property of version 3 Onion Services is considered a feature -- not a bug, but it makes it harder for you to achieve your goal.

You will have a bad time enumerating these addresses using brute force. The Onion Service addresses contain a 256-bit ed25519 public key encoded in the “.onion” hostname. Brute forcing thus means you must enumerate possible all values from 0 to \(2^{256}\), which is an incomprehensibly large number of addresses, with only a tiny fraction being actual, available, Onion Services. Remember, IPv6 is “only” \(2^{128}\) addresses available globally, and that is already a vast number of addresses.

I would guess the way others build their Onion Services index is likely from scanning for valid ".onion" strings in various public data sets on the internet. Thus slowly building up a corpus of available Onion Services.

The Rendezvous¶

The "rendezvous", in the Tor jargon, is a connection to an arbitrarily chosen Tor node between a client and an Onion Service.

This connection is done through a special way that offers many security and privacy properties.

Censorship resistance¶

Both location and IP address of an Onion Service are hidden, making it difficult to censor or identify who runs the service.
As a result they used to be called “hidden services”.
Tor exit nodes can block websites (rare), Onion Services never exit the Tor network.
It’s the most censorship-resistant technology available out there, as long as the Tor network is reachable to the user.

Metadata obfuscation and elimination¶

When you use the Tor network to browse the Web you are not sending any information by default of who you are or where you are connecting from.
The Onion Services use the Tor network to eliminate information about where they are situated.
Using them eliminates all metadata that may be associated with the service otherwise.

End-to-end encryption between user and website¶

Furthering the protection of sources and whistleblowers.

Contributing to the decentralized Web¶

To deploy an Onion Service, you don't need a static or dedicated IP address nor need to purchase a domain and submit it for approval.
Deployment is easy: you don't need to forward ports or configure your modem.
For smaller websites like blogs, there’s no need for expensive hardware.

Maximum harm reduction¶

Leaving the Tor network still puts users at risk of censorship and other security and privacy risks, Onion Services almost diminish these risks.
Even if websites are under DDoS (Distributed Denial of Service), Onion Services could still give access to content of the site (in the case that the onion service itself is not under DDoS (but there are protections if that happens!).
Visiting an Onion Service is more secure than visiting a regular website over Tor.
There are multiple cases of people prosecuted over digital evidence - particularly metadata and IP info - that can only be avoided if people used Onion Services.

Tor network sustainability¶

The traffic generated by Onion Services doesn't leave the Tor network, and therefore, these onion circuits free up exit relay bandwidth for others. This is important because exit relays are a limited resource, making up 20% of the 7000 relays (as of November 2022). As they're a small fraction of the network, in general, exit relays are overloaded and represent a bottleneck for Tor users' browsing experience.

Additional arguments¶

Here we gather other lists of properties and arguments in favour of Onion Services:

Notes¶

For the comparison with the estimated number of atoms in the galaxy, check atoms in the galaxy, 4.6×10^68 in base 2 and 2^228. ↩
This subsection originally appeared in the forum thread Is there an index for onion domains?, and here is presented with a few changes. ↩