Password Authentication Protocol

Password Authentication Protocol (PAP) is a simple, password-based authentication scheme used primarily within Point-to-Point Protocol (PPP) environments to verify user identities. PAP is widely supported across network operating systems, network access servers and PPPoE implementations for DSL authentication. Despite its broad compatibility and ease of use, PAP is considered a weak authentication method because PPP transmits data — including usernames and passwords — in cleartext. This characteristic exposes PAP-based sessions to interception whenever the underlying communication channel is not physically secure.
Although stronger alternatives exist, PAP continues to serve specific use cases where simplicity, interoperability and low computational overhead outweigh concerns around cryptographic protection. It also remains relevant in higher-layer protocols such as RADIUS and Diameter, where transport-layer encryption mitigates the inherent weaknesses of PAP.

Functional Overview and Protocol Characteristics

PAP operates through a straightforward two-way handshake between the client and the server during the initial establishment of a PPP link. Its design prioritises simplicity: the client transmits a username and password directly to the server, and the server accepts or rejects the authentication request based on these credentials.
Key characteristics include:

• Cleartext transmission: User credentials are sent without encryption, making them readable to any attacker capable of monitoring the PPP connection.
• Low computational overhead: PAP imposes minimal processing requirements compared to stronger security protocols.
• Dependence on physical link security: PAP is best suited to environments where interception is unlikely, such as home DSL connections or other controlled local links.

While PAP’s simple design facilitates compatibility and ease of deployment, it provides no protection from eavesdropping or replay attacks. For this reason, many modern frameworks treat PAP as a legacy or fallback mechanism rather than a primary authentication strategy.

Alternatives and Security Considerations

Several alternative authentication methods improve upon the security limitations of PAP:

• Challenge-Handshake Authentication Protocol (CHAP): Uses challenge–response cycles to reduce exposure of passwords during transmission.
• Extensible Authentication Protocol (EAP): Supports multiple authentication types and is widely used in modern, secure networking environments.
• Transport Layer Security (TLS): Provides encrypted communication channels suitable for scenarios where strong security is required.
• IPsec: Offers network-layer security for protecting data and credentials during transmission.

While stronger protocols provide enhanced protection, they may involve greater complexity, increased resource consumption or compatibility constraints, making PAP an acceptable choice in narrowly defined use cases.

PAP in Other Protocols

The term PAP is sometimes used in broader authentication contexts, such as within RADIUS or Diameter. In these systems, PAP refers to a method of passing user credentials in a password-based form. However, unlike PPP, these frameworks typically operate within encrypted tunnels or protected transport mechanisms. As such, PAP in these contexts does not inherit the same vulnerabilities associated with cleartext transmission over unsecured links.

Advantages of PAP

Despite its weaknesses, PAP offers several practical benefits:

• Secure storage flexibility: Because the server receives the password in cleartext, it can store a hashed form of the password using secure, one-way cryptographic functions. Even if an attacker compromises the password database, recovering original passwords may be computationally infeasible.
• Implementation simplicity: PAP is easy to configure and deploy, making it suitable for low-complexity or resource-constrained environments.
• Interoperability: PAP is universally supported by PPP implementations and many authentication servers.

These strengths make PAP particularly useful when transmission occurs over secure or trusted physical media and ease of implementation is a priority.

Working Cycle of PAP

The operational sequence of PAP during PPP link setup follows a predictable pattern:

1. Client transmits credentials: A username and password are sent directly to the authentication server. This message may be resent repeatedly until a response is received.
2. Server validates credentials: The server compares the received password with its stored version.
3. Server response:
   • Authentication-Ack: Sent when credentials match and the link is approved.
   • Authentication-Nak: Sent when credentials fail validation and the connection is denied.

The protocol involves no encryption, challenge-response mechanism or protection from credential capture. Its simplicity makes it accessible yet inherently insecure unless supplemented by external protections.

PAP Packet Structure

PAP operates within the PPP frame structure. PAP packets are identified by the PPP protocol field containing the hexadecimal value C023. Standard PAP packet types include authentication requests, acknowledgement responses and negative acknowledgements. These packets form part of the larger PPP negotiation process, which governs link establishment, authentication and network layer configuration.

Context and Use Cases

PAP remains relevant mainly in scenarios where:

• The physical link is considered secure.
• Simplicity and low overhead are desirable.
• Backward compatibility is necessary.