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What's the exact algorithm and where is it specified in documentation?

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The encoding of the onion service address is specified in section 6 of the rendezvous specification: https://github.com/torproject/torspec/blob/master/rend-spec-v3.txt#L2136

The onion address of a hidden service includes its identity public key, a version field and a basic checksum. All this information is then base32 encoded as shown below:

onion_address = base32(PUBKEY | CHECKSUM | VERSION) + ".onion"
CHECKSUM = H(".onion checksum" | PUBKEY | VERSION)[:2]

where:

  • PUBKEY is the 32 bytes ed25519 master pubkey of the hidden service.
  • VERSION is an one byte version field (default value \x03)
  • ".onion checksum" is a constant string
  • CHECKSUM is truncated to two bytes before inserting it in onion_address
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