Glossary
What is a proof-of-work CAPTCHA?
Definition
A proof-of-work CAPTCHA asks the visitor's device to solve a computational problem, often a hash challenge, in the background. The cost is small for one visit and adds up for someone sending thousands.
How it works
- The server issues a challenge.
- The browser spends a short time computing a solution, usually in a Web Worker.
- The server checks the solution, which is cheap to verify.
Where it matters
- Visitors see little or no challenge, which is why privacy-focused CAPTCHAs use it.
- It prices out bulk abuse but does not prove a person is present.
In CaptchaKit
CaptchaKit does not use proof of work. Its challenge is a game that the visitor plays.
Questions
Is proof of work visible to visitors?
Usually not. It runs in the background.
Does proof of work stop all bots?
No. It raises the cost per request. It does not tell a person from a script that is willing to compute.
Which tools use it?
Cap describes SHA-256 proof of work, and Private Captcha says it uses proof-of-work challenges.
Source
- Cap documentation (checked 2026-10-18)
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