http://ttevhjjsjxz6uqqcjkbig5cycd7n7xv7cmd6f5fcvrqaaa7f3bj36wad.onion/technology/security/pow/index.html
With the Proof of Work (PoW) mechanism, the introduction phase for Onion
Service connections is split in two parts: During the first part, A service under load can indicate a level of effort it expect clients
to employ when solving a client puzzle, as well as a random number (a
"seed") used to calculate a solution.