TL;DR
This post is about testing heuristic : “Do not keep your credit card and PIN together in your wallet”.
Heuristic is commonsense rule (or set of rules) intended to increase the probability of solving some problem [WolframAlpha]. Heuristic is fallible.
Captcha is [WolframAlpha]
a type of computer-administered test, usually in the form of distorted text or images, aimed at determining whether the respondant is a human or computer; used as a security measure on many websites to block automatically generated spam, since computers should be unable to respond correctly
Here is how we can apply this heuristic on capcha problem. Developer finished his captcha code. You hit Chrome developer tool Inspect feature on Captcha element, and you see this:
There is captcha question “odaberi kokice”, and three answers, radio values 1, 2 and 3 with appropriate image.
Can you apply credit card and PIN heuristic here?
Credit card is question, and PIN is radio image name. Image name contains PIN value. So it is possible, using simple algorithm, to automated answer to this captcha. In this example:
if captch_question is odaberi kokice the select img/captch-kokice.png.
In proper captcha, img src must not contain easy decodable captcha answer.
<img src="0efc2e4ab9e04bc9dc66833dbb98505438c26f5557713e53c69090b586e62c4ceff814a5def8174f5c6d417aec5c2d2d1829fafaa9d12b461b3b0fff0ab894a4.png">
Also, having more than three captcha answers helps in your fight against crawlers, spiders and robots.
