What Is an Onion Address?
An onion address is a special address ending in `.onion` that identifies a service on the Tor network. Unlike an ordinary domain such as `example.com`, a modern onion address is derived from cryptographic information associated with the onion service rather than being purchased from a conventional domain registrar.
An onion address is a Tor-only service address. Modern v3 addresses encode information derived from the onion service identity, including an Ed25519 public key plus checksum and version data, so Tor clients can verify the service without conventional DNS.
Current v3 onion addresses contain 56 characters before the `.onion` suffix and are designed so Tor clients can cryptographically validate important information about the service they are connecting to.
You normally open an onion address through Tor-aware software such as Tor Browser. Ordinary DNS does not resolve `.onion` addresses.
TL;DR
Quick answer
An onion address is a Tor-only service address. Modern v3 addresses encode information derived from the onion service identity, including an Ed25519 public key plus checksum and version data, so Tor clients can verify the service without conventional DNS.
Definition
Anatomy of a Modern .onion Address
A v3 onion hostname is a 56-character base32 string followed by `.onion`. Tor Project documentation describes v3 addresses as containing a full Ed25519 public key, with checksum and version information included in the encoded address.
That does not mean the address is a simple copy of a key. Tor uses defined encoding and validation rules, and later connection steps use Tor's onion service directory architecture to find descriptors and introduction points.
Onion Address vs Ordinary Domain
| Normal domain | Onion address | |
|---|---|---|
| Registration | Bought or assigned through a registrar and registry. | Generated from onion service cryptographic identity. |
| DNS | Uses the Domain Name System. | Ordinary DNS does not resolve .onion names. |
| Resolution | Domain resolves toward IP addressing through DNS. | Tor looks up onion service descriptors inside Tor's distributed architecture. |
| Address generation | Chosen by registrant if available. | Derived from cryptographic material; vanity prefixes require computation. |
| Normal-browser accessibility | Usually yes. | No, unless the browser is Tor-aware or configured for Tor. |
| Location exposure | DNS and hosting may reveal infrastructure. | Designed to hide service location from clients. |
| Cryptographic properties | TLS can authenticate a registered domain separately. | The address itself is tied to onion service identity. |
How an Onion Address Works
The service has cryptographic keys
An onion service generates or uses keys that identify the service.
The address derives from the service identity
The visible v3 hostname is produced from the service's public-key material plus validation information.
Descriptors are published inside Tor
The service makes connection information available through Tor's distributed onion service directory design.
The client asks Tor, not DNS
Tor Browser uses the onion address to locate the relevant descriptor and connection points.
The connection stays within Tor
The client and service connect through Tor circuits without a conventional DNS lookup for the onion name.
Why Are Onion Addresses So Long?
The length is a security tradeoff. V3 addresses are longer than older v2 names because they carry stronger identity information, including a full Ed25519 public key rather than only a shorter hash-based identifier.
Shorter, human-friendly names are easier to read but provide less room for the cryptographic identity and validation properties Tor wanted in the modern format.
V2 vs V3 Onion Addresses
Older v2 onion addresses were 16 characters long. Tor deprecated v2 onion services because the older design was no longer considered safe enough and was no longer being developed.
Tor's v2 deprecation timeline said Tor Browser stable would make v2 onion addresses completely unreachable in October 2021. Modern onion services should use v3 addresses.
Can Someone Create a Lookalike Onion Address?
An attacker cannot simply register a confusingly similar onion address the way they might register a typo domain. But that does not eliminate impersonation risk.
Vanity onion address generators can search for addresses with chosen prefixes, and phishing pages can rely on users checking only the first few characters, following bad links or trusting copied directories. Onion addresses reduce some naming attacks; they do not make users immune to lookalikes.
Can Google Index Onion Sites?
Google does not resolve .onion addresses through ordinary DNS, so direct crawling of onion services is not like crawling public HTTPS pages. That does not mean onion addresses are invisible everywhere.
References to onion addresses can appear on normal web pages and be indexed there. Specialized onion search engines and directories can also catalog some onion services if those services are discoverable. Discoverability is therefore separate from ordinary technical access.
Legitimate Uses of Onion Services
The Tor Project documents onion services as a way to provide location-hidden services through Tor. Established organizations have used them for censorship-resistant or privacy-preserving access.
Examples have included news organizations and whistleblower systems that publish official onion addresses through their normal public websites. The legitimacy comes from the use case and operator, not from the .onion suffix alone.
Questions people ask
Safety note
Educational, not operational
This guide is educational. It does not provide instructions for illegal activity, evading law enforcement, buying prohibited goods, or attacking systems. Laws and risks vary by country, so stay within your local rules and avoid interacting with unknown services.
Sources
Further reading
- Tor Project: V3 onion services usageThe Tor Project
- Tor Project: V2 onion services deprecation timelineThe Tor Project
- Tor Project Community: Onion servicesThe Tor Project
Glossary
Terms in this guide
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