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Server-side validation collates submitted value with a text representation of the captcha image, not the ascii sum or etc. Let's take a look at standard 5 a-z chars captcha example: Quantity of all combinations from aaaaa to zzzzz is 26^5 = 11881376 combinations. Quantity of all ascii sums of our captcha is 610-485+1 = 126 So, if we have an ascii sum ...


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You use CAPTCHA to prevent the use of automated scripts, sometimes called "bots". So you should add CAPTCHA to any part of your workflow that you are concerned will have problems from bots. Registration is one common place where this is done. You can't really add CAPTCHA to an API call as it is a web UI component. That said, you can add it to your website ...



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