Password Authentication Protocol, sometimes abbreviated PAP, is a simple authentication protocol used to authenticate a user to a remote access server or Internet service provider(ISP). PAP is underused by Point to Point Protocol (PPP).
Authentication is a process of validating a user, accessing the resources. Almost all Network operating system remote servers support PAP.
PAP transmits unencrypted ASCII passwords over the network and is therefore considered insecure, it is used as a last resort when the remote access server does not support a stronger authentication protocol.
See also
Point to Point Protocol has two protocols for authentications, which are PAP (this article) and
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PAP password is also sent in clear text which leaves it vulnerable for packet sniffers.