Password Authentication Protocol (PAP) is a password-based authentication protocol used by Point to Point Protocol (PPP) to validate users. Almost all network operating system remote servers support PAP. PAP is specified in RFC 1334.
PAP is considered a weak authentication scheme (weak schemes are simple and have lighter computational overhead but are much more vulnerable to attack; while weak schemes may have limited application in some constrained environments, they are avoided in general). Among PAP’s deficiencies is the fact that it transmits unencrypted passwords over the network. PAP is therefore used only as a last resort when the remote server does not support a stronger scheme such as CHAP or EAP.