Detailně jsem to nezkoumal, ale vycházím z toho, že je to otisk veřejného klíče, podobně jako u Bitcoin peněženky. A mojí domněnku potvrzuje i Wikipedia:
Addresses in the .onion TLD are generally opaque, non-mnemonic, 16-character alpha-semi-numeric hashes which are automatically generated based on a public key when a hidden service is configured. These 16-character hashes can be made up of any letter of the alphabet, and decimal digits from 2 to 7, thus representing an 80-bit number in base32. It is possible to set up a human-readable .onion URL (e.g. starting with an organization name) by generating massive numbers of key pairs (a computational process that can be parallelized) until a sufficiently desirable URL is found.