https://speakerdeck.com/gtank/solving-the-cloudflare-captcha-rwc2017
Solving the Cloudflare CAPTCHA (RWC2017) - Speaker Deck
Blind signatures to limit the number of CAPTCHAs presented to Tor users. Video: https://vimeo.com/199228698
solvingcloudflarecaptchaspeakerdeck