Challenge Handshake Authentication Protocol

Challenge Handshake Authentication Protocol

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The Challenge Handshake Authentication Protocol (CHAP) is a type of authentication protocol used by Point to Point Protocol (PPP) servers to validate the identity of remote clients.

Here’s how CHAP works:

  1. After the initial link establishment, the server sends a “challenge” message to the client.
  2. The client responds with a value obtained by using a one-way hash function (usually MD5) on the challenge and the client’s password.
  3. The server checks the response against its own calculation of the expected hash value. If the values match, the server authenticates the client; otherwise, the connection is terminated.
  4. This challenge-handshake process can be repeated at random intervals by the server to ensure the continued presence of the client.

This method avoids transmitting the password itself over the network, providing a more secure means of checking the client’s credentials.

One of the key features of CHAP over other authentication protocols is that it periodically verifies the client’s identity during a connection, minimizing the time that a third party has to capture the authentication credentials. In contrast, other authentication protocols, such as PAP (Password Authentication Protocol), only authenticate users upon initial connection.

As of my knowledge cutoff in September 2021, CHAP is used in various situations where PPP connections are established, such as in Virtual Private Networks (VPNs) and dial-up networking.

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