An authentication technique used by a system/server to authenticate a user. A server usually sends an unpredictable challenge (a set of numbers or letters) to the user, and the client/user will then compute a response using some special form of authentication token.

See Windows NT Challenge/Response Authentication.

A controversial method for limiting incoming e-mail. A sender of a message is sent an e-mail by the receiving mail server that requires a human response before the sender's original message will be delivered to the recipient.

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