Designing Proof of Human-Work Puzzles for Cryptocurrency and Beyond

Conference paper

DOI: 10.1007/978-3-662-53644-5_20

Volume 9986 of the book series Lecture Notes in Computer Science (LNCS)
Cite this paper as:
Blocki J., Zhou HS. (2016) Designing Proof of Human-Work Puzzles for Cryptocurrency and Beyond. In: Hirt M., Smith A. (eds) Theory of Cryptography. TCC 2016. Lecture Notes in Computer Science, vol 9986. Springer, Berlin, Heidelberg


We introduce the novel notion of a Proof of Human-work (PoH) and present the first distributed consensus protocol from hard Artificial Intelligence problems. As the name suggests, a PoH is a proof that a human invested a moderate amount of effort to solve some challenge. A PoH puzzle should be moderately hard for a human to solve. However, a PoH puzzle must be hard for a computer to solve, including the computer that generated the puzzle, without sufficient assistance from a human. By contrast, CAPTCHAs are only difficult for other computers to solve — not for the computer that generated the puzzle. We also require that a PoH be publicly verifiable by a computer without any human assistance and without ever interacting with the agent who generated the proof of human-work. We show how to construct PoH puzzles from indistinguishability obfuscation and from CAPTCHAs. We motivate our ideas with two applications: HumanCoin and passwords. We use PoH puzzles to construct HumanCoin, the first cryptocurrency system with human miners. Second, we use proofs of human work to develop a password authentication scheme which provably protects users against offline attacks.

Copyright information

© International Association for Cryptologic Research 2016

Authors and Affiliations

  1. 1.Purdue UniversityWest LafayetteUSA
  2. 2.Virginia Commonwealth UniversityRichmondUSA