[tor-dev] Probability of Guessing a v3 Onion Address

[procmem at riseup.net]

Hi I was wondering what the mathematical probability of guessing an
onion v3 address that is kept secret.

Or asked differently: what is the entropy of v3 addresses if an
adversary decides to bruteforce the entire keyspace?

I am struggling to come up with a usecase for authenticated v3 services
when keeping an address secret has the same effect and one can generate
multiple addresses for the same server and share them with different
entities.  The degraded usability of v3 auth services compared to v2 is
the reason I'm asking.