Password Authentication Protocol
PAP applies a two-way handshaking procedure. After the link is established the originator sends an id-password pair to the server. If authentication succeeds the server sends back an acknowledgement; otherwise it either terminates the connection or gives the originator another chance.
PAP is not a strong authentication method. Passwords are sent over the circuit "in the clear" and there is no protection against playback or repeated "trial and error" attacks. The originator is in total control of the frequency and timing of the attempts. Therefore, any server that can use a stronger authentication method, such as CHAP, will offer to negotiate that method prior to PAP. The use of PAP is appropriate, however, if a plaintext password must be available to simulate a login at a remote host.
PAP is defined in RFC 1334.