Password Authentication Protocol (PAP) Security Explained

PAP, or password authentication protocol, is a point-to-point protocol (PPP) authentication method that uses passwords to validate users. It is an internet standard (RFC 1334), password-based authentication protocol. 

Using PAP, data is not encrypted. It is sent to the authentication server as plain text. PAP uses a two-way handshake to authenticate users based on their provided username and password. 

When used in PPP, the password authentication protocol is considered a weak authentication scheme. Since data is unencrypted, it is vulnerable and visible to a bad actor who is able to view the PPP session. 

Using CHAP (challenge-handshake authentication protocol) can add an extra layer of security to the PPP session by adding a three-way handshake process. PAP is a standard login procedure used as a PPP method for authenticating users.

Understanding PAP security (password authentication protocol)

A PPP authentication method, password authentication protocol is a password-based, client-server authentication protocol. It is easy to implement and a simple authentication method. 

Using a two-way handshake, PAP authenticates users in two steps, which are as follows:

1. The user, or client, attempting to establish a PPP session with the server sends a username and password to the server through an authentication-request packet.
2. When the server is listening to requests, it will accept these credentials and verify that they match what is stored in the system. When a match is verified, an authentication-ack response packet is sent back to the user and the server will establish the PPP session between the server and user. If the credentials do not match, the PPP session is not established and an authorization-nak response packet is sent back to the user.

The password authentication protocol sends data in plain text, which can then be vulnerable to packet sniffer attacks where bad actors intercept network traffic, are able to view the PPP session, and can steal usernames and passwords. There are ways to send PAP authentication requests through encrypted channels, but alternative methods such as CHAP are often used instead.

Where PAP is used

With PAP, instead of the server sending a login request prompt and then waiting for the user to respond, the username and password are sent to a remote access server in an LCP (link control protocol) packet. Some uses for PAP include the following:

• Instances where CHAP is not supported (not all software supports CHAP)
• When simulating a login at a remote host requiring a simple plaintext password to be available
• In the event of inconsistency issues, such as when different vendors have varied implementations of CHAP

Difference between PAP & CHAP

PAP uses a two-way handshake process where the client sends their credentials to the server, the server verifies them, and the user is authenticated. CHAP uses a three-way handshake process. This adds an extra layer of security in the authentication process over password authentication protocol, helping to protect credentials from bad actors. 

CHAP was created to address security vulnerabilities within the PAP point-to-point authentication method. 

Unlike PAP, CHAP does not send the password across the network. Instead, CHAP uses cryptographic methods, which include the use of an encrypted hash for which both the server and client have the secret key.

CHAP can also be set up to run authentications repeatedly mid-session to keep threat actors from picking up a PPP connection mid-session when a port has been left open after the remote device was disconnected. CHAP adds security features to the PPP session that PAP does not have.

How CHAP works

The CHAP three-way handshake process works through the following three steps:

1. The authenticator sends an authentication challenge after the link is established. A hostname lookup is performed by the network access server on the user and sends an “ask challenge,” which includes a randomly generated challenge string, to the user to initiate the CHAP authentication.
2. The user then performs a hostname lookup. The user will use the password that is known to both the user and the server to create an encrypted one-way hash. This encrypted hash is based on the challenge string.
3. Next, the server verifies the hash by decrypting it to ensure that it matches the initial challenge string. When the strings match, an authentication-success message is sent, and the PPP session is established. If the strings do not match, an authentication-failure message is generated, and the session is terminated.

PPP can use either PAP or CHAP for authentication. While they cannot work directly together, protocols can interact with either. For example, an administrator can configure their communications protocol to try to authenticate using the more secure CHAP first, falling back to PAP when needed.

Additional resources

For details on the point-to-point protocol, see here. Check here for more information on the differences between PAP and CHAP, and here for information on how to edit the PPP path configuration file (ppp.conf). 

CHAP is considered to be more secure than password authentication protocol. Whenever possible, CHAP should be enabled first with PAP as the backup.

References

PPP Authentication Protocols. (October 1992). Internet Engineering Task Force (IETF).

Sniffing Attacks and How to Defend Against Them. (September 2021). Ciso Mag.

Point-to-Point Protocol Networking. (2012). Embedded Software (Second Edition).

Peer Authentication Using PAP and CHAP. (2010). Oracle Corporation.

Editing the PPP Path Configuration File (ppp.conf). (2010). Oracle Corporation.