There have been suggestions of proof of work requirements that exclude GPUs or ASICS (or try to) but I was wondering if anyone had suggested a proof of work that is not suited to being solved by computer at all, so that the miners would be humans working on proving their own work done - work that cannot (currently) be performed by a computer in a competitive amount of time.

This would presumably need to be quickly verifiable by computer, but not independently solvable by computer.

If this hasn't yet been proposed, is there any apparent reason this would not work?

  • It won't work because we haven't found anything simple enough for the majority of humans to do, while being both computer-verifiable and too difficult for a computer to do (see: all the failed variations on CAPTCHA).
    – Mark
    Oct 28, 2015 at 21:42

3 Answers 3


PoW needs to be hard to do but easy to verify (by a computer). There is very little left that humans are better at than computers (OCR, face and image recognition, IBM Watson [1] etc.). And probably nothing is easily computer verifiable.

One example are captchas. The ones that don't get read by OCR, simply get outsourced to children in our countries. You might say: still proof of human work, but there is no way for a computer to verify the result. On top of that that captchas don't satisfy some other requirements for PoW.

One such requirement is that the work is progress-free. Another: whatever the "puzzle" is, it must not be known beforehand as that would allow people to start the race early. This also implies there may not be one person or group of people that have access to the puzzles in advance as they would have to be trusted to not cheat (i.e. the system would not be trustless).

[1] https://en.m.wikipedia.org/wiki/Watson_(computer)


Yes, there is a paper about: "Designing Proof of Human-work Puzzles for Cryptocurrency and Beyond" https://eprint.iacr.org/2016/145.pdf

  • The linked paper addresses this exact question and is very interesting. However, it would be helpful to also have a description of the concepts here in the answer. Otherwise this seems more like a comment.
    – trichoplax
    Mar 23, 2016 at 14:05
  • The solution in the paper depends on cryptographic computer program obfuscation. Unfortunately, cryptographic code obfuscation is known to be impossible since around 2000 (there is a mathematical proof that these programs cannot be obfuscated). To make matters worse, cryptographic obfuscation goes far beyond any of the cryptography that we have today since a good enough cryptographic code obfuscator will be able to solve all of the problems in cryptography. The technology required to make this proof-of-work puzzle work simply does not exist. May 26, 2017 at 20:15

Something like this has been created and it is called Idena.

Idena is a novel way to formalize people on the blockchain. It does not collect or store personally identifiable information. Idena proves the humanness and uniqueness of its participants by running an AI-hard Turing test at the same time for everyone around the globe.

The Idena blockchain is driven by proof-of-person consensus: Every node is linked to a cryptoidentity, one single person with equal voting power.

Website: https://idena.io/

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