CloudFlare proposed digital signature using U2F signatrure instead of CAPTCHA. Pretty interesting, although from #privacy perspective it gives the website 100% unique identifier in the form of your public key - or to be more precise, the CAPTCHA provider, which is CloudFlare in this case.
https://blog.cloudflare.com/introducing-cryptographic-attestation-of-personhood/
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That actually works pretty cool, I've just tested with my U2F. As explained above, I wouldn't use it on a random website with no registration, but CAPTCHA is rarely used on anonymous pages.
