Paper 2016/145

Designing Proof of Human-work Puzzles for Cryptocurrency and Beyond

Jeremiah Blocki and Hong-Sheng Zhou


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 {\em 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.

Available format(s)
Publication info
A minor revision of an IACR publication in TCC 2016
Proof of Human WorkDistributed ConsensusCryptocurrencyHumanCoinBitcoinCAPTCHAPassword Protection
Contact author(s)
hszhou @ vcu edu
jblocki @ purdue edu
2016-08-24: revised
2016-02-16: received
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Creative Commons Attribution


      author = {Jeremiah Blocki and Hong-Sheng Zhou},
      title = {Designing Proof of Human-work Puzzles for Cryptocurrency and Beyond},
      howpublished = {Cryptology ePrint Archive, Paper 2016/145},
      year = {2016},
      note = {\url{}},
      url = {}
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