[mirimir]

The hash is generated from the onion services's private key.

Edit: I'm wrong. It's based on the hash of the public key. But it's rather confusing. After you create an onion service, you get a private key and an onion address. You don't actually get a public key, just the hostname based on it. I suspect that the public key could be found somehow. But it's not used for anything, that I know of.