An authentication scheme used in Pointto-Point Protocol (PPP) remote access servers (RASs) to validate the identity of a remote user. CHAP employs a challenge-response mechanism that challenges the remote user with a random number.The user responds with a digest, which is an encrypted password based on the random number challenge.The RAS then decrypts the password using that same random number key to verify the identity of the remote user. This approach is much more secure that the predecessor Password Authentication Protocol (PAP). See also authentication, encryption, PAP, password, PPP, and RAS.