TL;DR
Password Authentication Protocol (PAP) is a simple, password-based method used within Point-to-Point Protocol (PPP) connections to verify user identity via a two-way handshake. Because credentials are transmitted as unencrypted plain text, PAP is considered a weak authentication scheme and is vulnerable to interception. Challenge-Handshake Authentication Protocol (CHAP) was developed to address these weaknesses, using a three-way handshake and cryptographic hashing instead of sending passwords over the network. Best practice is to configure systems to attempt CHAP first and fall back to PAP only when necessary.
What is PAP and how does it work?
PAP is a PPP authentication method that uses passwords to validate users. It is an internet standard (Request for Comments (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. 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:
- 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.
- 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.
How does PAP transmit credentials?
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 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.
Where is PAP 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 a Link Control Protocol (LCP) 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
What is the difference between PAP and CHAP?
PAP and CHAP differ in their handshake process, how credentials are transmitted, and the level of security each provides. The table below summarizes the key differences derived from the existing content:
| Feature | PAP | CHAP |
|---|---|---|
| Handshake type | Two-way | Three-way |
| Password transmission | Sent as plain text over the network | Never sent over the network |
| Encryption method | None | Cryptographic one-way hash with shared secret key |
| Mid-session re-authentication | Not supported | Supported — can run repeatedly mid-session |
| Security level | Weak; vulnerable to packet sniffing | Stronger; addresses PAP vulnerabilities |
Why was CHAP created?
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 does CHAP work?
The CHAP three-way handshake process works through the following three steps:
- 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.
- 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.
- 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.
Frequently asked questions
Why is Password Authentication Protocol (PAP) considered a weak authentication method?
PAP transmits usernames and passwords as unencrypted plain text during the two-way handshake process. This makes credentials visible to anyone who can intercept the PPP session, leaving them vulnerable to packet sniffer attacks where bad actors can steal login information.
When would a system use Password Authentication Protocol (PAP) instead of Challenge-Handshake Authentication Protocol (CHAP)?
PAP is typically used when CHAP is not supported by the software in use, when a remote host requires a simple plaintext password to simulate a login, or when inconsistencies arise from different vendors implementing CHAP in varied ways.
What is the main technical difference between Password Authentication Protocol (PAP) and Challenge-Handshake Authentication Protocol (CHAP)?
PAP uses a two-way handshake and sends the password directly over the network as plain text. CHAP uses a three-way handshake and never transmits the password — instead, it uses a cryptographic one-way hash based on a challenge string, with both the server and client sharing a secret key.
Can Password Authentication Protocol (PAP) and Challenge-Handshake Authentication Protocol (CHAP) be used together in the same system?
PAP and CHAP cannot work directly together, but a system can be configured to attempt CHAP authentication first and fall back to PAP if CHAP is unavailable. This approach maximizes security while maintaining compatibility.
What is mid-session re-authentication in Challenge-Handshake Authentication Protocol (CHAP) and why does it matter?
CHAP can be configured to run authentication checks repeatedly during an active session, not just at login. This prevents threat actors from hijacking an open PPP connection mid-session — for example, if a port remains open after a remote device disconnects — a vulnerability that PAP does not address.
How does the Challenge-Handshake Authentication Protocol (CHAP) three-way handshake protect credentials?
After a link is established, the authenticator sends a randomly generated challenge string to the user. The user creates an encrypted one-way hash using that challenge string and a shared secret password, then sends the hash back. The server decrypts and verifies the hash against the original challenge string — the actual password is never transmitted across the network at any point.
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.
