Proof of Work for onion service introduction

The overall denial-of-service prevention strategies in Tor are described in the Denial-of-service prevention mechanisms in Tor document. This document describes one specific mitigation, the proof-of-work client puzzle for onion service introduction.

This was originally proposal 327, A First Take at PoW Over Introduction Circuits authored by George Kadianakis, Mike Perry, David Goulet, and tevador.