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A Tor v3 onion address is a compact, checksummed representation of a service’s public identity—not a disguised private key. It encodes the service’s 32-byte Ed25519 master public key, a two-byte checksum, and a one-byte version value, then Base32-encodes those bytes into 56 characters before .onion. The corresponding private master key is stored separately by the onion service. Decoding an address can reveal the public key, checksum, and version, but it cannot recover the private key.
What a Tor v3 onion address contains
The Tor Project defines the address as:
onion_address = base32(PUBKEY | CHECKSUM | VERSION) + ".onion"
In that formula, PUBKEY is the service’s long-term Ed25519 identity public key, CHECKSUM is a short error-detection value, and VERSION identifies the address format. The complete specification is documented in Tor’s onion-address encoding specification.
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| Field | Size | Purpose |
|---|---|---|
PUBKEY |
32 bytes | The Ed25519 master public key, also called the service identity public key. |
CHECKSUM |
2 bytes | A basic check that the address bytes were not mistyped or corrupted. |
VERSION |
1 byte | The address version; v3 uses the value x03. |
| Base32 result | 56 characters | The 35 bytes above encoded with Base32, followed by the literal .onion suffix. |
Thirty-five bytes contain 280 bits. Base32 represents five bits per character, so the encoded label is 56 characters long. The suffix is not part of the encoded key material; it tells software and users that the name is an onion service address.
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How the checksum is calculated
Tor calculates the checksum from the exact byte sequence:
SHA3-256(".onion checksum" | PUBKEY | VERSION)[:2]
The constant text .onion checksum, the 32 public-key bytes, and the one-byte version are concatenated in that order. Tor then computes SHA3-256 and keeps the first two bytes. Those two bytes are appended to the public key before the entire 35-byte value is Base32-encoded.
This checksum helps detect many transcription errors, such as a changed or missing character. It is not a certificate, reputation system, or proof that the operator is honest. A malicious party can publish a different valid onion address with its own key. Obtain an address through a channel you trust, and do not treat a correctly formed checksum as a trust guarantee.
Why the address does not reveal the private key
The public key is intentionally present in the address because clients need to identify the service they are trying to reach. The private master key is separate secret material held by the service operator. Public-key cryptography is designed so that knowing the public key does not allow practical recovery of the private key.
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Base32-decoding a v3 address therefore gives you three things:
- the 32-byte Ed25519 master public key;
- the two-byte checksum, which you can recompute to check the address; and
- the one-byte version value.
It does not give you the private master key, a password, or a way to impersonate the service. If someone claims that an onion address alone is enough to extract its private key, that claim conflicts with the address format specified by Tor.
Where an onion service stores its identity key
Tor’s documented hidden-service directory layout separates the public name from the secret key. The directory-format specification identifies these files:
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|---|---|---|
hostname |
The service’s onion address. | Public identity information; it is meant to be shared with clients. |
private_key_ed25519 |
The private master Ed25519 key. | Secret operator material; anyone who obtains it may be able to operate an indistinguishable service. |
The exact parent directory depends on how the service is configured, but the distinction between these two files is fundamental. Copying or publishing hostname does not expose the private key. Copying private_key_ed25519 is a compromise risk and should be handled like any other long-term signing or identity secret: restrict access, protect backups, and avoid placing it in logs, tickets, chat messages, or public repositories.
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The master identity key should also not be confused with every other key Tor uses. Onion-service operation involves introduction and rendezvous circuits, handshake keys, and, where configured, client-authorization keys. Those short-term or access-control keys serve different purposes from the long-term key that determines the onion address.
Tor’s ED25519-V3 control-protocol format
Tor exposes onion-service key operations through its control protocol. The control-protocol command specification describes an ED25519-V3 private-key blob as Base64 encoding of 64 bytes:
- A 32-byte Ed25519 secret scalar in little-endian form.
- A 32-byte pseudorandom-function (PRF) secret used by Tor’s key-blinding design.
That is not automatically the same representation used by a generic Ed25519 library or key file. A common alternative representation stores a 32-byte hash seed followed by the 32-byte public key. Tor explicitly warns that its v3 key-blinding scheme requires the scalar and PRF secret rather than simply storing that seed-plus-public-key pair.
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| Representation | Serialized secret material | Can it be pasted directly into the other format? |
|---|---|---|
Tor control protocol ED25519-V3 |
32-byte little-endian secret scalar plus 32-byte PRF secret, Base64-encoded. | No. It follows Tor’s key-blinding requirements and is not the generic seed-plus-public-key layout. |
| Common Ed25519 file example | 32-byte hash seed plus 32-byte public key. | No. A Tor import requires format-aware conversion and public-key recomputation. |
When Tor imports the control-protocol form, it recomputes the public key from the scalar. Renaming a file, changing Base64 padding, or pasting bytes into a tool that expects a different Ed25519 layout can produce an invalid key or, worse, a different identity. Use software that explicitly documents support for Tor’s ED25519-V3 format and validate the resulting public key against the intended onion address.
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Creating a service through ADD_ONION
The control protocol’s ADD_ONION command can create a service from an Ed25519 v3 key or ask Tor to generate a new key with NEW. When Tor generates a new keypair, its reply includes the private key unless the DiscardPK option is supplied.
DiscardPK changes the recovery situation, not merely the output formatting. The specification warns that if a generated key is discarded, the keypair and corresponding service cannot later be recreated from that discarded key. Treat this as a one-time operational choice, not as a backup method:
- For a persistent service, save the generated private key in a protected backup before ending the control session.
- Record the resulting onion address and verify that it matches the public identity you intend to distribute.
- Use
DiscardPKonly when losing the generated identity is acceptable and deliberate.
The control-protocol blob and the on-disk private_key_ed25519 file are two documented views of Tor’s identity-key handling. Do not assume that a Base64 value returned by ADD_ONION can be copied byte-for-byte into an on-disk file, or that an on-disk file can be pasted into a control command, without following the format required by that interface.
How the address participates in a connection
An onion address tells a client which service identity it is seeking; it is not the entire connection protocol. Tor’s protocol overview describes the high-level sequence:
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- The client uses the onion address to obtain the service’s introduction information and builds an anonymous circuit to an introduction point.
- The client sends an introduction request that identifies the rendezvous point it selected.
- The service builds its own circuit to that rendezvous point.
- Tor joins the two circuits at the rendezvous point.
- A cryptographic handshake establishes shared session keys and allows the client to confirm that it reached the intended service.
The long-term Ed25519 identity key anchors the service’s name, while the connection itself uses additional protocol keys and ephemeral material. Seeing an onion address therefore does not mean that every key used during a session is encoded in that address.
V3 addresses compared with historical v2 names
Tor’s protocol overview describes the older v2 naming scheme as an 80-bit truncated SHA-1 hash of a 1024-bit RSA key. V3 replaced that design with an Ed25519 identity public key, a checksum, and an explicit version byte. The visible result is also much longer: v3 uses 56 characters before .onion.
| Characteristic | V2 description in Tor’s historical protocol material | V3 |
|---|---|---|
| Identity basis | 80-bit truncated SHA-1 hash of a 1024-bit RSA key. | 32-byte Ed25519 master public key encoded directly in the address. |
| Address label length | Shorter historical format. | 56 Base32 characters before .onion. |
| Integrity field | Historical hash-based naming. | Two-byte checksum plus a one-byte version field. |
This comparison explains the structure of v3 addresses; it does not imply that an old-format service is currently usable. For modern onion-service work, identify the address version and follow the software’s current support policy.
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Practical checks when handling a v3 address or key
- Count the label correctly: a v3 address has 56 characters before the suffix. Do not include
.onionwhen checking the encoded label length. - Preserve exact spelling: Base32 is normally shown in lowercase for onion addresses, and changing even one character changes the decoded bytes or causes checksum failure.
- Verify the version: the decoded final byte should be the v3 value
x03. - Recompute the checksum: use the exact constant
.onion checksum, the decoded public-key bytes, and the version byte; compare the first two SHA3-256 bytes with the encoded checksum. - Keep public and private artifacts separate: share
hostnameonly when you intend to publish the address; protectprivate_key_ed25519and any control-protocol private-key response. - Validate conversions: if moving a key between Tor, a control client, and a general Ed25519 library, confirm the expected serialization, byte order, and whether the tool expects a seed, scalar, public key, or PRF secret.
The essential distinction
The onion address is a public, checksummed spelling of a service identity. Its key field is the 32-byte Ed25519 public key. The private master key lives separately and is represented differently depending on whether you are looking at Tor’s service-directory files or its control-protocol interface. Understanding those boundaries prevents the two most damaging mistakes: believing an address can be used to derive a private key, and importing a Tor ED25519-V3 blob as though it were an ordinary Ed25519 seed-and-public-key file.
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