A keypunch or key punch is a device used for precisely punching holes in specific locations on a stiff card. It was used in conjunction with early punched card computers. The punched card served as the program instruction for the computer. The locations of the punches were determined by the keys being struck by a human operator, much like typing...
Challenge-response authentication is a group or family of protocols characterized by one entity sending a challenge to another entity. The second entity must respond with the appropriate answer to be authenticated. A simple example of this is password authentication. The challenge is from a server asking the client for a password to authenticate the client's identity so that the client can be served.
Most smart card systems use challenge-response authentication. These systems require at least two things for authentication and entry: the smart card and the user’s password. Another challenge-response authentication example is the use of CAPTCHA, a form of reverse-Turing test for the system to determine if the client is a human or not. This is used to prevent spam and auto-registration of new accounts for a website or email. Biometric systems are another form of challenge-response authentication. In cryptography, zero-knowledge password proof and key agreement systems such as secure remote password, CRAM-MD5 and secure shell's challenge-response system based on RSA are considered to be very sophisticated challenge-response algorithms.
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