I fear there is no perfect "full crypto" system for that, so your easiest bet may be to just go with faceio or similar methods. But I just read this thread having previously discussed a related problem in
this thread about a Universal Basic Income on blockchain (basically a continuous airdrop each month or so), and maybe some of the following "proof of humanity" methods which were mentioned there may be interesting.
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Bitpeople - that would be a "full crypto solution" where users verify each other via videochat without a central party/verifier, but it seems to be in a very early state of development, so the dev costs would be very high. And the users would have to verify each other each month, so while there is no "cost" for them, they need to take 15 min of time. In addition, I have some doubts about the implications of AI video technology.
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Resilience, a protocol based on similar ideas than the original Ripple, where users to get airdrops need trust from other users. However, it seems to be only sybil proof at very high transaction rates and is also only a prototype.
- explore existing "UBI coin" systems like
Circles UBI, which also is based on the Ripple method; according to one user in the above thread it is inferior to Resilience but it actually already exists.
- perhaps explore early Ripple too.
One idea I also find interesting, but is very different, is let users verify each other sending a very small amount to a bank account on their name, and let them hash the bank data (Bisq does this). Do that two times with 2-week or 4-week distance to prevent someone using stolen accounts. Perhaps you can even use Bisq accounts as PoH, i.e. you have to trade twice on Bisq for full PoH? However it would not be really PoH but "proof of unique bank account".