A .onion address looks like a typing accident: 56 characters of lower-case letters and digits in no recognisable order, with no brand in them and nothing to memorise. That is not a quirk of design. The address is not a name that points at a service – it is the service’s public key, written in a form a browser can carry. Once that is clear, most of the familiar questions stop being mysterious: why the strings are so long, why they change, why you cannot look one up, and why a search index is needed on a network that is, in principle, just websites.
A current (v3) address encodes 35 bytes in base32: a one-byte version, a 32-byte ed25519 public key, and a two-byte checksum. Fewer characters and the checksum fails; one character changed and it fails. So the address verifies itself. On the ordinary web you lean on a registrar and a certificate authority to establish that a domain belongs to the company you think it does. A .onion has neither. Whoever can prove possession of the private key is that address, there is nobody to appeal to, and there is no transfer, no renewal invoice and no dispute process.
Earlier addresses were 16 characters, built from shorter key material and a hash. Tor retired that format in 2021, because the smaller key space was no longer a sensible trade-off. That history is a free diagnostic: a directory that still prints 16-character addresses has not been maintained since, and a list that mixes 16- and 56-character entries is a snapshot somebody stopped updating.
Your browser cannot resolve the address the way it resolves a domain, because there is no DNS entry to resolve. It hands the address to Tor, and four things happen:
- Tor derives the address’s descriptor identifier and fetches a signed descriptor from a hidden-service directory. The descriptor is the document that says which introduction points will accept contact for this service.
- Your client builds a three-hop circuit to one of those introduction points and asks it to pass a message: someone wants to meet at a particular rendezvous point.
- In parallel it builds a second three-hop circuit to that rendezvous point and waits there.
- The service, once the introduction point has told it, builds its own three-hop circuit to the same rendezvous point. Both sides meet in the middle.
Six relays end up between the two parties. Neither side ever learns the other’s IP address, and the rendezvous point sees two anonymous circuits that it cannot attribute to anyone. Nothing in that exchange resembles a lookup, which is why calling it “resolving” is misleading.
There is no zone file to walk and no certificate transparency log to scrape. The key space is 256 bits wide, so guessing an address you were not given is not a matter of patience. This is the structural difference from the open web. There, a crawler finds pages by following links, and short domain names can be guessed cheaply enough to be worth trying. On Tor a service exists for you only if its address reaches you from somewhere else. Somebody has to publish it.
In practice that means an index: a crawler that starts from published addresses and follows links between services, or a catalogue maintained by hand, or – most often – both. A search index such as OnionLand Search Engine is doing the job DNS would do if a registry existed. What matters in that map is not its size but its upkeep: whether dead entries are removed, whether mirrors are recorded, and whether the same service is listed once rather than five times under five addresses.
“The address is valid” is a low bar. It proves control of a key, and nothing else.
- That the service is safe. Anyone can publish an address. Impersonation on Tor works as it does anywhere: a look-alike is published under a different key, and the only tell is that the string itself is different.
- That it is the same operator as last year. Keys rotate. A service that returns on a new address is the old one only if the operator says so through a channel you already trust.
- That it is legal to use. Legality belongs to what a service does, not to the protocol carrying it. The same network serves journalists, researchers and people under censorship as it serves activities that are illegal in most countries.
- That it will still be there. Nothing renews an address. When the operator stops, the address is simply dead, with no notice and no error that distinguishes it from a temporary outage.
A .onion address is a public key in base32: 56 characters, self-verifying, unregistrable. Opening one is a rendezvous between two circuits, not a lookup. The network cannot be enumerated, so discovery depends on published lists and on the discipline of maintaining them – which is why indexes exist, and why a good one is judged by how it handles dead, duplicated and mirrored entries rather than by how long its list is. And you need Tor Project‘s browser to reach any of it: no other software resolves these addresses.
