In computer security, challenge-response refers to a secret that will lead to authentication of a user. After a user requests access to a system, the server sends back random data, at which point the user encrypts the data using a password. The server can then check the result for authentication. Challenge-response is critical in the process of identifying a remote source as either human or artificial (computer).
Farlex, Inc. The Free Dictionary: Challenge-response in Computer Security. [Online, 2004.] Farlex, Inc. Website. http://encyclopedia.thefreedictionary.com/ Challenge-response%20test.