  Decentralized unpermissioned peer-to-peer networks are inherently vulnerable
to spam when they allow arbitrary participants to submit content to a common
public index or registry; preventing this is difficult due to the absence of a
central arbitrator who can act as a gate-keeper. For this reason indexing of
new content even in otherwise decentralized networks (e.g. Bittorrent with DHT,
IPFS) has generally been left to centralized services such as torrent sites.
  Decentralized methods for spam prevention, such as Web of Trust, already
exist[1][2] but they require submitters to assume pseudonymous identities and
establish trust over time.
  In this paper we present a method of spam prevention that works under the
assumption that the participants are fully anonymous and do not want different
submissions of theirs to be linked to each other.
  By spam we do not specifically mean unsolicited advertising; rather it is the
practice of adding a large amount of content in a short time, saturating the
capacity of the network, and causing denial of service to peers. The purpose of
our solution is to prevent users from saturating the system, and can be
described as rate-limiting. The system should be censorship resistant: it
should not be possible for submissions to be excluded because of the content
itself, or for users to be excluded based on what they submit.
  We first discuss a solution based on a single, centralized rate-limiter that
is censorship resistant and anonymous, then we extend this to a fully
decentralized blockchain-based system and present methods to make it economical
and scalable.
