A Tor v3 onion address is a printable, checksummed encoding of an onion service’s public identity key. It contains a 32-byte Ed25519 master public key, a two-byte checksum and a one-byte version field, then adds the .onion suffix. The corresponding private master key is separate secret material stored by the operator; decoding an address can recover the public key, but never the private key.
What a Tor v3 onion address contains
Tor’s specification defines the address as base32(PUBKEY | CHECKSUM | VERSION) + ".onion". In that expression, PUBKEY is the onion service’s 32-byte Ed25519 master public key, CHECKSUM is two bytes, and VERSION is one byte. For the current v3 format, the version byte is x03. The three binary fields are concatenated and Base32-encoded; the text .onion is appended afterward. See the Tor Project’s ONIONADDRESS specification.
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| Address component | Size | Purpose |
|---|---|---|
| Ed25519 master public key | 32 bytes | Identifies the onion service publicly |
| Checksum | 2 bytes | Detects many transcription or decoding errors |
| Version | 1 byte | Identifies the address format; v3 uses x03 |
| Base32 label | 56 characters | Printable encoding of the 35 bytes above |
| Suffix | 5 characters | The literal .onion suffix |
Because 35 bytes are encoded together, a v3 onion label is 56 characters before .onion. The label is not a random password, an encrypted name or a lookup alias stored in a central directory. It is a compact spelling of the service’s public identity.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsThe Tor specification summarizes this relationship precisely: “The onion address of a hidden service includes its identity public key, a version field and a basic checksum.”
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How the checksum is calculated
The checksum is specified as the first two bytes of a SHA3-256 digest. Tor hashes an exact constant string, the raw public-key bytes and the version byte in that order:
CHECKSUM = SHA3_256(".onion checksum" | PUBKEY | VERSION)[:2]
The resulting two bytes are placed after the public key and before the version byte, and only then is the complete 35-byte sequence Base32-encoded. A decoder can therefore detect many accidental changes: it decodes the label, extracts the public key, checksum and version, recomputes the digest and compares the first two bytes.
Two bytes provide an integrity check, not a statement that an operator is honest. A correctly formed address can still point to a service you did not intend to visit, and a copied address can be deliberately substituted. Obtain important onion addresses through a channel you already trust, and treat the checksum as protection against mistakes rather than proof of legitimacy.
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What you can learn by decoding an address
- Remove the literal
.onionsuffix and Base32-decode the 56-character label. - Split the resulting 35 bytes into the first 32 bytes (the Ed25519 public key), the next two bytes (the checksum) and the final byte (the version).
- Confirm that the version byte is the expected v3 value,
x03. - Recompute
SHA3-256(".onion checksum" | PUBKEY | VERSION)[:2]and compare it with the two checksum bytes.
If the checksum does not match, the label is malformed, mistyped or being interpreted with the wrong format. If it does match, you have established that the characters are internally consistent with a public key and a v3 version field. You have not recovered any secret key, identified the operator or independently verified the service’s content.
Why the private key is not inside the address
The public and private halves have different jobs. The address deliberately exposes the public identity key so clients can name the service. The private master key remains with the service operator and is used as secret identity material. The public key is designed to be recoverable from the address; the private key is not encoded there.
In a filesystem-based onion-service directory, Tor documents two separate files. The hostname file contains the onion address, while private_key_ed25519 contains the private master Ed25519 key. The directory layout is described in Tor’s hidden-service directory format.
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The public v3 onion address | It may be distributed to clients |
private_key_ed25519 |
The private master Ed25519 key | The service operator or protected service storage |
Loss of the private master key is not repaired by knowing the address. The address identifies the public key, but it does not provide the secret material needed to reproduce the same identity. Conversely, possession of an address alone does not let someone impersonate the service by mathematically deriving its private key.
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Tor’s ED25519-V3 control-protocol format
The filesystem view and the control-protocol view describe related identity material, but they are not interchangeable text encodings. Tor’s control specification defines an ED25519-V3 key blob as Base64 encoding of 64 bytes:
- A 32-byte Ed25519 secret scalar, serialized in little-endian form.
- A 32-byte pseudorandom-function (PRF) secret used by Tor’s key-blinding scheme.
This is different from a commonly encountered Ed25519 representation that stores a 32-byte hash seed followed by the 32-byte public key. Tor explicitly warns that its v3 blinding design requires the scalar and PRF secret instead. When such a Tor key is imported, the public key is recomputed rather than simply read from a seed-plus-public-key record. The serialization and import behavior are specified in Tor’s control-protocol commands documentation.
| Representation | Serialized secret material | Import implication |
|---|---|---|
Tor ED25519-V3 control blob |
32-byte secret scalar + 32-byte PRF secret, Base64-encoded | Public key must be recomputed during import; it is not the seed-plus-public-key layout |
| Common Ed25519 example | 32-byte hash seed + 32-byte public key | Not automatically compatible with Tor’s v3 control format |
Do not rename a key file, paste its Base64 text into an unrelated Ed25519 tool or assume that every 64-byte Ed25519-looking value has the same meaning. Validate the format expected by the Tor interface you are using and convert deliberately when a conversion is supported.
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Tor’s control protocol can create an onion service with the ADD_ONION command. It can accept an Ed25519 v3 key or request a newly generated key with NEW. When Tor generates a new keypair, the reply includes the private key unless the DiscardPK option is supplied.
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DiscardPK is an explicit loss-of-key behavior, not a backup strategy. Tor’s specification warns that if a generated key is discarded, the keypair and its corresponding service cannot later be recreated from that discarded key. If the same onion identity must survive a restart, migration or recovery event, retain the private key in the format required by the control protocol and protect that secret separately from the public address.
Can anyone derive the private key from an onion address?
No. An address reveals the 32-byte public key, checksum and version only. The checksum itself is computed from public inputs, so it adds no secret information. The private key is a separate secret whose relationship to the public key is designed to resist practical inversion.
There are only two realistic ways an operator normally obtains the private identity material for an existing service: by retaining the key when the service was created, or by reading the protected service/control-protocol storage where Tor saved it. Reconstructing a service from the address alone is not an alternative. A newly generated keypair would represent a different public identity and therefore a different address.
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How the address participates in a Tor connection
The address is the starting identifier, not the entire authentication exchange. Tor’s protocol overview describes a client building an anonymous circuit to an introduction point and sending an introduction request. The onion service builds a separate circuit to a rendezvous point; Tor joins the circuits there. A cryptographic handshake establishes a shared key and allows the client to confirm that it is communicating with the intended service. See the Tor protocol overview.
This distinction matters because an onion service uses more than one kind of key during its lifetime. The long-term master identity key underlies the address, while introduction, rendezvous and handshake operations involve protocol keys with different roles. Client-authorization keys, where used, are also separate from the service’s public address and master private key. Treating every key mentioned in onion-service documentation as the same object leads to incorrect backups and failed imports.
V3 addresses versus historical v2 names
Tor’s protocol overview contrasts v3 naming with the older v2 scheme. V3 uses a 32-byte Ed25519 public identity and produces a 56-character label. Older names were based on an 80-bit truncated SHA-1 hash of a 1024-bit RSA key. That comparison explains the formats’ design lineage; it does not mean that an old-format service should be assumed usable today.
| Property | V3 | Historical v2 description |
|---|---|---|
| Identity basis | Ed25519 public identity key | 80-bit truncated SHA-1 hash of a 1024-bit RSA key |
| Label length | 56 characters before .onion |
Shorter historical naming format |
| Address construction | Public key + two-byte checksum + version, Base32-encoded | Hash-derived name |
Common mistakes and what they mean
“The onion address is encrypted.”
It is encoded, not encrypted. Base32 changes binary bytes into letters and digits suitable for a domain-like label. Anyone who has the address can decode the public key and verify the checksum.
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“The checksum proves the site is genuine.”
It only catches many errors in the label. It does not verify the operator’s identity or the safety of a site. Trust still depends on how you obtained the address and on Tor’s connection authentication.
“Any Ed25519 private key file can be used.”
Not necessarily. Tor’s ED25519-V3 control blob stores a scalar and PRF secret, unlike a seed-plus-public-key format. A tool that accepts one representation may reject or misinterpret the other.
“I can recreate the service after deleting the key because I still know the address.”
You cannot recreate the same identity from the address alone. Preserve the private key before migration or deletion, and do not use DiscardPK for a generated key when continuity matters.
A practical identity checklist
- Count the v3 label as 56 characters before
.onion. - Decode the label as Base32 into 35 bytes.
- Interpret the bytes as 32-byte public key, two-byte checksum and one-byte version.
- Recompute the SHA3-256 checksum with the exact
.onion checksumconstant. - Keep the public
hostnamevalue separate from the private key material. - When using the control protocol, preserve the Tor-specific
ED25519-V3serialization and its Base64 wrapper. - Back up the private key before moving or regenerating a service; an address is not a backup.
The essential distinction
A v3 onion address is a public, checksummed identifier derived from an Ed25519 master public key. Its 56-character label lets Tor and clients name that identity and detect many typing errors. The private master key lives separately, in service storage or a control-protocol representation that includes a secret scalar and PRF secret. Decoding the address confirms the public identity; it never supplies the private key needed to recreate or control the service.
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