Comment by e2le

There are two alternatives I'm aware of, one is Attestation of Personhood[1] proposed by Cloudflare, the other is a proof-of-work[2] which the Tor project have themselves introduced[3].

[1]: https://blog.cloudflare.com/introducing-cryptographic-attest...

[2]: https://github.com/mCaptcha/mCaptcha

[3]: https://blog.torproject.org/introducing-proof-of-work-defens...