Wireway Captcha

Wireway captcha is a free and public captcha service. It uses input the users reading and entering a text, alongside a cryptocraphic challange to solve.
How it works:

1: Browser GET /captcha
   The end user's browser fetches a PNG challenge and the response
   headers X-Captcha-Id, X-PoW-Prefix, X-PoW-Difficulty, X-Captcha-TTL.

2: Browser POST /captcha/solve  { id, answer, nonce }
   The end user types the captcha text and the browser solves the
   proof-of-work (find a nonce so SHA256(prefix + nonce) has
   X-PoW-Difficulty leading zero bits). The server replies with
   { authCode, captchaId, ts }.

3: Your backend POST /captcha/verify  { captchaId, authCode, ts }
   Your own server (not the browser) calls this to confirm the
   authCode is genuine and unexpired before granting access.

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