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Challenge–response authentication
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==Other non-cryptographic protocols== Challenge-response protocols are also used in non-cryptographic applications. [[CAPTCHA]]s, for example, are meant to allow websites and applications to determine whether an interaction was performed by a genuine user rather than a [[Web scraping|web scraper]] or [[Internet bot|bot]]. In early CAPTCHAs, the challenge sent to the user was a distorted image of some text, and the user responded by transcribing the text. The distortion was designed to make automated [[optical character recognition]] (OCR) difficult and prevent a computer program from passing as a human.
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