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 a 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 Here are a few example addresses: pg6mmjiyjmcrsslvykfwnntlaru7p5svn6y2ymmju6nubxndf4pscryd.onion sp3k262uwy4r2k3ycr5awluarykdpag6a7y33jxop4cs2lu5uz5sseqd.onion xa4r2iadxm55fbnqgwwi5mymqdcofiu3w6rpbtqn7b2dyn7mgwj64jyd.onion
For historical notes and rationales about this encoding, see this discussion thread.