challenge-response system

Challenge-response systems take a number of different approaches to the task of separating humans from machines. Typically, when a message is received, the system sends a reply that includes a URL linking the user to a Web site. At the Web site, the user is asked to perform some task that, while easy for a human, is beyond the capabilities of an automated spamming program. The system might ask the answer to a simple question, for example, or require the user to copy distorted letters or numbers displayed in an image.

Companies that provide free e-mail accounts often use a challenge-response system to ensure that their accounts aren't given out to spammer's programs. According to Carnegie Mellon's CAPTCHA Project, computerized programs can create thousands of new e-mail accounts per second, each of which can be used to send out reams of spam.

Read more about challenge-response system: - Carnegie Mellon's CAPTCHA Project offers more information in "Telling Humans and Computers Apart (Automatically) or How Lazy Cryptographers Do AI." - A TechNewsWorld article is called "Gates Wants to Give E-Mail Users Anti-Spam Weapons."

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