Password Authentication Protocol An authentication protocol that allows Point-to-Point Protocol (PPP) peers to authenticate one another. The remote router attempting to connect to the local router is required to send an authentication request. Unlike Challenge Handshake Authentication Protocol (CHAP), PAP passes the password and host name or username in the clear (unencrypted). PAP does not itself prevent unauthorized access, but merely identifies the remote end. The router or access server then determines if that user is allowed access. PAP is supported only on PPP lines. Compare with CHAP.