ISBN: | 978-5-5123-3354-9 |
High Quality Content by WIKIPEDIA articles! A challenge–response (or C/R) system is a type of spam filter that automatically sends a reply with a challenge to the (alleged) sender of an incoming e-mail. In this reply, the sender is asked to perform some action to assure delivery of the original message, which would otherwise not be delivered. The action to be performed is typically one that